CA1108316A - Integral nuclear fuel element assembly - Google Patents

Abstract

Case 4166 INTEGRAL NUCLEAR FUEL ELEMENT ASSEMBLY

A B S T R A C T

An integral nuclear fuel element assembly utilizes longitudinally finned fuel pins. The con-tinuous or interrupted pins of the fuel pins are brazed to fins of juxtaposed fuel pins or directly to the juxtaposed fuel pins or both. The integrally brazed fuel assembly is designed to satisfy the thermal and hydraulic requirements of a fuel assembly lattice having moderator to fuel atom ratios required to achieved high conversion and breeding ratios.

Description

- - Case 4166 .
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1. Field of the Invention This inyention relates to a fuel asse~bly design for use in a nuclear reactor and particularly to a fast breeder reactor utilizing plutonium as a fuel and pres-surlzed/hea~y water as a reactor coolant and moderator.

2, The ad~antages of utilizing nuclear ~reeder reactors which convert ~ertile material into fissile ma~erial and generate hea~, e~g. for power generation, ha~e been widely recognized in ~iew of the limited known fis3ionable material resources of the world. Develop-men~ o~ breeder reactors which convert the more abundant fertile uranium-238 into fissile plutonium-239 utilizing the latter as a fuel, possibly in conJunction wlth plu-tonium generated in other known reactors, and breed more ~issionable material~than is consumed, is highly desirable.
Since extens~ve tech~ological development and experience exists in the design and construction of pressurized light and heavy water reactor plants, use o~ the pressuri~ed ~ -water technology in a breeder application represents an attractive alterna~ive to development of other breeder options.
Heavy water, deuterium oxide (D2O), has essen-tially the same physical and chemical propertles as ligh~
water~ H2O. Its nuclear properties, howe~er, are dlf-ferent~ the neutron absorption cross-section and slowing - down power of D2O being mar~edly lower than that of H2O.
Hence~ the use of D2O as a coolant in a fast breeder - 2 ~
' Case 4166 ~f~3~$

application ls desirable due to its nuclear characteristics and the aprlicability of pressuri~ed water technology. In a pluton~um-uranium-deuterium oxide(Pu-U-D20~ reactor system, as the coolant to fuel atom ratio decreases~ it is known that the conversion or breeding ratios increase. The breed-ing ratio is the ratio of the number of fissile atoms produced to those consumed. High breeding ratics~ approach-ing a value o~ 1.40, may be realized in a Pu-U-D20 system i~ a fuel latt~ce geomekry is d~veloped wherein moderator to fuel volume ratios are ~d~us~ed to ~eld moderator ~o fuel atom rat~os approaching 1.0 or le~s. As the selection of a moderator to fuel atom ratio de~ines the volume of coolant per unit mass of fuel, it can be appreciated that difficulties arise in designing a ~uel lattice capable of passing adequa~e cooling flow rate at low moderator to fuel ratios. The high flow rates needed to assure adequate reactor core cooling necessitate high velocities in ~low :
channel3 tha~ are significantly restricted when achie~lng a low moderator to fuel ra~io. In the tightly packed fuel pin la~tices, the use of conventional spacer grids is dis-advantageous owing to inherent limits in fuel pin packing due to the interposed grids, a tendency to flow induced spacer grid ~ibration, the parasitic absorption of the grid plate ma~erial, and the increase in hydraulic pressure loss resulting from introduction of grids within ~he restricted flow passages.
The prior art teaches heavy water moderated and cooled reactor des~gns for particular fuel "rod" dia-meters and spacings wi~hin a moderator to fuel atom ratio range from 0.35 to 4.0 and suggests ~hat a moderator to fuel atom ratio of approxlmately 0.3 can be achieved

- 3 l~G`~3~ ~j case 4166 in a fuel lattice utilizing touchln~ ~uel rods arranged in a trian~ular pltch. Reduction of heat flux to the de~ree necess~ry to avoid potentially destructive hot spots at fuel pin contact polnks~ however, ~ould severely limit the capability of operating such a core at pres- - -surized water reactor conditio~s. ~urthermore, close spacing of the ~uel pins may lead to plugging by solid particles carried by the c~olant and prohibitiYely high reactor coolant pumping power requirements Other dif~
~iculties become readily apparent On the one hand, elim-in~tion o~ spacer grids ls desirable in order to permit the higher coolant flow velocities needed to approach the modera~or to fuel atom ratios yielding the high conversion ratio of the touching fuel rod configuration. On the other hand, elim~nation of spacer grids may result in impre~ise fuel pin spacing, flow induced vibration and unequal cool-ingi ~`~
SUMMARY OF THE INVENTION
- ~: , In accordance with the princlples of the inven-tion, the dlsad~antages of the prior art, discussed above, are effecti~ely surmou~ted by the practice of the in~
~ention. A fuel assem~ly, made in accordance with ~his inYention, utilizes longitudinally flnned fuel pin clad- i;
ding tubes arranged to form an integral fuel assembly by brazing together the continuous or interrupted ~ins of one fuel pin to the fins of other fuel pins. The in~
te~rally brazed fln ~uel pin assem~ly is designed to sat~sfy the thermal and hydraulic requirements of the very tight lattice required to achieve high breedlng ratios. ~:~
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~ Case 4166 In an alternate embod.iment the fins o~ some fuel pins may be connected directly to the tubular sect~on of other fuel ~ns 50 that the resulting assemblies have modera~or ~o fuel volume ratios which tend ko increase the breed~ng ratio in a Pu-U-D20 reac~or core.
In a further embodlment of the invent~on~ the core is fabricated from a solid mater~al ha~ng passages which are alternative~ suit,ed for coolant flo~ and ~uel retention.
Practice of the invention overcomes the dis~
advantages of the prior art by providing means ~or obtaining moderator to fuel ratios whlch are conducive to a high Pu-U-~20 reactor breeding ratio while assur-ing accurate spacing of the fuel pins without the para-sitic losses associated with the prior art~s use of spacer grlds. Furthermore~ the arrangemen~s of the ~ ~
inYention eliminate hydraulic pressure losses assoc- ;
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iated with conventional spacer grids~and reduce the - tendency o~ fuel pin~v~bration. ~he finned fuel pin arrangements, moreover, increase the strength of the pins, increase the available hea~ transfer surface and improve the overall heat transfer coeff~cient.
The various ~eatures of novelty which charac-terize the invention are pointed out with par~icularity ~-in the claims annexed to and forming a part of this specification. For a better understanding of the in-vention, its operatin~ ad~antages and ~peci~ic ob~ects ` attained by its u~e~ ~e~erence should be made to the .
accompanying drawings and descripti~e matter in which 3 there is illustrated and described a preferred embodi-~; ~ . ment of the lnvention .
; - 5 -Case 4166 BRIEF DESCRIPTION OF TH~ DRAWINGS

In the accompanying drawings, forming a part of this specification~ and in which reference numerals shown in the dra~ings designa~e like or corresponding parts throughout the same~
Figure 1 ~s a partial section in plan of a fuel ?
assembly;
Figure 2 is an ele~ation VieW of part of a number of finned fuel elements arranged ~n accordance 10with an alternate embodiment of the invention Figure 3 is an elevation vlew of part of a ~
number of fuel elements arranged in accordance with ~ -another alternate embodiment of the invention;
Figure 4 is a partial section plan of a ~uel assembly having ~uel elements arranged in accordance with still another embodiment of-the in~ention~ and Figure 5 is a part plan of a block core arrange-ment for a lo~ temperature reactor.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS :

Figure 1 shows part of a nuclear fuel asse~bly 10 of closely packed fuel plns 11 arranged in an array : with their longitudinal axes in parallel. ~ach fuel :--pin 1l consists of generally tubular cladding 12 which has a pluralIty of lcngltudinally extending fins 13 formed as part of the outer surface of the cladding and spaced circumferentially the~eabout. A nuclear fuel 14 consisting of a ~ixture of fisslle and fertile~material, is contained within the cladding 12. The fuel pins 11 :~ .
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`. Case 4166 in Figure 1 are arranged so that .the extremity of each ~in 13A ~buts ~h the extremity of a fin 13B of a ~uxta-posed fuel pin; fins of peripheral fuel pins may abut the fuel assembly can structure 15, ~he extremities of the fins shown in Figure 1 are joined to each other and to the reactor can structure by means of brazing at 16 and 17, respectiyely~ to form the integral fuel assembly 10.
The rins 13, in one embodiment, extend without interrup~ion along the longitudinal surface of the ~uel pin forming channels 20 in the interspaces o~ the fuel pins which direct reactor coolant flow (not shown) there-of the ~n within generally in parallel with ~he longitudinal axis/
The fins 13, however, need not extend continuously:along the len~th of the fuel pins but can be interrupted ~ins 21, as shown in Figures 2 and 3,so as to allow transverse flow and intermixing of the coolant through the fuel pin i~ter-spaces. The axially interrupted fins 21 of juxtaposed , fuel p~ns may be brazed to each other at 22 (Figure 2) org as shown in Figure 3 directly to the tubular portion of the fuel pin at 23. An assembly utilizin~ a combina- ~.
tion of both arrangements shown in Figures 2 and 3, i.e., fin .to fin contact and fin to tube contact, is also possible.
A finned fuel pin 26 design utilizing ~road rins 24 brazed to each other a~ 25 is shown in Figure 4. Broad .P
fins may be utilized to further 1imit the moderator volume frac~ion at some sacrifice of specific core power.
Elimination Or conventional spacer grids and the formation of fins ~s part of the tube. cladding:permits : ~
3 reduction of the reactor core moderator volume fraction Case 4166 3~

to ~alues cons~stent with the achievement o~ thé desired moderator ~o fuel ato~ ratios, Illus~rat~e physlcal design parameters are set forth in Table 1.

TABLE I

Example 1 2 3 ~
Fuel Pin Diame~er~ inches.35 .40 .40 : :
Fuel Pin Pitch~ inches ,39 - .43 .43 Clad Thickness, in~hes ,015 .020 .020 Clad Material Incoloy Type 316 Type 316 800 Stainless Stainless . Steel Steel Pitch - Diameter, inches.040 .030 .030 Number o~ ~ins per Pin 6 3 3 .~:
Fin height~ inches .020 .030 .030 ~:
Fin w~dth, inches .Q20 .030 .030 -.
Fin interruption, percent of 0 30 Fuel Volume Fraction - .6105 .6357 .6357 Structural ~olume Fraction .1381 .1659 .~1541 2C Coolant Volu~e Fraction.2514 .1984 .2102 Fuel/Coolant Volume Fraction Ratio 2.43 3.20 3.02 ~.
~ Moderator/Fuel Atom Ratio .82 .624 .66 : The fuel pins in the examples o~ ~able I are - formed in the shapes of rods~ The fuel plns o~ examples 1 and 2 are pro~ided with continuous ~in~ along their length, Example 3 ill~strates an alternate embodiment of example 2 wherein the fins traYerse approximately thirty percent o~ the }ength of the rods. The values - for the moderator to fuel atom ratios shown ln Table I
3 approxima~e normal pressurized water reactor operating - 8 - ;

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conditions including primary coolant .temperature and pressure~ fuel ~ellet shape~ clearances between the fuel pellets and clad, and percent of theoretical U02 density achieyed in the pelle~, The fuel assemblies of Table I would be typlcally formed by furnace brazing in a hydrogen atmosphere at 1950 to 2000F ~th a braæing alloy tradenamed "N~crobraz ~available ~rom the Wall-Colmonoy Corp.~ Detroit, Michigan) usin~ ~igs, ~ix~ures and methods of braze alloy placement known ln the furnace brazing art.
In still another embodiment, Figure 5 illustrates a desi~n ~or low temperature reactors suitable for breed-ing plutonium and low heat generation purpose, e.g.
residential heating. In this e~bodimen~ a fuel assembly is fabricated from a block 32 of metal, e.g. aluminum alloy. Parallel channels are formed for ~low passage and for fuei 30. The surfaces of the flow ~hannels may be roughened where needed to increase critical heat flux.
Illustrative design parameters ~or a block type reactor :
are shown in Table II.

TABLE II
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Example 1 2 Fuel channel diameter, inches .40 .325 ~-Fuel channel pitch, inches .500 O40 Coolant channel diameter, lnches .156 .125 Coolant channel pitch, inches .500 .40 Fuel Yolume fra~tlon ~ 0503 .518 Structure Yolu~e fr~ction ~421 .405 Coolant Volume fractlon ~o76 .0766 3` Fuel/Coolant Volume Frac~ion Ratio 6~62 6.76 Modera~or/Fuel Atom Ratio .44 ~43 : Case 4166 ~ 3~$

The moderator to fuel atom ratio o~ Table II
correspo~ds to a primary coolant water temperacure of about 250F at low pressure. Other process parameters are similar to those assumed ~or Table I.
The geometry of the coolant and fuel channels in the block type fuel assembly will produce a degree o~ what might be termed "moderator escape probab~llty" which wlll ser~e to ~arden the neutron spectrum and improYe the core conversion or breeding ratio. This occurs because each fuel channel is not completely surrounded by moderator.
Hence, some neutrons produced in a fuel channel can pass to another fuel channel without traversing a volume con-- taining moderator, thereby lmproving the breeding or conversion ratio since the average neutron energy at which fission occurs is increased. This, combined w1th a mod-erator to fuel ratio less than that which can be aohieved with touching uel pins, should yield a uniquely high breeding ratio for either H~O or D20 coolin~
By vlrtue of the moderator to fuel atom ratios 23 made possible by these approaches to fuel assembly design, fast reactor physics can be applied to pressurized water reactor tehnology. This combination has important ad-vanta~es including:

a. A~oidance of gas or liquid metal coolants - otherwise used for fast reactors.
. Reduced clad operating temperature.

c. AYailability of additional methods of reactivity control, namely, chem~cal shim and spectral shift control~
.~ 3a Availabllity of additional methods of reactivity control ... .
reduces the nor~.al dependence of ~ast reactors on control rods. They allow a general reduction in required control .
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Case 4166 rod worth and provide a means for continuous a~jus~ment of e~cess react:lvity to a minimum value, thereby greatly enhancing the safety of fast reactor cores. This would lnclude operation with higher worth rods out of the core.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fuel assembly for use in a pressurized water moderated and cooled nuclear fast breeder reactor comprising a nuclear fuel, a plurality of fuel pins disposed with parallel longitudinal axes in closely packed array, each fuel pin consisting essentially of a generally tubular cladding bearing said nuclear fuel and at least one longitudinally extending fin formed as part of the surface of the cladding of each of said fuel pins, and a brazed connection fixedly joining the extremity of said fin to the tubular cladding of a juxtaposed fuel pin to form an integral fuel assembly having a moderator to fuel atom ratio in the range from 0.624 to 0.82.

2. A fuel assembly according to claim 1 wherein at least one of the fuel pins includes a plurality of said fins extending continuously without interruption along the longitudinal surface of said fuel pin generally in parallel with the longitudinal axis of the pin.

3. A fuel assembly according to claim 1 wherein the nuclear fuel is plutonium and the pressurized water is heavy water.