CA1107409A - In-core fuel management for nuclear reactor - Google Patents
In-core fuel management for nuclear reactorInfo
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- CA1107409A CA1107409A CA312,530A CA312530A CA1107409A CA 1107409 A CA1107409 A CA 1107409A CA 312530 A CA312530 A CA 312530A CA 1107409 A CA1107409 A CA 1107409A
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
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Abstract
Abstract of the Disclosure An arrangement of square fuel assemblies in the first and subsequent cycles of a pressurized water nuclear reactor wherein shimmed feed assemblies and once-burned or equivalent assemblies occupy the same checkerboard component of alternating diagonals in an inner region of the core, and the highest burned or equivalent assemblies occupy the other checkerboard component. The remainder of the core consists of an outer region containing another checker-board of feed assemblies wherein the feed assemblies occupy less than one-half the assembly locations on the core periphery.
Description
; - "" 11074Q9 Field of Invention The present invention relates to fuel management for pressurized water nuclear reactors, and in particular to the arrangement of nuclear fuel assemblies within a reactor core.
Brief Description of the Drawings . Figure 1 shows two prior art in-core fuel management schemes for a core having 241 assembly locations. The upper - left quadrant (a) shows one symmetric quadrant of an OI scheme 'J 10 and the lower right quadrant (b) shows a symmetric quadrant of an IOI scheme.
Figure 2(a) shows a symmetric quadrant embodying the present invention in a manner that facilitates comparison with the prior art shown in Figure 1, and Figure 2(b) shows a schematic identifying the inner and outer region and the checkerboard components thereof.
Figure 3 shows a typical prior art OI scheme for the first cycle of a core having a total of 241 assembly locations.
Figure 4 shows the present invention in a second cycle scheme that immediately follows the first cycle scheme shown in Figure 3.
Figure 5 shows a schematic fuel assembly having a 16 x 16 fuel lattice, 5 water holes, and 8 lattice shims.
Figure 6 shows a typical relationship between the power fraction in the inner region of the core and the difference in K infinite between the outer and inner regions.
Figure 7 shows the present invention in a first cycle scheme for a core having 217 assembly locations.
Figure 8 shows the present invention in a later cycle 3Q for a core having 217 assembly locations.
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~ - 2 -U7~9 - Figure 9 shows the present invention in a second cycle fractional batch (z=2.62) scheme that immediately follows the f irst cycle scheme shown in Figure 3.
Figure 10 shows the present invention in a third cycle fractional batch scheme that immediately follows the scheme shown in Figure 9.
Background of the Invention Modern commercial nuclear power reactors are fueled with uranium having a slightly enriched U-235 content, which necessitates that portions of the core be periodically removed and replaced with fresher fuel. The plan of replacement and arrangement of fuel during the life of the reactor, known as in-core fuel management, is a major design consideration, having both safety and economic consequences. In a typical pressurized water nuclear power reactor (PWR), the initial core loading consists o~ three approximately equal sized batches of fuel assemblies having different enrichments. In conventional terminology, batch A has the lowest enrichment, batch B a higher enrichment, and batch C the highest enrich-ment. At the end of the first cycle, typically one year inlength, batch A is removed from the reactor, batches B and C
are rearranged, and a feed batch D of fresh fuel is placed in the reactor. This procedure is typical of three hatch in-core fuel management wherein an entire batch of fuel is : removed and replaced with the same number of feed fuel assemblies eyery year for the life of the plant. It is usually desirable to achieve an equilibrium in-core fuel management scheme as ; early as possible in the plant lifetime, such that the feed ; assemblies will always haye the same enrichment and will be placed in the same locations as the previous feed assemblies, - 2a -110~4C19 and the once-burned and twice-burned assemblies that remain in the core will be shuffled to identical locations occupied .
by the previously once and twice-burned assemblies.
Having introduced the nature of the art to which the invention per~ains, and before proceeding to a more detailed description of the background of the invention, a review of the terminology commonly used in the art of nuclear reactor fuel management will be . ~ .
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i presented with a view towards defining the terms for specific use . . . . .
herein.
A fuel assembly is a square array of fuel rods connected at their ends by end fittings to form a unit that is insertable and s removable from the core. Other structure that remains fixed ~ith ~-respect to the fuel rods and end fittings during a particular cycle is also considered part of the fuel assembly. The fuel lattice within the assembly is the array of fuel rod locations of the , ~
assembly, excluding water holes. Water holes are locations in the ;~ ; iO ~ fuel assembly where fuel rods are intentionally omitted, usually ~ in order to provide space forinstrumentation or for a control rod .: ~
guide tube. These tubes are part of the structural support of the assembly and provide guides wherein control rods may be reciprocated.
~ Fixed~burnable poison shims are solid material in the fuel assembly ,' 15 containing parasit~c neutron absorbing poison having a concentration I; ~ which permits most or all of the poison to be consumed during one or ,':r,~ more cycles in the reactor. The enrichment of the fuel rods relates to the fissile isotope content at the time of first introductlon into the reactor core, i.e., when it is fresh, or feed, fuel.
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~ A batch is d group of fuel assemblies that are placed into, and~then permanently removed from, the core together. A
ot~i;s a group of fuel assemblies that are placed into the core at the same time~, but which may or may not be permanently removed at the same time. A cycle is the time during which the arrangement .,, ~
;25 of fuel in the reactor core is unchanged, usually beginning with the plscement of a feed batch or lot of fresh fuel into the care, and ending with the removal of highly burned assemblies. Typical cycles range from 10 to 15 months in duration. The number of burns an individual fuel assembly or a lot of fuel has experienced is the number of cycles it has been in the reactor core.
~ ~ A checkerboard is a pattern, superimposed on a grid region )290 ~ - 3 -.
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- of adjacent parallel rows and columns of uniformly spaced squares, - that is similar to the red and black color pattern that appears on the checkers game board. A checkerboard is characterized in that . a line drawn through the diagonal of a single red square will, ifextended in either direction throughout the region, intersect only ~;~ red squares, and similarly for the black squares. In the present` context, checkerboarding fuel assemblies in the reactor core means that . . .
one type or types of assemblies correspond to the red component squares on the game board, and other types of assemblies correspond `~`
to the black component squares on the board. The core periphery `
consists of the fuel assembly locations in the core where more than ~; a mere corner of a fuel assembly borders on the neutron reflector jr i ~
~ at the outer boundary of the core.
... . . .
` ~ It is a primary purpose of in-core fuel management to minimize the amount of U-235 or other fissile material required for a given energy output during a given cycle. This can be appreciated ;1 ~ by the~rule of thumb that for every 0.1 effective weight percent (wt~ ) increase in requ1red core average enrichment, the increased cost of fuel for that cycle is over 2 million dollars. Typical 20 ~ ~equi;l~ibrium~cycle core average enrichments are about 3.3 wt% U-235.
It can~also be appreciated that the greatest savings in overall fuel ~ ~ , costs w11~1 be achieved by minimizing the feed enrichment required for an equilibrium fuel management scheme.
The major constraint on the flexibility of in-core fuel management is imposed by very strict power distribution limitations ,t ~
required by safety considerations. For example, the predicted ;` ratio of the powers produced in the hottest fuel rod to the core average fuel rod is typically not permitted to exceed l.40. This ,~ imposes correlative requirements on the ratio of power produced in a fuel assembly to the core average assembly power, and on the - maximum rod power within an assembly to the average power in the )74(~9 assembly containing that rod. In modern commercial PWR's, fixed burnable poison shims are ~requently located in selected assemblies to control the power distr;bution. These shims are strongly absorbent when the assembly is first placed in the core, and become S weaker the longer they are exposed to the operating core environ-ment. Although the shims are useful for controlling the power distribution and other core characteristics such as the moderator temperature coefficient, the presence of residual shim poison at the end of a cycle presents an inherent reactivity penalty, and requires a greater initial U-235 enrichment (and cost) at the beginning of each cycle in order to overcome the parasitic neutron absorbing effect of the residual.
l ~ The use of shims as a power shaping means has traditionally i, been directed primarily to controlling the power distribution within A I
and between assemblies, but the use of significant numbers of shims will also affect the gross power shape in the reactor. This has economic consequences in that a power shape that is peaked radially toward~the center of the core will be more efficient in conserving neutrons within the reactor so that they may produce additional ~ .,, ~fissions, than a power shape that is peaked near the core periphery, where neutrons will leak out of the reactor and never return. Thus, for the~same core average initial enrichment (and assuming zero end , ; of cycle shim~residual), a longer cycle can be achieved when the ; ~ power shape near end of cycle is centrally peaked than when it is more uniform or peripherally peaked.
Figure 1 symbolically shows two of the most common prior art fuel management techniques implemented in a core having 241 fuel assembly locations. Each is a three batch second cycle scheme :., ; ~ for achieving the same power level and cycle length, but the arrangement of the fuel types is characteristic of the respective schemes in other cycles. The highly reactive feed fuel (D) is shown `' ;~290 - S -.
: ~, as crosshatched squares 10, 10', the less reactive once^burned (C) fuel as open squares 12, and the least reactive once-burned (B) fuel as crossed squares 14. Note that in a first cycle all batches A, B, and C, would be fresh, but the different enrichments could be represented by the three symbols, and in cycles after the second the crossed squares 14 would represent twice-burned fuel.
In order to facllitate a later description of cycle-independent fuel management, fuel loadings wi11 be designated by their relative lots. Thus, lot L is the feed or fresh lot 10, 10', L-1 the previously loaded fresh fuel 12, L-2 the next previously loaded fresh fuel 14, etc., except that in the first cycle L, L-l, and L-2 correspond to the customary C, B, and A lots, respectively, and in second cycle L, L-l, L-2, L-3 correspond to D, C, B, and A, respectively. In equilibrium cycles, the numerical portion N of the L-N designation can be thought of as the number of cycles the lot has previously resided in the core, i.e. the number of burns it has experienced.
The upper left quadrant (a) of Figure 1 shows what is commonly referred to as the Out-In (OI) prior art fuel management scheme. This is characterized by unshimmed feed (L) fuel 10 placed at the core periphery to the extent possible. Any feed assemblies ; that are left over are located towards the periphery and surrounded to the extent possible by twice-burned (L-2) fuel 14. In the next cycle, the feed fuel will have become once-burned (L-l) fuel 12, and as shown in (a), the once-burned fuel 12 is concentrated in the core center. In general, the OI scheme has an inner region of once-burned fuel 12, a peripheral region of feed fuel 10, and an intermediate region of primarily twice-burned 14 mixed with some feed 10 and once-burned 12 fuel. It is noted that this concept for arranging fuel has also been used in first cycle designs.
The OI fuel management has the advantage of providing .~
a relatively flat cosine gross core radial power shape, which helps avoid excessive local peaking. But as the fuel is depleted during the cycle, the gross power tends to shift towards the core periphery where the fresh fuel is located. The relatively high peripheral ,~ ~ 5 power, however, produces a high neutron leakage, especially at end of cycle (EOC) when the interior of the core has been depleted and the .. ~ - .
exterior is still relatively highly reactive.
A prior art attempt to improve the neutron economy and reduce the required enr~chment of the OI scheme is shown in (b) of 10- ~ Figure 1, which will be referred to as the In-Out-In (IOI) scheme.
,, In this scheme, all feed (L) assemblies 10' contain burnable ~ poison sh1ms (represented by c1rcles) and are placed towards the ;i ~ center of the core in a checkerboard pattern that alternates components ~of L assemblies lO' with components of twice-burned (L-2) assemblies lS ~ 14. The L component is violated near the core center (assembly locations~ 50, 58) in order to accommodate the well known tendency of r j I ~ the power distribution to peak in this area. All once-burned (L-l) i fuel 12 i~s placed as far as possible towards the core periphery.
Thus,~the IOI~scheme is characterized by an inner checkerboard of ~feed fuel~and twice-burned fuel and an outer region of once-burned :, 1 : : : -fuei. The IOI scheme as practiced in the prior art requires that the~shims in the feed fuel assemblies be removed after one cycle.
The~101~scheme thus places fresh shimmed fuel in the center region, then removes the shims at the end of the cycle so that at the '~25~ ~ hginning of the next cycle the once-burned fuel contains no shims.
$; ~ Since the twice-burned fuel was previously a once-burned fuel, it ; also contains no shims.
~ The major advantage of the IOI scheme is the low neutron , ~ .
leakage from the core periphery resulting from the tendency of the power distribution to remain centrally peaked throughout the burnup cycle. In addition to permitting a lower initial enrichment for the ''' ~090 - 7 -~7409 . . ~ . .
same energy extraction, the centrally peaked power distribution produces a lower radiation exposure to the reactor internals and vessels surrounding the core, and has other advantages related to the stability of the power distribution.
^ 5 The prior art requirement in the IOI scheme that the `~ shims be removed at the end of the first`cycle of exposure of the feed assemblies has an inherent disadvantage which limits the x~l~; flexibility of fuel management. The purpose of shimming the feed fuel is to control the power shape so that the fresh fuel in the central region of the core will not produce power too great in relation to the core average power. This control requlres such a ,~.
high initial concentration of poison material in the shims that the , ~ poison does not burn out by EOC and therefore a significant parasitic ,~ effect remains. Nevertheless, the advantage of the lower neutron 3. 15 leakage from the periphery is greater than the disadvantage at end ` i cycle of having a significant posion shim residual. By removing these shims prior to the next cycle, the parasitic shim effect is :^ not carried over into the next cycle. In order for the shims tobe eas1ly removed, however, they are placed in guide tubes normally ~; 20 ~ ~reserved~for control rods rather than being permanently integrated ; in the~;fuel lattice. This precludes the placement of feed assemblies under control rods in the IOI scheme, and thus eliminates one-third .,,: . ~
or more core locations from use with feed assemblies. Such loss of fuel piacement flexibility can be particularly res~i ctive if the energy extraction or cycle length 1s to be varied from equilibrium ; ~ ~ IOI values. In order to best accommodate a non-equilibrium cycle ^ ~ or to optimize the return to equilibrium, fresh fuel might well be .: ~
- ideally placed in some of these locations yet cannot be without ~ - abandoning the prior art IOI scheme. In addition, the limited . ~
flexibility of the IOI scheme is even more evident if the scheme is used in cores employing more advanced control rod designs wherein '~ ' j:
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up to twelve control rod fingers are insertable into five adjacent assemblies by the action of a single drive mechanism. These control ` rods permit greater reactivity control and offer other significant - :
advantages, yet are generally impossible to use in cores having many of the control rod guide tubes occupied for other purposes, as in the IOI scheme.
If the prior art IOI scheme is modified so that the shims are left in each feed assembly at the end of cycle C and not removed when the assembly is shuffled into a once-burned location on the - 10 core periphery in cycle C+l, the residual shim absorption near the periphery will tend to accentuate the power near the core center, requiring that the cycle C+l feed batch have even stronger shims to `i control the power peak at the beginning of cycle C+l. The fresh fuel will than have an even greater shim residual at the end of cycle C+l and, when this fuel is placed on the periphery in cycle C~2 the peripheral power will be even further depressed requiring even stronger init~al shim loadings on the next batch of feed fuel. The end of cycle residual thus would become so large as to dissipate : . , ~ the advantage in the IOI scheme of low neutron leakage.
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~ 20 Summary of the Invention : .,i t is an object of the present invention to reduce the required core enrichment relative to the prior art OI fuel management, and to increase fuel placement flexibility relative to the prior art IOI fuel management, without exceeding acceptable power distribution limits. It is another object to achieve this improvement without the necessity of removing shims from the assemblies.
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The present invention exhibits the major benefits of the two important prior art PWR fuel management schemes, yet it is ; ~ independent of them. The invention is an in-core fuel management scheme for a PWR core wherein the arrangement of fuel assemblies `: :
n resulting from satisfying certain empirical reactivity relationships ,,. ~
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.; among assemblies can be characterized by reference to an imaginary . ~ . .
boundary between a radially inner region containing about two-thirds ~; of the assemblies in the core and a radially outer region containing the remainder of the assemblies. The inner region comprises a ~ 5 checkerboard pattern of fuel assemblies in which feed (L) assemblies `~ ~ and once burned (L-l) assemblies occupy at least two-thirds of one ~-~ component of the checkerboard, and once or greater burned (L-l, L-2, .
L-3,,..L-N) assemblies occupy the other comPonent. At least some of the feed assemblies in the inner region contain fixed lumped burnable , 10 poison shims.
;,``~ In the ideal embodiment, the first component of the inner ` checkerboard consists entirely of shimmed L assemblies and L-l as-'~:
; semblies, and the second component comprises twice or greater burned assemblies. The outer region is characterized by another checkerboard "
pattern, one component of which consists entirely of feed (L) assemblies and the other component of once and higher burned (L-l, L-2,.. L-N) assemblies. The feed (L) component on the outer region ~ ~ need not match the feed and once-burned component of the checkerboard ; ~ in the inner region.
When used for the first cycle of operation, where all fuel ; os~semblies are unburned, the highest enrichment (batch C) corresponds to the feed (L) assemblies and the next highest enrichment (batch B) ::
corresponds to the once-burned (L-l) assemblies. The invention may thus be used in any cycle of reactor operation and with any refueling , ., ` ~ 25 schedule.
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i ~ ~ The present invention offers typical core average enrichment savings relative to the prior art 0I schemes of over 0.1 wt% in , .. . ~
~ ~ equilibrium cycles, 0.2 wtX in second cycle, and about 0.05 wt% in :~ first cycle. In addition, the invention permits the shims to remain ~ 30 in place in the assemblies throughout their lifetimes in the core, `- without restricting the placement of the assemblies under control rods.
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)290 - - lO -The advantage of the invention over the IOI scheme is achieved by eliminating the requirement for a sizable poison residual to suppress the power in the feed (L) assemblies at the end of cycle. Therefore, the shims can be initially weak ~ enough to allow the poison material to burn out to essentially '' zero shim residual during the first cycle they are in the core.
In the prior art IOI, the high shim worth is needed at beginning and end of cycle to control the centrally peaked power distri-~, bution caused by the concentration of fresh fuel in the central region. By replacing some of the feed assemblies with once-burned fuel, the invention places slightly less reactive (L-l) assemblies in the feed fuel component of the inner checkerboard, thereby reducing the inherent reactivity of the central region of the core relative to a completely filled feed fuel com-ponent. Thus, the inherent tendency for central power peaking at beginning and end of cycle is lower than in the IOI scheme, and the need for control of this central power with poison shims is also reduced. With the present invention, the shim worth residual at the end of cycle is low enough, less than 0.5 2Q percent K, to have only a slight effect on the power distribu-tlon at the beginning of the next cycle. Therefore, the feed fuel in the next crcle does not require excessively strong shims in order to compensate for the residual remaining in the L-l assemblies, as would be the case in the IOI scheme if the shims in the L assemfilies were not removed prior to the start of the next cycle when the previous-ly fresh fuel assem-blies become L-l as-semfilies.
In accordance with the invention there is provided in a pressurized water nuclear reactor having a multiplicity of elongated, square fuel assemblies forming a generally , -~1~)74(:t9 :`:
cylindrical core which must be periodically refueled between ..:
burnup cycles by removing a fraction of burned assemblies, rearranging the remaining burned assemblies, and inserting fresh assemblies, each assem~ly belonging to one of lots L, L-l, L-2,...L-N, depending on whether the assembly resided in the core for 0, l, 2,...or N previous burnup cycles respect-ively, the arrangement of assemblies to begin a new burnup cycle after refueling, comprising: (a) a generally cylindrical inner core region consisting of approximately two-thirds the total assemblies in the core and forming a figurative checker-board array having (1) a first checkerboard component at least two-thirds of which consists of fresh assemblies (from lot L) and assemblies (from lot L-l) having burned through only one previous burnup cycle, at least some of the fresh assemblies containing fixed burnable poison shims, and (2) a second checkerboard component consisting of assemblies (~rom lots L-l, L-2...L-N) having burned through at least one previous burnup cycle; and (b) a generally annular outer region consist-ing of the remaining assemhlies and included at least some fresh assemblies.
Description of the Preferred Embodiment As set forth in the Background of the Invention, in-core fuel management is a very important feature of nuclear power plant design. Therefore, much time and money is spent by nuclear reactor vendors to optimize the fuel management for each particular reactor through the use of detailed com-puter simulation prior to fuel fabrication. All data pre-sented in the following description of the invention were , hl ~7"~f,.
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'. ' ' , ' ' .1074~9 generated in the course of a computer simulated verification that the inventive concept would indeed satisfy the above-recited objectives of the invention. The calculational models for implementing the ~` invention are well-known in the art of nuclear reactor fuel management, and the following description usedln conjunction therewith will enable one ordinarily skilled in this art to adapt the invention for use in any size PWR for any fuel cycle requirements ordinarily desired for large electric power generating stations.
In one embodiment,the invention is implemented in a reactor core that has previously been loaded with fuel for one or more cycles accordlng to some prior art scheme. Such an embodiment is illustrated in~ Figures 2(a) and 4, where a second cycle embodying the invention immediately follows the prior art OI first cycle shown in Figure 3.
The following table, used in conjunction with Figure 3, summarizes the important fuel design properties of the OI first cycle, and will serve as a reproducible starting point for practicing the embodiment of the invention described hereinbelow.
Table 1 ; Fuel Design for First Cycle Prior Art Out-In Scheme Shown in Figure 3 Assembly No. Shims in No. Assemblies Enrichment No. Fuel Shim Type Assembly (wt% U-235)Rods Loading (wt% B4C in B4C-A1 203 ) A 0 81 1.83 19116 BL 16 36 2.49 7920 2.76 ~ BH 16 52 2.49 11440 3 37 ;~ CL 16 24 2.95 5280 2 04 CH 16 8 2 95 1760 3.37 In Figure 3, the numeral 16 in the upper left corner of each ` assembly identifies an assembly location. Location number 69 is atthe core center, and the parts of the core not shown and the fuel contained therein are merely reflections along the major axes. A
schématic of a typical fuel assembly 18 is shown in Figure 5, where fuel rods 20, fixed burnable poison lattice shims 22, water holes 24, ~290 - 13 -- , . .
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`~ ilO'74~9 nd guide tubes 26 (one sho~n) are represented. More details of the ore and fuel assembly designs can be found in the Combustion En-gineering Standard Safety Analysis Report ~CESSAR)'Docket No. STN-' 50-470 Section 4.3 (1975). " ' ~ 5 In order to more clearly compare and distinguish the ; invention from the prior art, Figures l(a) and l(b) show how the prior art would be used to design a second cycle following the same 13,800 MWD/T first cycle shown in Figure 3. In the OI prior art second cycle scheme shown in Figure l(a), the A assemblies are . ~ 10 removed (except that the most reactive A assembly is moved to the co~re center), the B fuel 14 and C assemblies 12 relocated as shown, ~ and unshi~med D assemblies 10 having an average enrichment of 3.50 `~ wt70 are inserted. The beginning of cycle 2 (BOC2) core average initial enrichment is 2.97 wt~o, sufficient for a second cycle burnup 15~ ~ ¦ of 10,000 MWD/T.. In the IOI cycle 2 pri'or art scheme shown in ,,, ~
Figure l(b), the D assemblies 10' are shimmed and have an average enrichment of about 2.96 wt,~. The core average BOC2 enrichment is about 2.75 wt% for the same energy extraction as the OI scheme.
The second cycle scheme e~bodying the present invention ~is~shown in Figures 2(a) and 4. The inventive concept contained therein is derived from the discovery that the inner checkerboard in~the IOI~scheme of Figure l(b), which has one component of feed fuel 10 (L) and another component of B (L-2) fuel 14, can be sig-nificantly violated yet give an overall improvement in the gross ., power distribution and a decrease in the required shim worth, by an interchange of feed (L) fuel 10 and C (L-l) fuel 12 according :
~' ~ to a general procedure to be described below. The resulting ne~J
; in-core fuel management s~heme can'be characterized by reference to an imaginary'boundary 28 between an inner region 30 containing about two-thirds of the assemblies and an outer regi-on 32 as shown in Figures 2(a) and 2(b). The recbmmended outer boundary of the : , .
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inner region consists of all assemblies intersected by a circle ` drawn about the core center, havlng a radius equal to three-quarters the distance from the core center to the closest point on the outer ` edge ofthe core periphery. In Figure 2(b), the distance to the s periphery is labeled P and the boundary circle radius is labeled R.
`~ The following table summarizes the feed fuel assembly properties represented in Flgure 4. The numeral 34 in Figure 4 indicates the previous location of the A, B, and C assemblies.
The numeral 36 in the lower rîght corner of the D assemblies in- -dicates the type of shim loadings and distrlbution resulting from ~ ~:
application of the method to be described below.
Table 2 Feed Fuel Design for Second Cycle Using the Invention As Shown in Fi~ure 4 ~ Assembly No. Shims No. of Enrichment Shim Loading ; 15 Type per Assembly Assemblies D401 0 32 3.28 O
D*402 8 16 3.01 1.59 D*403 4 8 3.01 1.82 D~404 8 8 3.01 1.87 ~D*405 8 4 3.01 2.21 D*406 8 8 3.01 1.99 D*A07 4 4 3.01 3.12 The following is a detailed description of the method of implementing the invention. The intent is to satisfy certain reactivity and power relationships at the beginning of each cycle, which have~been found to consistently produce, particularly at EOC, the advantages of the invention as described above. This method 25 instructs one to arrange fuel at BOC by first determining what the limiting K infinite (hereinafter K) balance in the core can be at ÉOC and still satisfy the local fuel rod peaking limits, then working i. .
~`; back to the feed assembly enrichment, shim strength, and placement that will, with burnup, come within the EOC K balance. The steps 30 in the method are based more on the characteristics of the core and . fuel assembly design than on the specific fuel management scheme used . .
, ` l~V~409 ~ ~ the prior cycle. Thus, one familiar with the basic core and fuel .
~` assembly characteristic of a particular reactor in which prior art fuel management techniques have been used, can with relatively little effort implement the present invention.
First, the core geometry is divided into an inner region ; which cohtains approximately two-thirds of the assemblies, and an -`~ outer region containing the remainder of the assemblies. A rec-ommended boun~ary between the regions is a circle about the core ; center having a radius equal to three-quarters the shortest distance from the core center to the core periphery. From previous, commonly ~ available calculations, the ratio of the hottest fuel rod in the ; inner région 30 to the average rod in the inner region is determined.
- This ratio, Pi/Fi, is preferably obtained from existing fuel ~ .
; ~ management schemes which use the present invention or the IOI tech-n1que, but OI power distributions~can be used if the calculated ratio is augmented by the ratio of power of an EOC feed assembly to the power of an adjacent EOC twice-burned assembly. This augmentation factor can be determined from a checkerboard calculation having typical ; ~ end of cycle fuel characteristics.
The next step is to determine the relationship at EOC of ~, the~difference in K between the outer region 32 and the inner region ,, ~
30~(QKo j), and the resulting ratio of the average power in the inner region to the core average power, Pi/P. Figure 6 shows this relation-ship for the 241 assembly and the 217 assembly cores shown in various . , : .
other figures, where the basic fuel assembly design shown in Figure 5 is employed. This relationship is determined from surveying several end of cycle power distributions from any fuel management scheme i ,. . .
wherein the absorption of all shim poison material is cancelled from the calculation so as to represent zero shim residual.
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The designer then chooses the design target axially integrated radial peak fuel rod to core average rod power ratio, :
290 . - - 1 6 -.
- . . -.~ . ~ . . ..
1~7~9 `~ commonly known as Fr~ a value usually imposed on the designer as a , consequence of safety considerations. By dividing Fr by the ratio,~ Pi/Pi, the maximum permitted value of Pi/P consistent with the design ; , target Fr iscbtained. In the present example, Fr is 1.41 and Pi/P,~; 5 is 1.28. The required division indicates a permitted inner regionpower ratio Fi/p of about 1.10. Referring again to Figure 6, it can be seen that the end of cycle difference ~ _j(EOC) required to produce a P~/P equal to 1.10 is 9.2%.
In order to obta~n the same K difference at beginning of l ~ 10 ~ cycle QKo-i(Boc) to assure an Fr less than,1.41 based on the dif-ference;in K determined immediately above QKo j(EOC), a correction must~be made for the difference in regionwise exposure between end of cycle and beginning of cycle. The first step is the determination of~the difference in accumulated exposure between the inner and outer '15~ ~ regions over the cycle. This difference is just (Pi/F - Fo/p) *
CYCLE LENGTH. In the present example where Fi/~ is 1.1, Po/P for the outer one-third core is 0.8, ahd for a cycle length of 10,000 MWD/MTU, the~inner;region accumulates an additional 3,000 MWD/MTU relative to the outer~region. This difference between inner and outer region 20~ ~ exposure is converted into a reactivity difference according to well-known deriva~t~ves of the change in core K with exposure. In the ;present~exampie~, the adjusted reactivity difference (unshimmed) at BOC2 is ~about 6.5%. This BOC2'~K must be further adjusted to account for~the;shim residual poison carried over from the EOCl batch B and ~. ~
r~ 25~ a few C fuel assemblies. This EOCl shim residual poison is depleted ...,.
; during the course of cycle 2 and does not contribute to the difference ' ~ in regionwise K at the EOC 2. The adjusted reactivity difference ~ , at BOC2, allowing for the shim residual carried over from cycle 1, îs 'ç,''~ about 7.5% ~ K. As will be described below, the difference between this BOC2 value and the EOC2 regionwise reactivity difference of 9.2%
; ., ', ~K is accounted for in the design through the placement of shims in ~i290 ~! l 7 . .
.
11074(~9 `::
~ the fresh assemblies. It is the latter reactivity difference, of 9.2%
- ~K, that the designer strives for an order not to exceed a peak - fuel rod power Fr of 1.41. Experience shows that in a scheme arranged -` with the present method, the absolute value of the peak and the region-- 5 wise power density PilP remain fairly constant throughout the burnup . , ~ cycle.
. :i The next step is to make a rough estimate of the required fresh feed enrichment in the D-batch, which can be obtained by taking , ; the core average initial enrichment required to produce the desired lO ~ second~cycle length using the IOI scheme and adding about O.lS wt%, , :
or us1ng~the OI scheme and subtracting about 0.4 wt%. In the present example, the D feed enr~chment is about 3.12 wt%, and the beginning . 7~ BOC2 core average enrichment is about 2.85 wt%.
At this point, the following target characteristics have '.` I ~
15~ ~ been estimated for BOC2~: the reactivity difference between the outer and~inner regions (9.2X), the amount of this reactivity difference that should be distributed as shims in the fresh assemblies in the inner region (1.7%), and the average enrichment of the fresh fuel (3.~12~wt~ It remains to choose the specific shim loadings (boron , i - .
~ content)for the fresh assemblies, and to arrange all the assemblies in~the reactor core.
This can be facilitated by performing a few preliminary tr1al and error hand calculations of ~Ko j(BOC) based on known values of~K for each fuel assembly in the core at BOC2. The assemblywise 25 ~ K's can be obtained by performing a single core reactivity calculation ; at the estimated BOC2 so1uble boron concentration with noxenon and peak samarium in the burned (L-l, L-2,...L-N) assemblies. Fresh assemblies having a variety of shim loadings are included in this ,::
~!1 calculation, so that a relation between K and shim loading is deter-~ 30 mined. The adequacy of specific shim loadings and fuel assembly -~ arrangements can be estimated through trial and error according to . ~
.. - : ... . . .
- .. . . .
.,. . . . : ' . , ' :
; the following plan.
The inner region of the core is filled with a quarter core symmetric checkerboard having one component of L and a second component of L-2 assemblies. L-l assemblies are placed toward the core periphery. An arithmetic reactivity difference is calculated between the outer and inner regions of the core, which will generally be smaller than the target ~Ko j(BOC). Shims are located in L
assemblies such that the inner region contains about 2.7% more shim worth than the outer region(l.7% in fresh assemblies and 1.0% carry-.;
over from first cycle). The inner region reactivity must be further reduced, and this is accomplished through the key step of interchanging L-l assemblies from the outer region with L assemblies from the inner region. It will be generally found advantageous to place L assemblies in several peripheral locations. The L-l and L assemblies are ~l 15 interchanged, and the shim loadings and placement are manipulated, '$:'. until the hand calculation indicates the desired QKo j(BOC) (9.2%) and the desired L assembly shim worth in the inner region (1.7%) have been achieved. At thls point, customary computer calculations , can be employed to fine-tu;ne the power distribution and to verify ,l~ 20 the estimated enrichment.
Figure 4 and Table 2 include information showing the resulting change in location of the L-l and L-2 (and a single L-3) assemblies from EOCl to BOC2. Also shown are the number of shims in each L
assembly and the shim loading in wt% of B4C (containing natural boron) in B4C-AL203 shim material. The invention is not limited to " ~ .
use with B4C shim material, however, and can be practiced, for example, with lattice shims composed of an admixture of gadolinium and fuel material (U02), or with removable shims whether or not located in the guide tube. It is well within the skill of an ordinary nuclear fuel management engineer to substitute other shim material, or other fuel lattices, without departing from the scope of the invention.
~290 - 19 -- .. ~
110741~9 ::.
It is to be understood that once the target BOC arithmetic ~Ko j difference is achieved, a computer calculation of the power ~ djstrjbution during the cycle is to be made. It is expected that - several iterations in which minor adjustments of shim loadings,fuel enrichment, or fuel assembly placement are made may be needed before satisfactory power distributions and EOC reactivity are obtained. After practicing the present invention a few times, however, one having ordinary skill will need only about two or three such iterations.
Referring again to Figure 2(a) the differences in the arrangement of fuel assemblies with the present embodiment of the ^~ invention can be identified relative to the arrangements of the prior art OI scheme shown in Figure l(a) and the IOI scheme shown in Figure l(b). With respect to the boundary between the inner and outer region indicated by a heavy line 28, the present invention consists of a checkerboard pattern in the inner region having one component consisting of L (lO, lO') and L-l (12) assemblies and a second component consisting of L-2 assemblies (14). The core geometry of Figure 2(a) is shown in Figure 2(b) where first component 40 and second component 42 lines of the inner checkerboard and third component 44 and fourth component 46 lines of the outer checkerboard (to be later described) are indicated. The prior art does not show ; a checkerboard wherein the first component 40 consists mostly of~- L and L-l assemblies. In the embodiment shown, the second component 42 of the inner checkerboard consists entirely of L-2 fuel and, when the center assembly is included, L-3 fuel.
It is also seen that the outer region 32 consists of a checkerboard of L assemblies on the third component 44 alternating with a fourth component 46 of L-l and L-2 assemblies. The OI scheme of Figure l(a) intentionally avoids checkerboarding L fuel in the outer region. There is no discernable checkerboard pattern in the ~2gO - 20 -1~7409 outer region of the IOI scheme shown in Figure l~b), since adjacent components of L-l fuel near the periphery have no L fuel.
With respect to the OI scheme of Figure l(a), none of the L assemblies is shimmed, whereas in the present invention at least some of the L assemblies 10' are shimmed. Furthermore, in the present invention less than two-thirds of the L assemblies are in the outer region, whereas in the OI scheme almost all L assemblies are in the ,~ outer region.
With respect to the IOI scheme shown in Figure l(b), no L assemblies are on the core peripher~ whereas in the present invention there are several L assemblies on the periphery. Further-., ~
more, every L assembly 10' is shimmed in the IOI scheme, whereas in the present invention the outer region includes at least some .' unshimmed assemblies 10.
The abov~ comparison of the present invention with the prior art is based on the preferred embodiment of the invention. As will be described below, different fuel management objectives may require .different relative fractions of L, L-l, L-2,.. L-N assemblies in the core, and the cheskerboard components may therefore not be as perfectly filled as in the present embodiment. Nevertheless, the essential characteristic of the present invention, the first component of the inner checkerboard consisting mostly of L and L-l fuel, is found in all embodiments of the invention.
Figure 7 shows the invention practiced in the first cycle j 25 of a core havlng 217 assembly locations. The core contains unshimmed ` A fuel 14, shimmed ~BS) 12' and unshimmed B fuel 12 and shimmed (CS) ; 10' and unshimmed C fuel 10. In this embodiment, the first component 40 of the inner region consists of L (10, 10') and L-l (12, 12') assemblies, and the second component 42 consists of L-2 (14) assemblies.
In the outer region the third component 44 is chosen from L, L-l assemblies and the fourth component 46 is chosen for L, L-l, and L-2 assemblies. Although a few minor modifications are required to .
:, the outline of steps discussed previously for implementing the inventive scheme, an ordinarily skilled nuclear reactor fuel manage-ment engineer can easily adapt the above procedures for use in designing the first cycle. For example, it is well known that in the first ; 5 cycle most or all of the B (L-l) as well as the C (L) assemblies require substantial shim loadings.
Figure 8 shows a later cycle scheme in the 217 assembly core in which the first component of the inner checkerboard contains four L-2 assemblies in each quadrant (assembly locations 16, 23, 43 and 51). This deviation from a perfect L and L-l first component is sometimes the best way to accommodate peculiarities of the core power distribution in which certain assembly locations exhibit high power peaks. It is believed that a minimum of two-thirds of the first component locations must contain L and L-l assemblies, and that at teast one-third of all L assemblies be in the inner region, in order not to depart from the inventive concept. It is noted ; that it may not be necessary to use shims in every L assembly of the first component, especially if several different enrichments are used in each batch. This would permlt concentrating the desired shim worth in only a few L assemblies in the inner region.
Although such an arrangement falls within the scope of the invention, it is believed that the power distribution cannot be controlled if more than one-third of the L assemblies in the inner region are un-shimmed.
Referring now to Figures 9, and 10, there is shown an ; application of the present invention in the 241 assembly core designed for fractional batch fuel cycles. In fractional batch management, the distinction between a lot of fuel and a batch of fuel becomes im-portant. In the normal three batch fuel management, a batch and a lot are synonymous because all the assemblies in a batch remain in the core for the same number of cycles and are removed together.
,'~
J2~0 - 2~ -' ' -: . ' ~1~)7409 In the fractional batch scheme shown in Figures 9, and 10.
some assèmblies of a batch are removed while others remain in the i core for the next cycle. For example, in the third cycle fractional batch scheme shown in Figure 107 the L or feed lot, 10, 10' contains -~ S 92 assemblies, the L-l lot 12 contains 92 assemblies, and the L-2 1 lot 14 contains only 57 assemblies. This means that before the L-l ,"~ assemblies are shuffled for the next cycle, 35 are permanently removed from the reactor, leaving only 57 as L-2 assemblies.
In the second cycle shown in Figure 9, the first component 40 of the inner checkerboard consists of L and L-l assemblies, and il the second component 42 consists of L-2 assemblies. In the outer region, the third component 44 consists of L and L-l assemblies and the fourth component 46 comprises assemblies chosen from lots , L-l and L-2.
lS In the third cycle embodiment shown in Figure 10, the first component 40 of the inner region checkerboard consists of L
and L-l assemblies, and the second component 42 consists of L-l and L-2 assemblies. In the outer region checkerboard, the third com-ponent 44 consists of L and L-l assemblies and the fourth component 46 consists of L-l assemblies.
It can be appreciated that, as fuel management schemes become more tailored to the individual needs of particular utilities, - the use of fractional batch fuel management will be more common.
~levertheless, the present invention finds application in such use and the procedures outlined above for implementing the inventive scheme can easily be adapted for use with the more complex schemes.
:
.:.
_ . . , . .. .. , .. . , __.
Brief Description of the Drawings . Figure 1 shows two prior art in-core fuel management schemes for a core having 241 assembly locations. The upper - left quadrant (a) shows one symmetric quadrant of an OI scheme 'J 10 and the lower right quadrant (b) shows a symmetric quadrant of an IOI scheme.
Figure 2(a) shows a symmetric quadrant embodying the present invention in a manner that facilitates comparison with the prior art shown in Figure 1, and Figure 2(b) shows a schematic identifying the inner and outer region and the checkerboard components thereof.
Figure 3 shows a typical prior art OI scheme for the first cycle of a core having a total of 241 assembly locations.
Figure 4 shows the present invention in a second cycle scheme that immediately follows the first cycle scheme shown in Figure 3.
Figure 5 shows a schematic fuel assembly having a 16 x 16 fuel lattice, 5 water holes, and 8 lattice shims.
Figure 6 shows a typical relationship between the power fraction in the inner region of the core and the difference in K infinite between the outer and inner regions.
Figure 7 shows the present invention in a first cycle scheme for a core having 217 assembly locations.
Figure 8 shows the present invention in a later cycle 3Q for a core having 217 assembly locations.
'':' ~ '.
~ - 2 -U7~9 - Figure 9 shows the present invention in a second cycle fractional batch (z=2.62) scheme that immediately follows the f irst cycle scheme shown in Figure 3.
Figure 10 shows the present invention in a third cycle fractional batch scheme that immediately follows the scheme shown in Figure 9.
Background of the Invention Modern commercial nuclear power reactors are fueled with uranium having a slightly enriched U-235 content, which necessitates that portions of the core be periodically removed and replaced with fresher fuel. The plan of replacement and arrangement of fuel during the life of the reactor, known as in-core fuel management, is a major design consideration, having both safety and economic consequences. In a typical pressurized water nuclear power reactor (PWR), the initial core loading consists o~ three approximately equal sized batches of fuel assemblies having different enrichments. In conventional terminology, batch A has the lowest enrichment, batch B a higher enrichment, and batch C the highest enrich-ment. At the end of the first cycle, typically one year inlength, batch A is removed from the reactor, batches B and C
are rearranged, and a feed batch D of fresh fuel is placed in the reactor. This procedure is typical of three hatch in-core fuel management wherein an entire batch of fuel is : removed and replaced with the same number of feed fuel assemblies eyery year for the life of the plant. It is usually desirable to achieve an equilibrium in-core fuel management scheme as ; early as possible in the plant lifetime, such that the feed ; assemblies will always haye the same enrichment and will be placed in the same locations as the previous feed assemblies, - 2a -110~4C19 and the once-burned and twice-burned assemblies that remain in the core will be shuffled to identical locations occupied .
by the previously once and twice-burned assemblies.
Having introduced the nature of the art to which the invention per~ains, and before proceeding to a more detailed description of the background of the invention, a review of the terminology commonly used in the art of nuclear reactor fuel management will be . ~ .
... .
:' . .
;
,~ .
:
J
- 2b -~ --` 1107409 ,~
i presented with a view towards defining the terms for specific use . . . . .
herein.
A fuel assembly is a square array of fuel rods connected at their ends by end fittings to form a unit that is insertable and s removable from the core. Other structure that remains fixed ~ith ~-respect to the fuel rods and end fittings during a particular cycle is also considered part of the fuel assembly. The fuel lattice within the assembly is the array of fuel rod locations of the , ~
assembly, excluding water holes. Water holes are locations in the ;~ ; iO ~ fuel assembly where fuel rods are intentionally omitted, usually ~ in order to provide space forinstrumentation or for a control rod .: ~
guide tube. These tubes are part of the structural support of the assembly and provide guides wherein control rods may be reciprocated.
~ Fixed~burnable poison shims are solid material in the fuel assembly ,' 15 containing parasit~c neutron absorbing poison having a concentration I; ~ which permits most or all of the poison to be consumed during one or ,':r,~ more cycles in the reactor. The enrichment of the fuel rods relates to the fissile isotope content at the time of first introductlon into the reactor core, i.e., when it is fresh, or feed, fuel.
.: :
~ A batch is d group of fuel assemblies that are placed into, and~then permanently removed from, the core together. A
ot~i;s a group of fuel assemblies that are placed into the core at the same time~, but which may or may not be permanently removed at the same time. A cycle is the time during which the arrangement .,, ~
;25 of fuel in the reactor core is unchanged, usually beginning with the plscement of a feed batch or lot of fresh fuel into the care, and ending with the removal of highly burned assemblies. Typical cycles range from 10 to 15 months in duration. The number of burns an individual fuel assembly or a lot of fuel has experienced is the number of cycles it has been in the reactor core.
~ ~ A checkerboard is a pattern, superimposed on a grid region )290 ~ - 3 -.
- - , .
~. . ' ~'~ ' . -'-' 1107409 ,. ~
- of adjacent parallel rows and columns of uniformly spaced squares, - that is similar to the red and black color pattern that appears on the checkers game board. A checkerboard is characterized in that . a line drawn through the diagonal of a single red square will, ifextended in either direction throughout the region, intersect only ~;~ red squares, and similarly for the black squares. In the present` context, checkerboarding fuel assemblies in the reactor core means that . . .
one type or types of assemblies correspond to the red component squares on the game board, and other types of assemblies correspond `~`
to the black component squares on the board. The core periphery `
consists of the fuel assembly locations in the core where more than ~; a mere corner of a fuel assembly borders on the neutron reflector jr i ~
~ at the outer boundary of the core.
... . . .
` ~ It is a primary purpose of in-core fuel management to minimize the amount of U-235 or other fissile material required for a given energy output during a given cycle. This can be appreciated ;1 ~ by the~rule of thumb that for every 0.1 effective weight percent (wt~ ) increase in requ1red core average enrichment, the increased cost of fuel for that cycle is over 2 million dollars. Typical 20 ~ ~equi;l~ibrium~cycle core average enrichments are about 3.3 wt% U-235.
It can~also be appreciated that the greatest savings in overall fuel ~ ~ , costs w11~1 be achieved by minimizing the feed enrichment required for an equilibrium fuel management scheme.
The major constraint on the flexibility of in-core fuel management is imposed by very strict power distribution limitations ,t ~
required by safety considerations. For example, the predicted ;` ratio of the powers produced in the hottest fuel rod to the core average fuel rod is typically not permitted to exceed l.40. This ,~ imposes correlative requirements on the ratio of power produced in a fuel assembly to the core average assembly power, and on the - maximum rod power within an assembly to the average power in the )74(~9 assembly containing that rod. In modern commercial PWR's, fixed burnable poison shims are ~requently located in selected assemblies to control the power distr;bution. These shims are strongly absorbent when the assembly is first placed in the core, and become S weaker the longer they are exposed to the operating core environ-ment. Although the shims are useful for controlling the power distribution and other core characteristics such as the moderator temperature coefficient, the presence of residual shim poison at the end of a cycle presents an inherent reactivity penalty, and requires a greater initial U-235 enrichment (and cost) at the beginning of each cycle in order to overcome the parasitic neutron absorbing effect of the residual.
l ~ The use of shims as a power shaping means has traditionally i, been directed primarily to controlling the power distribution within A I
and between assemblies, but the use of significant numbers of shims will also affect the gross power shape in the reactor. This has economic consequences in that a power shape that is peaked radially toward~the center of the core will be more efficient in conserving neutrons within the reactor so that they may produce additional ~ .,, ~fissions, than a power shape that is peaked near the core periphery, where neutrons will leak out of the reactor and never return. Thus, for the~same core average initial enrichment (and assuming zero end , ; of cycle shim~residual), a longer cycle can be achieved when the ; ~ power shape near end of cycle is centrally peaked than when it is more uniform or peripherally peaked.
Figure 1 symbolically shows two of the most common prior art fuel management techniques implemented in a core having 241 fuel assembly locations. Each is a three batch second cycle scheme :., ; ~ for achieving the same power level and cycle length, but the arrangement of the fuel types is characteristic of the respective schemes in other cycles. The highly reactive feed fuel (D) is shown `' ;~290 - S -.
: ~, as crosshatched squares 10, 10', the less reactive once^burned (C) fuel as open squares 12, and the least reactive once-burned (B) fuel as crossed squares 14. Note that in a first cycle all batches A, B, and C, would be fresh, but the different enrichments could be represented by the three symbols, and in cycles after the second the crossed squares 14 would represent twice-burned fuel.
In order to facllitate a later description of cycle-independent fuel management, fuel loadings wi11 be designated by their relative lots. Thus, lot L is the feed or fresh lot 10, 10', L-1 the previously loaded fresh fuel 12, L-2 the next previously loaded fresh fuel 14, etc., except that in the first cycle L, L-l, and L-2 correspond to the customary C, B, and A lots, respectively, and in second cycle L, L-l, L-2, L-3 correspond to D, C, B, and A, respectively. In equilibrium cycles, the numerical portion N of the L-N designation can be thought of as the number of cycles the lot has previously resided in the core, i.e. the number of burns it has experienced.
The upper left quadrant (a) of Figure 1 shows what is commonly referred to as the Out-In (OI) prior art fuel management scheme. This is characterized by unshimmed feed (L) fuel 10 placed at the core periphery to the extent possible. Any feed assemblies ; that are left over are located towards the periphery and surrounded to the extent possible by twice-burned (L-2) fuel 14. In the next cycle, the feed fuel will have become once-burned (L-l) fuel 12, and as shown in (a), the once-burned fuel 12 is concentrated in the core center. In general, the OI scheme has an inner region of once-burned fuel 12, a peripheral region of feed fuel 10, and an intermediate region of primarily twice-burned 14 mixed with some feed 10 and once-burned 12 fuel. It is noted that this concept for arranging fuel has also been used in first cycle designs.
The OI fuel management has the advantage of providing .~
a relatively flat cosine gross core radial power shape, which helps avoid excessive local peaking. But as the fuel is depleted during the cycle, the gross power tends to shift towards the core periphery where the fresh fuel is located. The relatively high peripheral ,~ ~ 5 power, however, produces a high neutron leakage, especially at end of cycle (EOC) when the interior of the core has been depleted and the .. ~ - .
exterior is still relatively highly reactive.
A prior art attempt to improve the neutron economy and reduce the required enr~chment of the OI scheme is shown in (b) of 10- ~ Figure 1, which will be referred to as the In-Out-In (IOI) scheme.
,, In this scheme, all feed (L) assemblies 10' contain burnable ~ poison sh1ms (represented by c1rcles) and are placed towards the ;i ~ center of the core in a checkerboard pattern that alternates components ~of L assemblies lO' with components of twice-burned (L-2) assemblies lS ~ 14. The L component is violated near the core center (assembly locations~ 50, 58) in order to accommodate the well known tendency of r j I ~ the power distribution to peak in this area. All once-burned (L-l) i fuel 12 i~s placed as far as possible towards the core periphery.
Thus,~the IOI~scheme is characterized by an inner checkerboard of ~feed fuel~and twice-burned fuel and an outer region of once-burned :, 1 : : : -fuei. The IOI scheme as practiced in the prior art requires that the~shims in the feed fuel assemblies be removed after one cycle.
The~101~scheme thus places fresh shimmed fuel in the center region, then removes the shims at the end of the cycle so that at the '~25~ ~ hginning of the next cycle the once-burned fuel contains no shims.
$; ~ Since the twice-burned fuel was previously a once-burned fuel, it ; also contains no shims.
~ The major advantage of the IOI scheme is the low neutron , ~ .
leakage from the core periphery resulting from the tendency of the power distribution to remain centrally peaked throughout the burnup cycle. In addition to permitting a lower initial enrichment for the ''' ~090 - 7 -~7409 . . ~ . .
same energy extraction, the centrally peaked power distribution produces a lower radiation exposure to the reactor internals and vessels surrounding the core, and has other advantages related to the stability of the power distribution.
^ 5 The prior art requirement in the IOI scheme that the `~ shims be removed at the end of the first`cycle of exposure of the feed assemblies has an inherent disadvantage which limits the x~l~; flexibility of fuel management. The purpose of shimming the feed fuel is to control the power shape so that the fresh fuel in the central region of the core will not produce power too great in relation to the core average power. This control requlres such a ,~.
high initial concentration of poison material in the shims that the , ~ poison does not burn out by EOC and therefore a significant parasitic ,~ effect remains. Nevertheless, the advantage of the lower neutron 3. 15 leakage from the periphery is greater than the disadvantage at end ` i cycle of having a significant posion shim residual. By removing these shims prior to the next cycle, the parasitic shim effect is :^ not carried over into the next cycle. In order for the shims tobe eas1ly removed, however, they are placed in guide tubes normally ~; 20 ~ ~reserved~for control rods rather than being permanently integrated ; in the~;fuel lattice. This precludes the placement of feed assemblies under control rods in the IOI scheme, and thus eliminates one-third .,,: . ~
or more core locations from use with feed assemblies. Such loss of fuel piacement flexibility can be particularly res~i ctive if the energy extraction or cycle length 1s to be varied from equilibrium ; ~ ~ IOI values. In order to best accommodate a non-equilibrium cycle ^ ~ or to optimize the return to equilibrium, fresh fuel might well be .: ~
- ideally placed in some of these locations yet cannot be without ~ - abandoning the prior art IOI scheme. In addition, the limited . ~
flexibility of the IOI scheme is even more evident if the scheme is used in cores employing more advanced control rod designs wherein '~ ' j:
., -~`` 11~)746)9 .
up to twelve control rod fingers are insertable into five adjacent assemblies by the action of a single drive mechanism. These control ` rods permit greater reactivity control and offer other significant - :
advantages, yet are generally impossible to use in cores having many of the control rod guide tubes occupied for other purposes, as in the IOI scheme.
If the prior art IOI scheme is modified so that the shims are left in each feed assembly at the end of cycle C and not removed when the assembly is shuffled into a once-burned location on the - 10 core periphery in cycle C+l, the residual shim absorption near the periphery will tend to accentuate the power near the core center, requiring that the cycle C+l feed batch have even stronger shims to `i control the power peak at the beginning of cycle C+l. The fresh fuel will than have an even greater shim residual at the end of cycle C+l and, when this fuel is placed on the periphery in cycle C~2 the peripheral power will be even further depressed requiring even stronger init~al shim loadings on the next batch of feed fuel. The end of cycle residual thus would become so large as to dissipate : . , ~ the advantage in the IOI scheme of low neutron leakage.
.. . .
~ 20 Summary of the Invention : .,i t is an object of the present invention to reduce the required core enrichment relative to the prior art OI fuel management, and to increase fuel placement flexibility relative to the prior art IOI fuel management, without exceeding acceptable power distribution limits. It is another object to achieve this improvement without the necessity of removing shims from the assemblies.
... ~
The present invention exhibits the major benefits of the two important prior art PWR fuel management schemes, yet it is ; ~ independent of them. The invention is an in-core fuel management scheme for a PWR core wherein the arrangement of fuel assemblies `: :
n resulting from satisfying certain empirical reactivity relationships ,,. ~
"':' ~'--` 11`~7409 ~.
.; among assemblies can be characterized by reference to an imaginary . ~ . .
boundary between a radially inner region containing about two-thirds ~; of the assemblies in the core and a radially outer region containing the remainder of the assemblies. The inner region comprises a ~ 5 checkerboard pattern of fuel assemblies in which feed (L) assemblies `~ ~ and once burned (L-l) assemblies occupy at least two-thirds of one ~-~ component of the checkerboard, and once or greater burned (L-l, L-2, .
L-3,,..L-N) assemblies occupy the other comPonent. At least some of the feed assemblies in the inner region contain fixed lumped burnable , 10 poison shims.
;,``~ In the ideal embodiment, the first component of the inner ` checkerboard consists entirely of shimmed L assemblies and L-l as-'~:
; semblies, and the second component comprises twice or greater burned assemblies. The outer region is characterized by another checkerboard "
pattern, one component of which consists entirely of feed (L) assemblies and the other component of once and higher burned (L-l, L-2,.. L-N) assemblies. The feed (L) component on the outer region ~ ~ need not match the feed and once-burned component of the checkerboard ; ~ in the inner region.
When used for the first cycle of operation, where all fuel ; os~semblies are unburned, the highest enrichment (batch C) corresponds to the feed (L) assemblies and the next highest enrichment (batch B) ::
corresponds to the once-burned (L-l) assemblies. The invention may thus be used in any cycle of reactor operation and with any refueling , ., ` ~ 25 schedule.
~ : ~
i ~ ~ The present invention offers typical core average enrichment savings relative to the prior art 0I schemes of over 0.1 wt% in , .. . ~
~ ~ equilibrium cycles, 0.2 wtX in second cycle, and about 0.05 wt% in :~ first cycle. In addition, the invention permits the shims to remain ~ 30 in place in the assemblies throughout their lifetimes in the core, `- without restricting the placement of the assemblies under control rods.
,.
)290 - - lO -The advantage of the invention over the IOI scheme is achieved by eliminating the requirement for a sizable poison residual to suppress the power in the feed (L) assemblies at the end of cycle. Therefore, the shims can be initially weak ~ enough to allow the poison material to burn out to essentially '' zero shim residual during the first cycle they are in the core.
In the prior art IOI, the high shim worth is needed at beginning and end of cycle to control the centrally peaked power distri-~, bution caused by the concentration of fresh fuel in the central region. By replacing some of the feed assemblies with once-burned fuel, the invention places slightly less reactive (L-l) assemblies in the feed fuel component of the inner checkerboard, thereby reducing the inherent reactivity of the central region of the core relative to a completely filled feed fuel com-ponent. Thus, the inherent tendency for central power peaking at beginning and end of cycle is lower than in the IOI scheme, and the need for control of this central power with poison shims is also reduced. With the present invention, the shim worth residual at the end of cycle is low enough, less than 0.5 2Q percent K, to have only a slight effect on the power distribu-tlon at the beginning of the next cycle. Therefore, the feed fuel in the next crcle does not require excessively strong shims in order to compensate for the residual remaining in the L-l assemblies, as would be the case in the IOI scheme if the shims in the L assemfilies were not removed prior to the start of the next cycle when the previous-ly fresh fuel assem-blies become L-l as-semfilies.
In accordance with the invention there is provided in a pressurized water nuclear reactor having a multiplicity of elongated, square fuel assemblies forming a generally , -~1~)74(:t9 :`:
cylindrical core which must be periodically refueled between ..:
burnup cycles by removing a fraction of burned assemblies, rearranging the remaining burned assemblies, and inserting fresh assemblies, each assem~ly belonging to one of lots L, L-l, L-2,...L-N, depending on whether the assembly resided in the core for 0, l, 2,...or N previous burnup cycles respect-ively, the arrangement of assemblies to begin a new burnup cycle after refueling, comprising: (a) a generally cylindrical inner core region consisting of approximately two-thirds the total assemblies in the core and forming a figurative checker-board array having (1) a first checkerboard component at least two-thirds of which consists of fresh assemblies (from lot L) and assemblies (from lot L-l) having burned through only one previous burnup cycle, at least some of the fresh assemblies containing fixed burnable poison shims, and (2) a second checkerboard component consisting of assemblies (~rom lots L-l, L-2...L-N) having burned through at least one previous burnup cycle; and (b) a generally annular outer region consist-ing of the remaining assemhlies and included at least some fresh assemblies.
Description of the Preferred Embodiment As set forth in the Background of the Invention, in-core fuel management is a very important feature of nuclear power plant design. Therefore, much time and money is spent by nuclear reactor vendors to optimize the fuel management for each particular reactor through the use of detailed com-puter simulation prior to fuel fabrication. All data pre-sented in the following description of the invention were , hl ~7"~f,.
' ' .
'. ' ' , ' ' .1074~9 generated in the course of a computer simulated verification that the inventive concept would indeed satisfy the above-recited objectives of the invention. The calculational models for implementing the ~` invention are well-known in the art of nuclear reactor fuel management, and the following description usedln conjunction therewith will enable one ordinarily skilled in this art to adapt the invention for use in any size PWR for any fuel cycle requirements ordinarily desired for large electric power generating stations.
In one embodiment,the invention is implemented in a reactor core that has previously been loaded with fuel for one or more cycles accordlng to some prior art scheme. Such an embodiment is illustrated in~ Figures 2(a) and 4, where a second cycle embodying the invention immediately follows the prior art OI first cycle shown in Figure 3.
The following table, used in conjunction with Figure 3, summarizes the important fuel design properties of the OI first cycle, and will serve as a reproducible starting point for practicing the embodiment of the invention described hereinbelow.
Table 1 ; Fuel Design for First Cycle Prior Art Out-In Scheme Shown in Figure 3 Assembly No. Shims in No. Assemblies Enrichment No. Fuel Shim Type Assembly (wt% U-235)Rods Loading (wt% B4C in B4C-A1 203 ) A 0 81 1.83 19116 BL 16 36 2.49 7920 2.76 ~ BH 16 52 2.49 11440 3 37 ;~ CL 16 24 2.95 5280 2 04 CH 16 8 2 95 1760 3.37 In Figure 3, the numeral 16 in the upper left corner of each ` assembly identifies an assembly location. Location number 69 is atthe core center, and the parts of the core not shown and the fuel contained therein are merely reflections along the major axes. A
schématic of a typical fuel assembly 18 is shown in Figure 5, where fuel rods 20, fixed burnable poison lattice shims 22, water holes 24, ~290 - 13 -- , . .
..
`~ ilO'74~9 nd guide tubes 26 (one sho~n) are represented. More details of the ore and fuel assembly designs can be found in the Combustion En-gineering Standard Safety Analysis Report ~CESSAR)'Docket No. STN-' 50-470 Section 4.3 (1975). " ' ~ 5 In order to more clearly compare and distinguish the ; invention from the prior art, Figures l(a) and l(b) show how the prior art would be used to design a second cycle following the same 13,800 MWD/T first cycle shown in Figure 3. In the OI prior art second cycle scheme shown in Figure l(a), the A assemblies are . ~ 10 removed (except that the most reactive A assembly is moved to the co~re center), the B fuel 14 and C assemblies 12 relocated as shown, ~ and unshi~med D assemblies 10 having an average enrichment of 3.50 `~ wt70 are inserted. The beginning of cycle 2 (BOC2) core average initial enrichment is 2.97 wt~o, sufficient for a second cycle burnup 15~ ~ ¦ of 10,000 MWD/T.. In the IOI cycle 2 pri'or art scheme shown in ,,, ~
Figure l(b), the D assemblies 10' are shimmed and have an average enrichment of about 2.96 wt,~. The core average BOC2 enrichment is about 2.75 wt% for the same energy extraction as the OI scheme.
The second cycle scheme e~bodying the present invention ~is~shown in Figures 2(a) and 4. The inventive concept contained therein is derived from the discovery that the inner checkerboard in~the IOI~scheme of Figure l(b), which has one component of feed fuel 10 (L) and another component of B (L-2) fuel 14, can be sig-nificantly violated yet give an overall improvement in the gross ., power distribution and a decrease in the required shim worth, by an interchange of feed (L) fuel 10 and C (L-l) fuel 12 according :
~' ~ to a general procedure to be described below. The resulting ne~J
; in-core fuel management s~heme can'be characterized by reference to an imaginary'boundary 28 between an inner region 30 containing about two-thirds of the assemblies and an outer regi-on 32 as shown in Figures 2(a) and 2(b). The recbmmended outer boundary of the : , .
g . .
~ ._....
- . - .
.
~' ~1074(:~9 ., .
inner region consists of all assemblies intersected by a circle ` drawn about the core center, havlng a radius equal to three-quarters the distance from the core center to the closest point on the outer ` edge ofthe core periphery. In Figure 2(b), the distance to the s periphery is labeled P and the boundary circle radius is labeled R.
`~ The following table summarizes the feed fuel assembly properties represented in Flgure 4. The numeral 34 in Figure 4 indicates the previous location of the A, B, and C assemblies.
The numeral 36 in the lower rîght corner of the D assemblies in- -dicates the type of shim loadings and distrlbution resulting from ~ ~:
application of the method to be described below.
Table 2 Feed Fuel Design for Second Cycle Using the Invention As Shown in Fi~ure 4 ~ Assembly No. Shims No. of Enrichment Shim Loading ; 15 Type per Assembly Assemblies D401 0 32 3.28 O
D*402 8 16 3.01 1.59 D*403 4 8 3.01 1.82 D~404 8 8 3.01 1.87 ~D*405 8 4 3.01 2.21 D*406 8 8 3.01 1.99 D*A07 4 4 3.01 3.12 The following is a detailed description of the method of implementing the invention. The intent is to satisfy certain reactivity and power relationships at the beginning of each cycle, which have~been found to consistently produce, particularly at EOC, the advantages of the invention as described above. This method 25 instructs one to arrange fuel at BOC by first determining what the limiting K infinite (hereinafter K) balance in the core can be at ÉOC and still satisfy the local fuel rod peaking limits, then working i. .
~`; back to the feed assembly enrichment, shim strength, and placement that will, with burnup, come within the EOC K balance. The steps 30 in the method are based more on the characteristics of the core and . fuel assembly design than on the specific fuel management scheme used . .
, ` l~V~409 ~ ~ the prior cycle. Thus, one familiar with the basic core and fuel .
~` assembly characteristic of a particular reactor in which prior art fuel management techniques have been used, can with relatively little effort implement the present invention.
First, the core geometry is divided into an inner region ; which cohtains approximately two-thirds of the assemblies, and an -`~ outer region containing the remainder of the assemblies. A rec-ommended boun~ary between the regions is a circle about the core ; center having a radius equal to three-quarters the shortest distance from the core center to the core periphery. From previous, commonly ~ available calculations, the ratio of the hottest fuel rod in the ; inner région 30 to the average rod in the inner region is determined.
- This ratio, Pi/Fi, is preferably obtained from existing fuel ~ .
; ~ management schemes which use the present invention or the IOI tech-n1que, but OI power distributions~can be used if the calculated ratio is augmented by the ratio of power of an EOC feed assembly to the power of an adjacent EOC twice-burned assembly. This augmentation factor can be determined from a checkerboard calculation having typical ; ~ end of cycle fuel characteristics.
The next step is to determine the relationship at EOC of ~, the~difference in K between the outer region 32 and the inner region ,, ~
30~(QKo j), and the resulting ratio of the average power in the inner region to the core average power, Pi/P. Figure 6 shows this relation-ship for the 241 assembly and the 217 assembly cores shown in various . , : .
other figures, where the basic fuel assembly design shown in Figure 5 is employed. This relationship is determined from surveying several end of cycle power distributions from any fuel management scheme i ,. . .
wherein the absorption of all shim poison material is cancelled from the calculation so as to represent zero shim residual.
`
The designer then chooses the design target axially integrated radial peak fuel rod to core average rod power ratio, :
290 . - - 1 6 -.
- . . -.~ . ~ . . ..
1~7~9 `~ commonly known as Fr~ a value usually imposed on the designer as a , consequence of safety considerations. By dividing Fr by the ratio,~ Pi/Pi, the maximum permitted value of Pi/P consistent with the design ; , target Fr iscbtained. In the present example, Fr is 1.41 and Pi/P,~; 5 is 1.28. The required division indicates a permitted inner regionpower ratio Fi/p of about 1.10. Referring again to Figure 6, it can be seen that the end of cycle difference ~ _j(EOC) required to produce a P~/P equal to 1.10 is 9.2%.
In order to obta~n the same K difference at beginning of l ~ 10 ~ cycle QKo-i(Boc) to assure an Fr less than,1.41 based on the dif-ference;in K determined immediately above QKo j(EOC), a correction must~be made for the difference in regionwise exposure between end of cycle and beginning of cycle. The first step is the determination of~the difference in accumulated exposure between the inner and outer '15~ ~ regions over the cycle. This difference is just (Pi/F - Fo/p) *
CYCLE LENGTH. In the present example where Fi/~ is 1.1, Po/P for the outer one-third core is 0.8, ahd for a cycle length of 10,000 MWD/MTU, the~inner;region accumulates an additional 3,000 MWD/MTU relative to the outer~region. This difference between inner and outer region 20~ ~ exposure is converted into a reactivity difference according to well-known deriva~t~ves of the change in core K with exposure. In the ;present~exampie~, the adjusted reactivity difference (unshimmed) at BOC2 is ~about 6.5%. This BOC2'~K must be further adjusted to account for~the;shim residual poison carried over from the EOCl batch B and ~. ~
r~ 25~ a few C fuel assemblies. This EOCl shim residual poison is depleted ...,.
; during the course of cycle 2 and does not contribute to the difference ' ~ in regionwise K at the EOC 2. The adjusted reactivity difference ~ , at BOC2, allowing for the shim residual carried over from cycle 1, îs 'ç,''~ about 7.5% ~ K. As will be described below, the difference between this BOC2 value and the EOC2 regionwise reactivity difference of 9.2%
; ., ', ~K is accounted for in the design through the placement of shims in ~i290 ~! l 7 . .
.
11074(~9 `::
~ the fresh assemblies. It is the latter reactivity difference, of 9.2%
- ~K, that the designer strives for an order not to exceed a peak - fuel rod power Fr of 1.41. Experience shows that in a scheme arranged -` with the present method, the absolute value of the peak and the region-- 5 wise power density PilP remain fairly constant throughout the burnup . , ~ cycle.
. :i The next step is to make a rough estimate of the required fresh feed enrichment in the D-batch, which can be obtained by taking , ; the core average initial enrichment required to produce the desired lO ~ second~cycle length using the IOI scheme and adding about O.lS wt%, , :
or us1ng~the OI scheme and subtracting about 0.4 wt%. In the present example, the D feed enr~chment is about 3.12 wt%, and the beginning . 7~ BOC2 core average enrichment is about 2.85 wt%.
At this point, the following target characteristics have '.` I ~
15~ ~ been estimated for BOC2~: the reactivity difference between the outer and~inner regions (9.2X), the amount of this reactivity difference that should be distributed as shims in the fresh assemblies in the inner region (1.7%), and the average enrichment of the fresh fuel (3.~12~wt~ It remains to choose the specific shim loadings (boron , i - .
~ content)for the fresh assemblies, and to arrange all the assemblies in~the reactor core.
This can be facilitated by performing a few preliminary tr1al and error hand calculations of ~Ko j(BOC) based on known values of~K for each fuel assembly in the core at BOC2. The assemblywise 25 ~ K's can be obtained by performing a single core reactivity calculation ; at the estimated BOC2 so1uble boron concentration with noxenon and peak samarium in the burned (L-l, L-2,...L-N) assemblies. Fresh assemblies having a variety of shim loadings are included in this ,::
~!1 calculation, so that a relation between K and shim loading is deter-~ 30 mined. The adequacy of specific shim loadings and fuel assembly -~ arrangements can be estimated through trial and error according to . ~
.. - : ... . . .
- .. . . .
.,. . . . : ' . , ' :
; the following plan.
The inner region of the core is filled with a quarter core symmetric checkerboard having one component of L and a second component of L-2 assemblies. L-l assemblies are placed toward the core periphery. An arithmetic reactivity difference is calculated between the outer and inner regions of the core, which will generally be smaller than the target ~Ko j(BOC). Shims are located in L
assemblies such that the inner region contains about 2.7% more shim worth than the outer region(l.7% in fresh assemblies and 1.0% carry-.;
over from first cycle). The inner region reactivity must be further reduced, and this is accomplished through the key step of interchanging L-l assemblies from the outer region with L assemblies from the inner region. It will be generally found advantageous to place L assemblies in several peripheral locations. The L-l and L assemblies are ~l 15 interchanged, and the shim loadings and placement are manipulated, '$:'. until the hand calculation indicates the desired QKo j(BOC) (9.2%) and the desired L assembly shim worth in the inner region (1.7%) have been achieved. At thls point, customary computer calculations , can be employed to fine-tu;ne the power distribution and to verify ,l~ 20 the estimated enrichment.
Figure 4 and Table 2 include information showing the resulting change in location of the L-l and L-2 (and a single L-3) assemblies from EOCl to BOC2. Also shown are the number of shims in each L
assembly and the shim loading in wt% of B4C (containing natural boron) in B4C-AL203 shim material. The invention is not limited to " ~ .
use with B4C shim material, however, and can be practiced, for example, with lattice shims composed of an admixture of gadolinium and fuel material (U02), or with removable shims whether or not located in the guide tube. It is well within the skill of an ordinary nuclear fuel management engineer to substitute other shim material, or other fuel lattices, without departing from the scope of the invention.
~290 - 19 -- .. ~
110741~9 ::.
It is to be understood that once the target BOC arithmetic ~Ko j difference is achieved, a computer calculation of the power ~ djstrjbution during the cycle is to be made. It is expected that - several iterations in which minor adjustments of shim loadings,fuel enrichment, or fuel assembly placement are made may be needed before satisfactory power distributions and EOC reactivity are obtained. After practicing the present invention a few times, however, one having ordinary skill will need only about two or three such iterations.
Referring again to Figure 2(a) the differences in the arrangement of fuel assemblies with the present embodiment of the ^~ invention can be identified relative to the arrangements of the prior art OI scheme shown in Figure l(a) and the IOI scheme shown in Figure l(b). With respect to the boundary between the inner and outer region indicated by a heavy line 28, the present invention consists of a checkerboard pattern in the inner region having one component consisting of L (lO, lO') and L-l (12) assemblies and a second component consisting of L-2 assemblies (14). The core geometry of Figure 2(a) is shown in Figure 2(b) where first component 40 and second component 42 lines of the inner checkerboard and third component 44 and fourth component 46 lines of the outer checkerboard (to be later described) are indicated. The prior art does not show ; a checkerboard wherein the first component 40 consists mostly of~- L and L-l assemblies. In the embodiment shown, the second component 42 of the inner checkerboard consists entirely of L-2 fuel and, when the center assembly is included, L-3 fuel.
It is also seen that the outer region 32 consists of a checkerboard of L assemblies on the third component 44 alternating with a fourth component 46 of L-l and L-2 assemblies. The OI scheme of Figure l(a) intentionally avoids checkerboarding L fuel in the outer region. There is no discernable checkerboard pattern in the ~2gO - 20 -1~7409 outer region of the IOI scheme shown in Figure l~b), since adjacent components of L-l fuel near the periphery have no L fuel.
With respect to the OI scheme of Figure l(a), none of the L assemblies is shimmed, whereas in the present invention at least some of the L assemblies 10' are shimmed. Furthermore, in the present invention less than two-thirds of the L assemblies are in the outer region, whereas in the OI scheme almost all L assemblies are in the ,~ outer region.
With respect to the IOI scheme shown in Figure l(b), no L assemblies are on the core peripher~ whereas in the present invention there are several L assemblies on the periphery. Further-., ~
more, every L assembly 10' is shimmed in the IOI scheme, whereas in the present invention the outer region includes at least some .' unshimmed assemblies 10.
The abov~ comparison of the present invention with the prior art is based on the preferred embodiment of the invention. As will be described below, different fuel management objectives may require .different relative fractions of L, L-l, L-2,.. L-N assemblies in the core, and the cheskerboard components may therefore not be as perfectly filled as in the present embodiment. Nevertheless, the essential characteristic of the present invention, the first component of the inner checkerboard consisting mostly of L and L-l fuel, is found in all embodiments of the invention.
Figure 7 shows the invention practiced in the first cycle j 25 of a core havlng 217 assembly locations. The core contains unshimmed ` A fuel 14, shimmed ~BS) 12' and unshimmed B fuel 12 and shimmed (CS) ; 10' and unshimmed C fuel 10. In this embodiment, the first component 40 of the inner region consists of L (10, 10') and L-l (12, 12') assemblies, and the second component 42 consists of L-2 (14) assemblies.
In the outer region the third component 44 is chosen from L, L-l assemblies and the fourth component 46 is chosen for L, L-l, and L-2 assemblies. Although a few minor modifications are required to .
:, the outline of steps discussed previously for implementing the inventive scheme, an ordinarily skilled nuclear reactor fuel manage-ment engineer can easily adapt the above procedures for use in designing the first cycle. For example, it is well known that in the first ; 5 cycle most or all of the B (L-l) as well as the C (L) assemblies require substantial shim loadings.
Figure 8 shows a later cycle scheme in the 217 assembly core in which the first component of the inner checkerboard contains four L-2 assemblies in each quadrant (assembly locations 16, 23, 43 and 51). This deviation from a perfect L and L-l first component is sometimes the best way to accommodate peculiarities of the core power distribution in which certain assembly locations exhibit high power peaks. It is believed that a minimum of two-thirds of the first component locations must contain L and L-l assemblies, and that at teast one-third of all L assemblies be in the inner region, in order not to depart from the inventive concept. It is noted ; that it may not be necessary to use shims in every L assembly of the first component, especially if several different enrichments are used in each batch. This would permlt concentrating the desired shim worth in only a few L assemblies in the inner region.
Although such an arrangement falls within the scope of the invention, it is believed that the power distribution cannot be controlled if more than one-third of the L assemblies in the inner region are un-shimmed.
Referring now to Figures 9, and 10, there is shown an ; application of the present invention in the 241 assembly core designed for fractional batch fuel cycles. In fractional batch management, the distinction between a lot of fuel and a batch of fuel becomes im-portant. In the normal three batch fuel management, a batch and a lot are synonymous because all the assemblies in a batch remain in the core for the same number of cycles and are removed together.
,'~
J2~0 - 2~ -' ' -: . ' ~1~)7409 In the fractional batch scheme shown in Figures 9, and 10.
some assèmblies of a batch are removed while others remain in the i core for the next cycle. For example, in the third cycle fractional batch scheme shown in Figure 107 the L or feed lot, 10, 10' contains -~ S 92 assemblies, the L-l lot 12 contains 92 assemblies, and the L-2 1 lot 14 contains only 57 assemblies. This means that before the L-l ,"~ assemblies are shuffled for the next cycle, 35 are permanently removed from the reactor, leaving only 57 as L-2 assemblies.
In the second cycle shown in Figure 9, the first component 40 of the inner checkerboard consists of L and L-l assemblies, and il the second component 42 consists of L-2 assemblies. In the outer region, the third component 44 consists of L and L-l assemblies and the fourth component 46 comprises assemblies chosen from lots , L-l and L-2.
lS In the third cycle embodiment shown in Figure 10, the first component 40 of the inner region checkerboard consists of L
and L-l assemblies, and the second component 42 consists of L-l and L-2 assemblies. In the outer region checkerboard, the third com-ponent 44 consists of L and L-l assemblies and the fourth component 46 consists of L-l assemblies.
It can be appreciated that, as fuel management schemes become more tailored to the individual needs of particular utilities, - the use of fractional batch fuel management will be more common.
~levertheless, the present invention finds application in such use and the procedures outlined above for implementing the inventive scheme can easily be adapted for use with the more complex schemes.
:
.:.
_ . . , . .. .. , .. . , __.
Claims (12)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a pressurized water nuclear reactor having a multiplicity of elongated, square fuel assemblies forming a generally cylindrical core which must be periodically refueled between burnup cycles by removing a fraction of burned assemblies, rearranging the remaining burned assem-blies, and inserting fresh assemblies, each assembly belong-ing to one of lots L, L-1,L-2, ...L-N, depending on whether the assembly resided in the core for 0, 1, 2...or N previous burnup cycles respectively, the arrangement of assemblies to begin a new burnup cycle after refueling, comprising:
(a) a generally cylindrical inner core region consisting of approximately two-thirds the total assemblies in the core and forming a figurative checkerboard array having (1) a first checkerboard component at least two-thirds of which consists of fresh assemblies (from lot L) and assemblies (from lot L-1) having burned through only one previous burnup cycle, at least some of the fresh assemblies containing fixed burn-able poison shims, and (2) a second checkerboard component consisting of assemblies (from lots L-1, L-2...L-N) having burned through at least one previous burnup cycle; and (b) a generally annular outer region consisting of the remaining assemblies and included at least some fresh assemblies.
(a) a generally cylindrical inner core region consisting of approximately two-thirds the total assemblies in the core and forming a figurative checkerboard array having (1) a first checkerboard component at least two-thirds of which consists of fresh assemblies (from lot L) and assemblies (from lot L-1) having burned through only one previous burnup cycle, at least some of the fresh assemblies containing fixed burn-able poison shims, and (2) a second checkerboard component consisting of assemblies (from lots L-1, L-2...L-N) having burned through at least one previous burnup cycle; and (b) a generally annular outer region consisting of the remaining assemblies and included at least some fresh assemblies.
2. The arrangement of Claim 1 wherein less than two-thirds of the lot L assemblies in the core are in said outer region.
3. The arrangement of Claim 2 wherein at least one but less than one-half of the lot L assemblies in the core are on the core periphery.
4. The arrangement of Claim 3 wherein at least some of the lot L assemblies in said outer region are unshimmed.
5. The arrangement of Claim 4 wherein the shimmed assemblies in lot L have lattice shims.
6. The arrangement of Claim 1 wherein at least two-thirds of the lot L assemblies in said inner region contain fixed burnable poison shims.
7. The arrangement of Claim 1 wherein the first components of the inner checkerboard consist of lot L and lot L-1 assem-blies, each of said lot L assemblies in said first component containing fixed burnable poison shims.
8. The arrangement of Claim 7 wherein said outer region consists of an outer checkerboard having a third component consisting of assemblies from lots L and L-1 and a fourth component comprising assemblies chosen from lots L-1, L-2, ...L-N.
9. The arrangement of Claim 7 wherein said outer region consists of an outer checkerboard having a third component consisting of lot L assemblies, and a fourth component con-sisting of assemblies chosen from lot L-1, L-2,...L-N.
10. The arrangement of Claim 7 wherein said second component comprises assemblies chosen from lots L-2,...L-N.
11. The arrangement of Claim 10 wherein said outer region consists of an outer checkerboard having a third component consisting of lot L and lot L-1 assemblies and a fourth component comprising assemblies chosen from lot L-1, L-2,...L-N.
12. The arrangement of Claim 10 wherein said outer region consists of an outer checkerboard having a third component consisting of lot L assemblies and a fourth component comprising assemblies chosen from lot L-1, L-2, ...L-N.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US87495578A | 1978-02-03 | 1978-02-03 | |
| US874,955 | 1978-02-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1107409A true CA1107409A (en) | 1981-08-18 |
Family
ID=25364950
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA312,530A Expired CA1107409A (en) | 1978-02-03 | 1978-10-02 | In-core fuel management for nuclear reactor |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS54111086A (en) |
| CA (1) | CA1107409A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6233302B1 (en) * | 1996-11-15 | 2001-05-15 | The United States Of America As Represented By The United States Department Of Energy | Mox fuel arrangement for nuclear core |
-
1978
- 1978-10-02 CA CA312,530A patent/CA1107409A/en not_active Expired
-
1979
- 1979-02-01 JP JP978379A patent/JPS54111086A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6233302B1 (en) * | 1996-11-15 | 2001-05-15 | The United States Of America As Represented By The United States Department Of Energy | Mox fuel arrangement for nuclear core |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS54111086A (en) | 1979-08-31 |
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