JPH0378599B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates generally to nuclear reactors, and more specifically to fuel assembly configurations for boiling water nuclear reactors.

BACKGROUND OF THE INVENTION In a nuclear reactor, a fissile fuel atom, e.g. Generate energy with a few neutrons.

In a typical boiling water reactor (BWR), the nuclear fuel is in the form of fuel rods, each consisting of a number of sintered pellets contained within an elongated cladding tube. Such fuel rods are supported in groups between upper and lower tie plates to form individually replaceable fuel assemblies or fuel bundles. A sufficient number of fuel assemblies are arranged in a matrix close to a right cylindrical cylinder to form a nuclear reactor core capable of carrying out a self-sustaining nuclear fission reaction. The kinetic energy of the fission products is dissipated as heat within the fuel rods. Energy is also stored in the fuel structure and moderator by neutrons, gamma rays and other radiation resulting from the fission process. The core is submerged in a coolant (eg, water) that extracts heat that can then be extracted to perform useful work. When the coolant is water, water also acts as a neutron moderator, slowing down the neutrons and making it easier for them to initiate the fission reaction.

The fuel commonly used in water-cooled and moderated power generation electronic reactors consists of turquoise dioxide, approximately 0.7 to 5.0% of which is fissile U-235 mixed with the fuel parent element U-238. During operation of the reactor, a portion of the parent fuel element U-238 is converted into fissile Pu-239 and Pu-241. U-238 is also fissile, but only to high-energy neutrons. Produced fissile material (e.g. Pu-239 and Pu-241) vs. decayed fissile material (e.g. U-235, Pu-239)
and Pu−241) is defined as the “conversion ratio”.

If a nuclear reactor is required to operate at steady-state power levels, the number of fission-inducing neutrons must be maintained constant. That is, each fission reaction must produce a net neutron, which in turn causes the next fission reaction, so that the reaction is self-sustaining. Nuclear reactor operation is characterized by the effective multiplication factor Keff, which must be 1 in steady-state operation. Here, the effective multiplication factor Keff is the neutron regeneration rate of the nuclear reactor considered as a whole, and the local or infinite multiplication factor that defines the neutron regeneration of an infinitely large system having the same composition and properties as the local region of the reactor core in question. It should be distinguished from the magnification Kinf.

During operation, the fissile fuel is depleted and, in fact, some of the fission products are themselves neutron absorbers, or "poisons." To counteract this, the reactor is usually initially loaded with excess nuclear fuel. This results in excessive initial reactivity. This initial excess reactivity reduces the effective multiplication factor by 1 during reactor operation.
A control system is required to maintain the effective multiplication factor below 1 when the reactor needs to be shut down. A typical control system uses a neutron absorbing material that acts to control the number of neutrons by non-fissile absorption or capture of neutrons.

At least a portion of the neutron-absorbing material is introduced into a plurality of selectively actuable control rods that are inserted axially from the bottom of the core as needed to adjust the power level and distribution. , also shuts down the reactor core. Combustible absorbent material is introduced in some of the fuel rods to minimize the need for mechanical controls. Combustible absorbers are neutron absorbers, and upon absorption of neutrons, they are converted into substances with low neutron absorption capacity. Gadolinium is a well-known combustible absorbent and is usually used in the form of gadolinia. Odd isotopes (Gd-155 and Gd-157) have extremely large capture cross sections for thermal neutrons. Combustible absorbers that are effective by design exhibit undesirable residual neutron absorption reactivity at the end of the fuel reload cycle due to residual isotope neutron absorption by the small neutron cross section absorber. For example, when using gadolinium as a combustible absorber, large cross-sectional area isotopes (Gd-155 and Gd-157) are rapidly depleted, while even isotopes (Gd-154, Gd-156 and Gd-158) are depleted rapidly. Due to continued neutron capture, residual absorption remains.

As is well known, combustible absorbers such as gadolinium act in a self-shielding mode when present in sufficient concentrations. That is, upon exposure to a neutron flux, neutron absorption occurs substantially at the outer surface of the absorber, such that the volume of the absorber contracts radially at a rate determined by the concentration of the absorber. Thus, by appropriately choosing the number of absorber-containing regions and the absorber concentration therein, it is possible to achieve a desired variation in absorption value over one or more reactor operating cycles.

During operation, the proportion of steam voids increases towards the top of the reactor, resulting in reduced deceleration in the top region,
The power distribution is therefore skewed towards the lower region of the core. It is a well known technique to compensate for this distortion by distributing the combustible absorbent material non-uniformly in the axial direction. Combustible absorbent material is incorporated into a number of fuel rods in a distribution skewed toward the axial region of highest reaction rate during hot operation. A typical configuration is U.S. Patent No.
No. 3799839.

However, in a cold stop state, the situation is completely different. More specifically, in cold conditions, the top of an irradiated BWR core is more reactive than the bottom. The reason for this is that plutonium production is greater at the top of the reactor during operation, and there is less U-235 decay (the conversion ratio is greater at the top of the core, and burnup, or burnup, is smaller). In cold shutdown conditions, steam voids in the top of the core are removed, thus making the top of the core more reactive than the bottom. Typical approval standards require a reactivity shutdown margin of 0.38% (Keff less than 0.9962) when any one control rod is removed from the core. A design criterion of 1% predicted outage margin (less than Keff 0.99) is used in most cases to provide margin to account for prediction uncertainties, given by control rods and combustible absorbers. It is being

The axial power distribution can be adjusted by increasing the amount of combustible absorber in the lower part of the core, but the optimal absorber distribution for optimizing the axial power distribution at maximum power is the correct cold shutdown. Doesn't help maintain margin. To meet cold shutdown constraints, it is typically necessary to design with excess combustible absorbent remaining, and such excess combustible absorbent remains due to initial enrichment and uranium impose unfavorable mineral requirements and increase reactor fuel cycle costs;

Another problem is that gadolinia reduces the thermal conductivity of fuel rods and increases fission gas release. As a result, gadolinia-containing fuel rods are often the most constrained fuel rods in fuel assemblies and must have lower power ratings, with a corresponding negative impact on local power distribution. The amount of power derating required depends on gadolinia concentration, but is a significant issue in long-burn fuel bundle designs and/or high-energy cycle designs where increasing gadolinia concentration is required to provide adequate cold shutdown margin. Become.

Accordingly, the margins required for hot operating conditions and cold shutdown conditions, respectively, have tended to impose competitive constraints on the reactor core design and thus prevent the realization of an optimal core structure.

DISCLOSURE OF THE INVENTION The present invention provides a nuclear reactor fuel assembly structure that can meet cold shutdown margins with minimal negative impact on operating efficiency. The present fuel assembly structure minimizes the end-of-cycle reactivity of the combustible absorber, minimizes initial enrichment requirements, makes the use of higher concentrations of combustible absorbent more feasible and optimal, and Maximize flexibility of combustible absorbent distribution for directional power distribution control.

The fuel assembly of the present invention, which achieves the above-mentioned advantages, comprises a fissile material component and a neutron-absorbing material component distributed over substantially the entire axial length of the fuel assembly, and the neutron-absorbing material component is distributed along the axial direction of the fuel assembly. A relatively short axial region (referred to as the cold shutdown control region) whose distribution corresponds to at least a portion of the axial region of highest cold shutdown reactivity (the region where the neutron flux peaks in cold shutdown conditions). characterized by an enhancement in . To this end, the fuel assembly is configured such that the concentration of combustible absorbent material in the cold shutdown control zone is increased or the number of cross-sectional distribution areas containing combustible absorbent material is increased. The presence of a cold shutdown control zone can reduce combustible absorbent material in the top and other regions of the core. This reduces the total combustible absorbent content of the fuel assembly and facilitates optimization of the combustible absorbent distribution. In the present invention, "fissile material component" means the fissile material in all of the fuel rods of the fuel assembly. For example, at any axial location in a fuel assembly, the total amount of fissile material is the sum of the amount of fissile material contained in all fuel rods at that axial location. Similarly, in the present invention, the term "neutron absorbing material component" refers to the neutron absorbing material in all of the absorber-containing fuel rods of the fuel assembly. For example, at any axial location in the fuel assembly, the total amount of neutron absorbing material is the sum of the amount of neutron absorbing material contained in all absorber-containing fuel rods at that axial location.

The axial distribution of the neutron-absorbing material component typically includes multiple portions extending over most or all of the axial length of the fissile material to achieve an axial power distribution and a representative In particular, it is characterized by an additional enhancement in the axial region corresponding to at least part of the axial region of highest reactivity during hot operation. This last zone is referred to as the "hot run control zone" and is typically longer than the cold shut down control zone and is located near the bottom of the fuel assembly.

Advantageously, a neutron absorbing material component is incorporated into at least a portion of the fuel rod. Enhancement in the cold shutdown control zone can be achieved, at least in part, by having the fuel rod or rods contain absorbent material only in the cold shutdown control zone. If gadolinia is used, the gadolinia concentration in these fuel rods can be higher than other gadolinia-containing fuel rods. This is because the contribution of the short high concentration gadolinia portion to the total gas internal pressure is small.

It can be seen that the cold stop value of the combustible absorbent is maximized by configuring the fuel assembly so that the highest concentration and greatest number of combustible absorbent areas are in the cold stop control zone. At the same time, this absorber enhancement is in the region of low neutron importance of the core under hot operating conditions and therefore has minimal impact on the axial power distribution under hot operating conditions. Furthermore, the gadolinia residual absorption penalty in the cold reactivity zone is minimal when hot, but maximal when cold. This has the effect of minimizing undesirable fuel cycle effects.

Even if a fuel rod with only a short gadolinia section in the cold shutdown control zone is placed in a grid position normally prohibited for gadolinia, such as a grid position diagonally adjacent to a fuel rod at a corner of a fuel assembly, the core There is no negative impact on instrumentation readings, and there is no need to reduce enrichment in these fuel rods to address fission gas emissions due to gadolinia.

According to another aspect of the invention, the enhancement of neutron absorbing material in the cold shutdown control zone can be compensated by a reduction in fuel enrichment in the cold shutdown control zone. It is preferable to reduce enrichment only to gadolinia-containing fuel rods to simplify fuel production. By reducing the enrichment of the cold shutdown control zone, neutrons can be utilized more efficiently than by increasing the gadolinia content of this zone. The fissile material inventory requirements required to achieve a given design burnup are relaxed. The reduction in reactivity due to the reduction in enrichment extends throughout the residence period of the fuel assembly;
Combustible absorbers reduce reactivity primarily during the first refueling cycle of the residence period. but,
If the peak enrichment is fixed, reducing the enrichment in the cold shutdown control zone will reduce the burnup and make the axial power peaking more severe than increasing the gadolinia in this zone. It is therefore advantageous to use a combination of reduced enrichment and increased gadolinia content.

BRIEF DESCRIPTION OF THE DRAWINGS In order to better understand the configuration and advantages of the present invention, the present invention will be described below with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic, partially cutaway longitudinal side view of a boiling water type water-cooled and moderated nuclear reactor system 10. The system includes a pressure vessel 11 in which a reactor core 12 is placed submerged in a coolant and moderator, for example light water. The core 12 has a structure in which a plurality of fuel cells 13 are surrounded by an annular shroud 14. Each fuel cell 13 includes four fuel assemblies (or bundles) 15 and one control rod 16.
The fuel cells are held in spaced relationship by an upper core grid 18 and a lower core plate 19, each supported at the bottom by suitable supports 20. Control rods 16 are selectively insertable between fuel assemblies to control core reactivity. Associated with each control rod 16 is a control rod guide tube 2
1 is provided, and this guide tube guides the control rod when it is withdrawn below the reactor core. FIG. 2 is a diagrammatic top plan view showing the arrangement of the fuel cells 13 within the core 12. Typical cores are 300 to 900
including fuel assemblies.

The portion of the pressure vessel 11 below the core 12 defines a coolant supply chamber 22 , while the portion above the core includes a brackish water separator and dryer structure 25 . During operation, the coolant supply chamber 2 is supplied by the coolant circulation pump 27.
The coolant in the reactor core 12 is pressurized and the coolant is pumped upward through the reactor core 12 . The coolant absorbs the heat generated by the nuclear fission reaction occurring in the reactor core, and a portion of the coolant is converted to steam, which is then passed through the steam separator/dryer structure 25 to the utilization equipment,
For example, it reaches the turbine 30. The exhaust steam is condensed in a condenser 32 connected to the turbine, and the condensed water is returned to the inlet side of the coolant circulation pump 27 by a condensed water return pump 35 as feed water.

FIG. 3 shows the structure of one fuel cell 13 in detail. The control rod 16 is cross-shaped in cross section and has control blades 40, each control blade 40 being sandwiched between two adjacent fuel assemblies. Each fuel assembly 15 comprises a number of elongated fuel rods 42 held between an upper tie plate 45 and a lower tie plate 46 and housed within a tubular flow channel 48 of rectangular cross section. The lower portion of the fuel assembly is provided with a nose 50 through which cooling water flows through a plurality of openings 52 into channels 48 and upwardly along fuel rods 42 . The nose 50 has an appropriate shape, and the fuel assembly support 20
It fits into a correspondingly shaped socket (not shown) provided in the.

As is well known, each fuel rod 42 is comprised of a cylindrical cladding containing a number of sintered pellets of enriched uranium oxide and/or plutonium fuel. Enrichment varies from fuel rod to fuel rod in a single fuel assembly, typically ranging from about 0.7 to 5.0 W/O (fissile nuclide weight percent);
It exhibits an average value of about 1.5-3.5 W/O (naturally occurring uranium is 0.7 W/O). The fuel rod is approximately 1.3cm in diameter.
(1/2 inch) can be approximately 12 to 15 feet long.

FIG. 4 is a schematic diagram showing the cross-sectional (horizontal) distribution of fuel rods in a typical reloaded fuel assembly of the present invention. The fuel rods 42 are arranged in an 8×8 matrix,
The two center fuel rod positions are occupied by water channels 55 (sometimes referred to as "water rods"). The ten fuel rods with reference numeral 57 contain combustible absorbent material in the form of gadolinia and are designated by a circle with the symbol G1, G2, G3 or G4 within the circle. The remaining 52 fuel rods do not contain gadolinia and are represented by circles with numbers indicating the weight percent of U-235 within the circle, in this example ranging from 1.60 to 3.95 W/O. Changes in enrichment in the transverse region of the fuel assembly 15 and gadolinia-containing fuel rods 57
The specific position of is determined by well-known considerations, but these conditions do not form part of the present invention and will not be described in detail.

The specific structure of G1, G2, G3 and G4 fuel rods is
Determine the axial power distribution and cold shutdown control characteristics of the fuel assembly. Various specific examples are shown below. Figures 5A-5G show the compositions of seven example fuel assemblies, and Figures 6A-6G show the compositions of these seven example gadolinium-containing fuel rods. A relative enhancement of gadolinia in a given axial region of the fuel assembly can be achieved by increasing the number of gadolinia-containing rods in that region or by increasing the gadolinia concentration in a given number of fuel rods in that region. It should be noted that At the concentration used in the present invention (2 to 5 W/O),
All gadolinia are self-shielding.

FIG. 5A is a schematic diagram showing the longitudinal composition of the first embodiment of the fuel assembly according to the present invention. This fuel assembly has an axial dimension of 12.5 feet or 381 cm.
(150 inches), including the top and bottom 15
It includes a 6 inch (cm) natural uranium blanket 59 and a 351 cm (11.5 ft) enriched section 60. The uranium blanket 59 will not be further described and the following discussion will be directed to the enrichment section 60.

The fuel assembly includes a gadolinia component provided by gadolinia-containing fuel rods, which has two purposes: axial power distribution adjustment during hot operating conditions and cold shutdown reactivity control.
For this purpose, the fuel assembly has a long section 62 called the "hot operating control section" and a relatively short section 6 called the "cold shutdown control section".
Gadolinia is enhanced within 5. Zone 62 is located at or near the bottom of concentrating section 60, while zone 65 is located near the top of concentrating section 60. In this particular embodiment, the length of area 62 is 137
cm (54 inches), the length of area 65 is 30 cm (12 inches). Thus, the cold shutdown control area 65 in the embodiment of FIGS. 5A and 6A is 76 to 84 percent of the overall height of the fuel rods in the fuel assembly, or 290 inches from the bottom.
It will extend over a range of ~320 cm (114-126 inches). The enhancement in zone 62 is achieved by increasing the gadolinia concentration, and the enhancement in zone 65 is achieved by increasing the gadolinia concentration and increasing the number of gadolinia-containing fuel rods.

FIG. 6A shows the longitudinal composition of a gadolinia-containing fuel rod. In the G1 and G4 fuel rods, the gadolinia component is distributed throughout the enrichment section 60,
The G4 fuel rod has a high gadolinia concentration in the hot operation control zone 62 (4 W/O compared to 2 W/O in other zones). The G4 fuel rod also has a high gadolinia concentration in the cold shutdown control zone 65 (4W/O), and the additional gadolinia enhancement in the cold shutdown control zone 65 results in a relatively high concentration only in zone 65 (5W/O).
This has been achieved with G2 and G3 fuel rods containing gadolinia. Note that the G2 and G3 fuel rods are characterized in that the uranium enrichment in the cold shutdown control zone 65 is somewhat lowered. The significantly reduced enrichment in this zone is characteristic of some of the examples described below, but G2 and
If the enrichment of G3 fuel rods is reduced only, the average enrichment reduction of the entire fuel assembly is small. An important aspect of this embodiment is that it is advantageous to produce fuel pellets having predetermined standard gadolinia and U-235 concentrations.

By increasing the gadolinia content in the cold shutdown control zone 65, the gadolinia content in areas other than zones 65 and 62 can be lowered to provide desired axial power distribution adjustment. In conventional fuel rods without zone 65, the gadolinia content in zone 62 had to be increased to achieve axial power distribution adjustment. Furthermore, in the prior art, the need to increase cold stop margin required increasing gadolinia loading in all axial zones to maintain axial power distribution.

FIG. 5B is a schematic diagram showing the longitudinal composition of a second embodiment of the fuel assembly according to the present invention. This embodiment differs from the embodiment of FIG. 5A in that the hot run control zone is slightly shorter (122 cm or 48 inches) and the cold shutdown control zone is longer (76 cm or 30 inches); The cold shutdown control zone extends from 68 to 88% of the fuel rod height, or 259 to 335 cm (102 to 132 inches) from the bottom. It can also be seen that the gadolinia component in the cold shutdown control area is not uniform, but rather has a graded distribution with the highest enhancement in the central portion 67. FIG. 6B shows the longitudinal composition of a gadolinia-containing fuel rod. It can be seen that the gadolinia distribution in the cold shutdown control zone is adjusted by making the reinforced sections of G2 and G3 fuel rods relatively shorter than the reinforced sections of G4 fuel rods.

FIG. 5C is a schematic diagram showing the longitudinal composition of a third embodiment of a fuel assembly according to the present invention. This embodiment differs from the embodiment of FIG. 5B in that the cold shutdown control area 65 also has a stepped configuration;
68~88% of the fuel rod height, i.e. 259~ from the bottom
It extends over a range of 335 cm (102 to 132 inches), with its highest gadolinia enhancement portion 68 being longer. Figure 6C shows the longitudinal composition of the gadolinia-containing fuel rod. It can be seen that the gadolinia augmentation sections of the G2 and G3 fuel rods are longer than in the embodiments of FIGS. 5B and 6B, which explains the difference in length of section 68.

FIG. 5D is a schematic diagram showing the longitudinal composition of a fourth embodiment of a fuel assembly according to the present invention. This embodiment is very similar to the example of FIG. 5B, except that the central highest enhancement part of the cold shutdown control zone is somewhat shorter, and the cold shutdown control zone 65 is
72~88% of the fuel rod height, i.e. 274~ from the bottom
Extends over a range of 335 cm (108 to 132 inches). FIG. 6D is a schematic diagram showing the longitudinal composition of a gadolinia-containing fuel rod.

FIG. 5E is a schematic diagram showing the longitudinal composition of a fifth embodiment of a fuel assembly according to the present invention. This embodiment is very similar to the example of FIG.
%, i.e. extending from 274 to 335 cm (108 to 132 inches) from below, the control area 62
and gadolinia distributions other than 65 are slightly different. More specifically, Sections 5A-D
The embodiment shown has eight gadolinia-containing fuel rods outside the control area, four of which have a gadolinia concentration of 4W/
This fifth embodiment uses eight identical fuel rods having a gadolinia concentration of 3 W/O, whereas four of the rods have a gadolinia concentration of 2 W/O. Therefore, although the absolute gadolinia content is the same, the combustion and production properties are different. FIG. 6E is a schematic diagram showing the longitudinal composition of a gadolinia-containing fuel rod.

FIG. 5F is a schematic diagram showing the longitudinal composition of the sixth embodiment of the fuel assembly according to the present invention. This embodiment has the same zone length and gadolinia distribution as the embodiment of FIG. 5D. However, this example differs in that the concentration in the cold shutdown control zone 65 is significantly lower than the concentration outside the control zone. 6th F showing the longitudinal composition of gadolinia-containing fuel rods
As can be seen, this reduction in enrichment is achieved by using natural uranium in the gadolinium-enhanced portion of the G1, G2, G3, and G4 fuel rods. The reduction in enrichment is not uniform, but is gradual in a manner similar to the gradual progression of gadolinium enhancement.

FIG. 5G is a schematic diagram showing the longitudinal composition of a seventh embodiment of the fuel assembly according to the present invention. This embodiment differs from the example of FIG. 5F only in that the hot operation control area 62 is slightly longer. This difference is achieved by correspondingly reconfiguring the configuration of the G4 fuel rods, as can be seen in Figure 6G.

The above-described embodiment is characterized by adjusting the axial power distribution by enhancing the gadolinia distribution in the hot operation control area 62, but the cold stop control achieved by the present invention can be achieved by adjusting the axial power distribution. No need to combine. FIG. 5H is a schematic diagram showing the longitudinal composition of the eighth embodiment of the fuel assembly according to the present invention. This example is based on the above-mentioned 7
From the two examples, the gadolinia distribution except for the enhancement in the cold shutdown control zone 65, which extends from 68 to 84% of the fuel rod height, or 259 to 320 cm (102 to 126 inches) from the bottom. They differ in that they are uniform. As is clear from Figure 6H, which shows the longitudinal composition of the gadolinia-containing fuel rods, the G1 and G4 fuel rods have a uniform gadolinia content, and the G2
and cold shutdown control area 6 with G3 fuel rods
This has resulted in an increase of 5. This embodiment differs from the embodiments of FIGS. 5C and 6C only in the manner in which the G4 fuel rods are constructed.

The effectiveness of the enhanced gadolinia in the cold shutdown control zone 65, which extends between 68% and 88% of the fuel rod height, will vary between different embodiments and will also vary depending on the operating history of the reactor. is clear. In Figure 7A, curve 85 represents the relative power output of a nuclear reactor having a core made of fuel assemblies of the type shown in Figures 5A and 6A, operating with all control rods withdrawn at BOC (beginning of the cycle). Shown as a function of axial position of the furnace. This graph is normalized to unit average power. For comparison purposes,
In FIG. 7A, the axial power distribution in a core with a uniform gadolinia distribution is shown by an imaginary curve 86. As can be seen, the gadolinia boost in the hot run control zone has the effect of making the axial power distribution somewhat more uniform, while the gadolinia boost in the cold shutdown control zone has the effect of slightly reducing the power in this zone.

Curve 88 in FIG. 7B shows relative power as a function of axial position for a reactor core in cold shutdown at BOC with all but one control rod inserted. This graph is also standardized to unit average output. In the drawing, the power distribution in the core in which the gadolinia component is uniform in the axial direction is further shown by a virtual curve 89. It should be noted that the absolute neutron flux in Figure 7B is typically less than the absolute neutron flux in Figure 7A. The effect of gadolinia enhancement in the cold shutdown control zone is manifested as a significant reduction in neutron flux and power in the cold shutdown zone 65.

The choice of fuel assembly example depends on the specific requirements of the equipment operating the nuclear power plant. This is because the mode of operation strongly influences specific cold shutdown requirements. Some equipment requires high availability and at the same time must adhere to strict timetables that require early shutdown for excessive reactivity. For example, such time constraints may be imposed by regulations governing inspection intervals or by the possibility of seasonal use of alternative generation sources such as hydroelectric power. In such cases, sufficient cold shutdown control is maintained by providing a longer cold shutdown control section or a greater number of gadolinia-containing fuel rods, especially when the limiting situation is early in the cycle. Reducing the uranium enrichment in the cold outage control zone is an appropriate technique under these circumstances. Note that although the reduction in enrichment has a fairly uniform effect throughout the cycle, the gadolinia number varies throughout the cycle. On the other hand, other equipment tends to be operated over long cycles. This mode of operation always has a high cost of alternating power;
Therefore, it is required in situations where replacement power costs exceed fuel cycle costs. In such cases, it may be appropriate to increase the concentration of gadolinia in the cold shutdown control zone to maintain sufficient gadolinia numbers near the middle and end of the cycle.

As is clear from the above, the fuel assembly configuration of the present invention has an increased cold shutdown margin,
It is characterized by adjustable cold shutdown margins and low impact on combustible absorber residual reactivity penalties and axial power distribution. The requirements of cold shutdown and axial power distribution adjustment are both optimally met in conjunction with several configurations made possible by the present invention. In view of the embodiments described above, it is apparent from the present invention that fuel assemblies suitable for a wide variety of operating conditions can be obtained by reconfiguring only (and not necessarily all) gadolinia-containing fuel rods. Flexibility is quickly demonstrated.

Although the foregoing description is a full and complete description of the preferred embodiment of the invention, various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. For example, in an example of increasing gadolinia and decreasing enrichment in a cold shutdown control area, it is said that the gadolinia enhancement and enrichment decrease are performed in the same area, but there is no absolute requirement that this is the case. . Thus, the gadolinia enhancement can be applied to a first part of the zone and the enrichment reduction can be applied to a second part, possibly overlapping the first part. Additionally, while the cross-sectional distribution of gadolinia-containing fuel rods as shown is appropriate for reloaded fuel assemblies, this configuration is typically different for initially loaded fuel assemblies. Furthermore, it is advantageous to minimize the number of "special" fuel rods characterized by gadolinia content and reduced enrichment by limiting these characteristics to a small set of fuel rods; There is no absolute requirement that this must be done. Accordingly, the first set of fuel rods may be provided with a cold shutdown control zone gadolinia enhancement and the second set of fuel rods may be provided with a cold shutdown control zone enrichment reduction. Therefore,
The description and specific examples herein should not be construed as limiting the scope of the invention.

[Brief explanation of drawings]

Figure 1 is a partial cross-sectional schematic diagram of a water-cooled and moderated nuclear reactor, Figure 2 is a plan view showing the normal layout of a fuel assembly in the core, and Figure 3 is a diagram showing one fuel cell in the core. FIG. 4 is a cross-sectional view of the fuel assembly according to the present invention, and FIG.
Figures 6A-6H are diagrams showing longitudinal compositions of various embodiments of fuel assemblies according to the present invention;
Figure 5H is a diagram showing the longitudinal composition of gadolinia-containing fuel rods in a fuel assembly, Figure 7A is a graph showing the relative power in a nuclear reactor under hot operating conditions, and Figure 7B is a graph with one control rod removed. 3 is a graph showing relative power in a nuclear reactor in a cold shutdown state. 10...Water cooling and moderation reactor, 15...Fuel assembly, 42...Fuel rod, 57...Gadolinia-containing fuel rod, 60...Enrichment section, 62...Hot operation control area, 65...Cold shutdown control area.

Claims (1)

[Scope of Claims] 1. Core operation is characterized by a substantial proportion of steam voids such that deceleration decreases toward the top of the core, and as a result of decreased deceleration, combustion slows and transitions occur in the upper core region. A fuel assembly for use in a boiling water reactor core in which a reactivity profile peaks in the upper core region with a cold shutdown where the ratio increases and therefore the deceleration in the upper core region increases relatively. ; having a fissile material component distributed over substantially the entire axial length of the fuel assembly and having a neutron absorbing material component, the axial distribution of the neutron absorbing material component being in an axial direction called the cold shutdown control zone; characterized by an enhancement in a zone, the cold shutdown control zone corresponding to at least a portion of the axial region where the infinite reactivity profile in the cold shutdown peaks, and wherein the total amount of neutron absorbing material in the cold shutdown control zone is greater than the total amount of neutron absorbing material in the axial regions of the fuel assembly immediately above and below the cold shutdown control zone, and the reactivity peak in the cold shutdown state is higher than the reactivity of the regions immediately above and directly below the cold shutdown control zone. and wherein the cold shutdown control zone extends axially within a range of 68 to 88% from the bottom relative to the height of the fissile material of the nuclear fuel assembly. 2. The fuel assembly according to claim 1, wherein the neutron absorbing material is a combustible absorbing material. 3. The fuel assembly according to claim 2, wherein the combustible absorbent material is made of cadrinium. 4. The fuel of claim 1, wherein the enhancement of neutron absorbing material in the cold shutdown control zone is achieved at least in part by a relative increase in the number of cross-sectional distribution regions containing combustible absorbing material. Aggregation. 5. The enhancement of neutron absorbing material in the cold shutdown control zone is achieved at least in part by a relative increase in absorber concentration in a constant number of cross-sectional distribution areas containing combustible absorber. The fuel assembly according to item 1. 6. The fuel assembly according to claim 1, wherein the fissile material component is distributed substantially homogeneously in the axial direction. 7. The fuel assembly of claim 1, wherein the axial distribution of fissile material components is characterized by a relative decrease in enrichment in the cold shutdown control zone. 8. The fuel assembly according to claim 1, wherein the axial distribution of the neutron absorbing material component includes a plurality of portions that substantially overlap the entire axial length of the fissile material. 9. The axial distribution of the neutron-absorbing material component is characterized by an additional enhancement in an axial region corresponding to at least a portion of the axial region where the hot operating reactivity is highest. fuel assembly. 10 The fuel assembly comprises a plurality of substantially parallel spaced apart fuel rods, wherein a first set of fuel rods is enriched with neutron absorbing material in a cold shutdown control zone and a second set of fuel rods is enriched with neutron absorbing material; 2. A fuel assembly according to claim 1, in which the cold shutdown control zone is not enriched with neutron absorbing material. 11. The fuel assembly of claim 10, wherein at least some of the fuel rods of the second set are essentially free of neutron absorbing material. 12. The fuel assembly of claim 10, wherein at least some of said first set of fuel rods contain neutron absorbing material over an axial extent beyond said cold shutdown control zone. 13. The fuel assembly of claim 10, wherein at least some of the fuel rods of the first set contain neutron absorbing material only in the cold shutdown control zone. 14 The axial length of the fissile material component is 305
2. The fuel assembly of claim 1, wherein the axial length of the cold shutdown control zone is 0.5 to 3 feet. 15. Claim 1, wherein the axial length of the cold shutdown control zone is within the range of 68% to 84% of the height of the fissile material of the fuel assembly, measured from the bottom of the fuel assembly. Fuel assembly as described in section. 16. Claim 1, wherein the axial length of the cold shutdown control zone is within 72% to 88% of the height of the fissile material of the fuel assembly, measured from the bottom of the fuel assembly. Fuel assembly as described in section. 17. Claim 1, wherein the axial length of the cold shutdown control zone is within 76-84% of the height of the fissile material of the fuel assembly, measured from the bottom of the fuel assembly. Fuel assembly as described in section.