JP3339768B2 - Light water reactor core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light water reactor (hereinafter referred to as "light water reactor") such as a boiling water reactor.

[0002]

2. Description of the Related Art FIG. 7 is a schematic sectional view showing a boiling water reactor which is a conventional light water reactor. As shown in FIG.
A large number of fuel assemblies 1 are located at the center of the reactor pressure vessel 11.
3 is stored therein, and cooling water 14 is injected into the reactor pressure vessel 11 up to above the core 12.

The reactor pressure vessel 11 includes a core 1
A steam separator 15 and a steam dryer 16 are accommodated in the upper part of the reactor 2, and a main steam outlet nozzle 17 is provided above the reactor wall of the reactor pressure vessel 11, and a water supply nozzle 18 is provided below the main steam outlet nozzle 18. Is provided.

[0004] As shown in FIG. 8, a unit lattice (cell) 19 is constituted by four fuel assemblies 13 as a set, and a cross section is formed between these four fuel assemblies 13. A control rod 20 in the shape of a cross is arranged. By inserting or pulling out the control rod 20,
The core reactivity is controlled.

In general, the reactor core 12 of the nuclear reactor is configured by arranging a large number of cells 19 in a grid as shown in FIG. 9, and the reactor core 12 includes a neutron source 21 and an in-core monitor 22 in addition to these devices. And so on. Further, in the core 12, a control rod 20 for performing a control rod operation for adjusting the reactivity during the power operation is predetermined, and a cell 19 corresponding to the control rod 20 is, as shown in FIG. 23.

In the conventional light water reactor having the above configuration, when the operation is started, the fission of the fuel material in the reactor core 12 causes the cooling water 14 in the reactor pressure vessel 11 to boil. After passing through the steam separator 15 and the steam dryer 16, it is taken out through the main steam outlet nozzle 17 for driving the turbine of the power plant. The steam after working in the turbine is cooled by a condenser (not shown), and then returned to the reactor pressure vessel 11 from the water supply nozzle 18 as cooling water 14.

FIG. 10 is a perspective view showing the structure of the fuel assembly. As shown in FIG. 10, the fuel assembly 13 has a plurality of fuel rods 25 filled with fuel pellets 24 and water rods 26 arranged in a square lattice to form a fuel bundle. It is configured to be housed in the channel 27.

An upper tie plate 28 and a lower tie plate 29 for supporting the upper and lower ends of the fuel rod 25 and the water rod 26 are attached to the upper and lower ends of the channel 27, respectively.
The water rod 26 holds the spacer 30 at a plurality of positions between the positions 8 and 29 so that the distance between the spacers 30 is kept constant. The channel 27 is fixed to the fuel bundle by a channel fastener 31 and forms a coolant flow path of the fuel assembly 13.

FIG. 11A is a configuration diagram showing a cross section of a fuel assembly, and FIG. 11B is a diagram showing enrichment, plutonium enrichment, and gadolinia concentration distribution in the axial direction of each fuel rod. . FIG. 11A shows a plurality of fuel rods 2 having several types of gadolinia concentration, uranium enrichment, and plutonium enrichment.
5 and the water rods 26 are arranged in a square lattice. In the figure, P1, P2, P3, and P4 indicate fuel rods (hereinafter, referred to as {O} fuel rods) in which UO ₂ and plutonium oxide PUO ₂ extracted and reprocessed are taken out. The enrichment is different. Here, the plutonium enrichment is usually calculated as (plutonium + americium) weight / (uranium +
(Plutonium + Americium) Defined by weight.

Reference numeral G denotes a uranium fuel rod mixed with gadolinia, and the enrichment and gadolinia concentration are different for each code. As shown in FIG. 11B, MO rods P1, P2, P3, and P4 are P1, P2, P3, and P4, respectively.
4 and the uranium fuel rod G has axial enrichment and gadolinia concentration distributions of e0 and e1, respectively.

The MOX fuel assembly shown in FIG. 11 is composed of four types of MOX fuel rods of different enrichment, uranium fuel rods containing gadolinia, and one large diameter water rod.

FIG. 12 is an explanatory diagram showing an example of a fuel loading pattern. As shown in FIG. 12, the core 12 has {O}
The fuel assemblies 32 and the uranium fuel assemblies 33 are loaded in a mixed state, and the numbers in the figure indicate the number of cycles in the core (loading years).
Is shown. Numbers that are circled and displayed are ΜO
X fuel assembly 32.

In FIG. 12, a {O} fuel assembly having three or less plutonium enrichment types of {O} fuel rods is defined as a first group of OX fuel assemblies, and a plutonium enrichment type of {O} fuel rods having four plutonium enrichment types is four. More than one type of {O} fuel assembly is referred to as a second group of OX fuel assemblies.

[0014]

In the fuel assembly and the core configured as described above, the MOΧ fuel assembly 3
In order to obtain a thermal margin equivalent to that of the uranium fuel assembly 33 when the fuel rod 2 is loaded into the core 12, the output of each fuel rod is averaged, and in order to prevent the local peaking coefficient from becoming excessive, the {O} fuel rod It is necessary to adjust the Pu enrichment or the enrichment distribution of the uranium fuel rods.

Generally, the handling of the OX fuel assembly 32 such as transportation and storage is complicated from the viewpoint of radioactivity and nuclear material management, and therefore, it is necessary to reduce the number of bodies to be handled. Therefore, it is desirable to incorporate as many MOX fuel rods as possible into one body.

As shown in FIG. 11 (A), when about 2/3 or more of all the fuel rods are MOX fuel rods, it is necessary to use four or more MOX fuel rods. , Μ
When loaded into the core 12, a {O} fuel assembly having three or less types of enrichment of OΧ fuel rods has thermal characteristics as shown by a curve A in FIG. 13 and makes it difficult to operate the reactor.

As described above, the enrichment of the {O} fuel rods constituting the MO fuel assembly 32 generally needs to be four or more types. However, if the enrichment type increases, the manufacturing cost of the MOX fuel rods increases. There was a problem that it became high.

Further, when manufacturing a fuel assembly, it is necessary to prepare spare fuel rods. For example, FIG.
In the fuel assemblies shown in (A) and (B), symbols P1 and P
2, 50 MOX fuel rods indicated by P3 and P4, and 10 gadolinia-containing uranium fuel rods indicated by reference symbol G.

Here, 104 per operation cycle
Assuming that a fuel assembly is loaded and 3% of spare fuel rods are prepared, there are 50 MO fuel rods per OX fuel assembly, so the number of spare fuel rods is Is 156.

In fact, since only a small part of these spare fuel rods are used, the four types of fuel rods having different enrichment types are each provided with several to several tens of excess fuel rods (hereinafter, referred to as surplus fuel rods). Excess fuel rods) will remain.

On the other hand, in the case of the uranium fuel assembly 33, the isotope composition of the uranium oxide does not change during storage of the surplus fuel rods outside the core 12 and molding of the next batch fuel. It can be used in molding and processing the next batch fuel.

However, in the OX fuel assembly 32, the half-life of the fissile substance plutonium 241 in the uranium-plutonium mixed oxide used is as short as 14.4 years,
It breaks down to Americium 241 which is a neutron absorbing substance. The weight ratio of the isotope composition in plutonium 241 in plutonium is about 4 to 9%, and the fluctuation of the isotope composition occurs while the surplus fuel rods are stored outside the core 12 and the next batch fuel is molded.

Therefore, P of the original fuel rod and the surplus fuel rod
It is necessary to use surplus fuel rods in consideration of the influence on local peaking due to the difference in u composition and the decrease in reactivity due to accumulation of americium 241. Further, the above-described fluctuation does not occur according to the number ratio of each enrichment level,
Depending on the degree of enrichment, excess fuel rods may be excessive or deficient, and the same configuration as the original assembly cannot be made.

As described above, the following problems have been encountered in the conventional light water reactor core 12 loaded with the MO 課題 fuel assembly 32. That is, the ΜOX fuel assembly 32
It is difficult to use the surplus fuel rods of the MOX fuel rods for the fuel of the next batch, because the molding processing cost and the handling cost are high.

The present invention has been made in view of the above-mentioned circumstances, and has as its object to provide a light water reactor core which makes it possible to use MOX fuel assemblies economically.

[0026]

In order to solve the above-mentioned problems, a first aspect of the present invention relates to a light water reactor core loaded with a MOX fuel assembly using a uranium-plutonium mixed oxide. The MOΧ fuel assembly has the first plutonium enrichment type of MOX fuel rods of three or less types.
A MOX fuel assembly of the first group, and a second group of MOX fuel assemblies having four or more plutonium-enrichment types of MOΧ fuel rods . The number of MOX fuel assemblies of the second group is smaller than the number of loaded bodies of the MOX fuel assemblies of the second group, and the MOX fuel assemblies of the first group are provided in the outermost peripheral region of the core and at least one of the innermost outermost layers for at least two cycles. For a period of time.

A second aspect of the present invention is the light water reactor core according to the first aspect, wherein plutonium enrichment of all MOΧ fuel rods constituting the first group of MOX fuel assemblies is equal to M of the second group.
It is characterized in that it is the same as any one of the MO fuel rods constituting the O fuel assembly.

According to a third aspect of the present invention, there is provided the light water reactor core according to the first aspect, wherein the uranium fuel assembly using uranium dioxide is loaded and only one type is used. First group of M with uranium fuel rods
The enrichment of the uranium fuel rods constituting the OΧ fuel assembly is determined by the enrichment of one of the uranium fuel rods constituting the second group MOΧ fuel assembly or the fuel of one of the uranium fuel assemblies.
It is characterized in that it has the same concentration as the rod .

According to a fourth aspect, in the light water reactor core according to the first aspect, the MOX fuel assemblies of the first group and the second group are:
Each is equipped with gadolinia-filled fuel rods.
The number of gadolinia-containing fuel rods of the fuel assembly is smaller than the number of gadolinia-containing fuel rods of the second group of MOX fuel assemblies.

According to a fifth aspect of the present invention, in the light-water reactor core according to the first aspect, in the light-water reactor core according to the first aspect, the sum of the weight of plutonium and the weight of americium is calculated by subtracting the weight of uranium and the weight of plutonium. The value of the first group of MOX fuel assemblies having the same value divided by the total weight of americium is
MOX fuel of the MOX fuel rods and MOX fuel assemblies of the second group
The ratio of the contents of americium 241 and plutonium 241 in the plutonium enrichment of the rod is the MOX of the first group.
Those of MOX fuel rods of the fuel assembly is in the second group of fuel of the united
It is characterized by being larger than MOX fuel rods .

[0031]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory view showing a fuel loading pattern of one embodiment of a light water reactor core according to the present invention. Parts that are the same as or correspond to the conventional configuration will be described using the same reference numerals as in FIGS.

In FIG. 1, the core 12 is denoted by reference numerals 1, 2,
Uranium fuel assemblies 33 indicated by reference numerals 3 and 4,
, And the {O} fuel assembly 32 shown in FIG. The {O} fuel assembly 32 uses a uranium-plutonium mixed oxide. Here, code 1,
Numerical values of 2, 3, 4, and, indicate the number of core stay cycles of the fuel assembly.

The MOX fuel assembly 32 has a plutonium enrichment of the first group having three or less types of MΟ
A fuel assembly, and a second group of M fuel assemblies having plutonium enrichment of four or more types indicated by the symbols ，, ΟΧ, and the outermost peripheral region of the core 12 (or the region inside the outermost one layer). ) May be loaded with a first group of M # fuel assemblies. FIGS. 2 to 4 show examples of the first group of MOX fuel assemblies, respectively.
, Are shown in FIG.

The first group MΟΧ shown in FIGS. 2A and 2B
The fuel assembly is composed of fuel rods having only one type of plutonium enrichment. The fuel rod having plutonium enrichment P1 has the same design as one of the fuel rods P1 to P4 of the second group of MOX fuel assemblies shown in FIGS. 11A and 11B.

The first group MΟΧ shown in FIGS. 3A and 3B
The fuel assembly is composed of a total of two types of fuel rods, one MOΧ fuel rod having one plutonium enrichment and one uranium fuel rod having one enrichment using uranium dioxide. The fuel rods 25 having the plutonium enrichment P1 have the same design as one of the fuel rods P1 to P4 of the MO group fuel assembly of the second group, and the fuel rods having the enrichment U are denoted by reference numerals in FIG. The uranium fuel assemblies loaded at the same time as the uranium fuel assemblies shown at 1, 2, 3, and 4, or the fuel rods P1 to P4 of the second group MOX fuel assemblies shown at (A) and (B) in FIG. It has the same design as one of the fuel rods in the assembly.

The first group MＢ shown in FIGS. 4A and 4B
The fuel assembly is composed of fuel rods having three types of plutonium enrichment. Plutonium enrichment P
The fuel rods P1, P2, and P3 have the same design as any one of the fuel rods P1 to P4 of the second group of MOX fuel assemblies. The uranium fuel rod G containing gadolinia is shown in FIG.
It has the same design as the gadolinia-containing uranium fuel rod G constituting the second group of MOX fuel assemblies shown in FIGS. 1 (A) and 1 (B). In FIG. 4A, reference symbol P1 ′ denotes an excess MOΧ fuel rod having a plutonium enrichment P1.

As described above, the MO of the first group shown in FIGS.
The fuel assembly has three or fewer MOX fuel rods and the same design as some of the fuel rods used in the second group of MO fuel assemblies. The manufacturing cost of the bar can be reduced.

Next, the operation of the present embodiment will be described with reference to FIGS.

FIG. 5 shows the change in local peaking combustion, and FIG.
7 shows the combustion change at infinite multiplication factor in the first group (examples of FIGS. 2 to 4) and the second group, respectively.

As shown in FIGS. 5 and 6, the local peaking and infinite multiplication factor of the first group of fuels at the beginning of combustion are as follows:
They are about 15% and about 5% larger than the second group, respectively. Thus, the first group of MOX fuel assemblies
Although the local peaking and the infinite multiplication factor tend to be higher than those of the MOΧ fuel assemblies of the second group, the thermal margin is higher when the fuel assemblies are loaded in the outer peripheral region lower than the core average. Increases, and the maximum linear power density increases as shown in FIG.
As shown in the curve の of No. 3, the limit value can be sufficiently satisfied. On the other hand, the ΜOX fuel assemblies of the second group have low local peaking, and can satisfy the limit value even when loaded at the center of the core.

As described above, the MO group fuel assemblies of the first group have at least a local peaking, a high infinite multiplication factor, and a period in which the thermal load of the fuel tends to be severe, that is, about two years of an operation cycle of about one year. Core 12 during the cycle
By loading the outer periphery of the fuel cell, fuel integrity can be ensured and used.

Further, in the periodic inspection of the reactor core 12 for about one year, the rearrangement of the continuously loaded fuel is performed together with the removal of the burned fuel and the loading of the new fuel, and the output of the entire reactor core 12 is made as uniform as possible. However, such a change in the fuel position is not always necessary because the power is originally low in the outer peripheral region of the core 12.

Therefore, if the first group of MOX fuel assemblies are loaded at one position in the outer peripheral region for a period of two or more cycles and then taken out, the fuel transfer work during the periodic inspection period can be reduced, This can contribute to shortening of the inspection process.

Further, as shown in FIG. 4A, the number of Gd fuel rods in the first group of MOX fuel assemblies is eight, which is the same as that in the fuel assembly shown in FIG. 11A. Number 1
This is less than zero, and the number of MOΧ fuel rods is correspondingly large. For this reason, the MOX fuel assembly shown in FIG.
The infinite multiplication factor at the beginning of combustion is higher than that of the MOΧ fuel assembly shown in FIG. 11, and the output tends to increase. When such a {O} fuel assembly is loaded on the outer peripheral portion of the core having a low output, the output of the outer peripheral portion is relatively increased, and as a result, the radial power distribution in the core 12 is flattened.

Also, since the number of {O} fuel rods is large,
More plutonium can be used in the body fuel assembly. When a certain amount of plutonium is used, reducing the number of fuel assemblies to be handled is advantageous in terms of handling such as transportation of the fuel assemblies.

One or two or all of the MOX fuel rods P1, P2 and P3 of the MOΧ fuel assembly shown in FIGS. 2 to 4 are the MOX fuel rods of the second group shown in FIG.
Excess fuel rods used when manufacturing the X fuel assembly can also be used. In this case, the initial plutonium enrichment is
The same as the {OX} fuel rods of the {O} fuel assemblies of the group,
Since the fissile nuclide plutonium 241 is β-decayed into the non-fissile nuclide Americium 241, the ratio of the contents of Americium 241 and Plutonium 241 is higher in the first group {O} fuel assembly than in the second group. It is getting bigger. That is, the number of fissile nuclides is relatively reduced, and the number of nuclides having a strong absorption is increasing, which affects the power distribution of each fuel rod in the fuel assembly and may increase local peaking.

However, the MOΧ fuel assemblies of the first group are loaded on the outer periphery of the core 12 having a relatively low output, so that the surplus fuel rods can be used effectively without greatly affecting the characteristics of the entire core 12. can do.

As described above, in the reactor core of the present embodiment, the ΜOX fuel assembly having three or less plutonium enrichment and large local peaking is used as the outermost core region of the core having a low output or the region inside one layer of the outermost core. , Safe driving satisfying a predetermined limit value is possible. Further, such a {O} fuel assembly with a small enrichment type has a lower production cost than that of a fuel with a large enrichment type, and can improve economic efficiency.

[0050]

As described above, according to the first aspect of the present invention,
According to the above, the MOΧ fuel assembly has a first group of MO 燃料 fuel assemblies in which MOX fuel rods have three or less plutonium enrichment types, and four or more MOΧ fuel rods have plutonium enrichment types. A second group of MOX fuel assemblies,
The number of loaded bodies of the first group of MOX fuel assemblies is
The number of MOX fuel assemblies in the group is less than the number of loaded bodies, and the first group of MOX fuel assemblies is loaded in the outermost peripheral region of the core and / or at least one region inside the outermost layer for a period of at least two cycles. By doing so, the {O} fuel assembly whose manufacturing cost has been reduced is loaded on the outer peripheral portion of the core having a relatively low fuel output, so that the MOX fuel assembly can have a sufficient thermal margin and can be used safely.

Further, surplus fuel rods generated at the time of fuel production can be used effectively, and the economics in the production of ΜOX fuel assemblies can be improved comprehensively. further,
The OX fuel assembly of minority enrichment type is continuously loaded at the same position on the outer periphery of the core for two or more cycles, so the fuel transfer work during the periodic inspection is reduced and the periodic inspection process is reduced. Can be.

According to the second aspect, in the light water reactor core according to the first aspect, the plutonium enrichment of all MOΧ fuel rods constituting the first group of MOX fuel assemblies is equal to or less than the second.
By being the same as any of the MO fuel rods constituting the MO fuel assembly of the group, the manufacturing cost of the fuel rods can be reduced.

According to a third aspect, in the light water reactor core according to the first aspect, a uranium fuel assembly using uranium dioxide is loaded, and only one type of uranium fuel rod is provided.
To enrichment of uranium fuel rods <br/> constituting the MOΧ fuel assemblies in the first group, the enrichment or uranium fuel assembly of any of the uranium fuel rods that constitute the MOΧ fuel assemblies of the second group When the enrichment is the same as that of any one of the fuel rods , the production cost of the fuel rods can be reduced as in the second aspect.

According to a fourth aspect, in the light water reactor core according to the first aspect, the first and second groups of MOX fuel assemblies each include a fuel rod containing gadolinia, and the first group of MOX fuels. Since the number of gadolinia-containing fuel rods in the assembly is smaller than the number of gadolinia-containing fuel rods in the second group of MOX fuel assemblies, the number of MOΧ fuel rods in the first group of MOX fuel assemblies is correspondingly large. Become. As a result, the output tends to increase, and when the core is loaded on the outer periphery of the core having a low output, the output of the outer periphery relatively increases, and as a result, the radial power distribution in the core is flattened.

Also, since the number of {O} fuel rods is large, 1
More plutonium can be used in the body fuel assembly. As a result, the number of fuel assemblies to be handled can be reduced, which is advantageous in terms of handling such as transportation of the fuel assemblies.

According to claim 5, in the light water reactor core according to claim 1, the value obtained by dividing the sum of the weight of plutonium and the weight of americium by the sum of the weight of uranium, the weight of plutonium and the weight of americium is obtained. M of the same first group
The ratio of the contents of americium 241 and plutonium 241 in the plutonium enrichment of the MOX fuel rods of the OX fuel assembly and the MOX fuel rods of the second group of MOX fuel assemblies is determined by the MOX fuel rod of the first group. Since the MOX fuel rods of the fuel assemblies are larger than the MOX fuel rods of the second group of fuel assemblies, the surplus fuel rods can be used effectively without significantly affecting the characteristics of the entire core.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a fuel loading pattern of an embodiment of a light water reactor core according to the present invention.

FIG. 2A is a configuration diagram showing a cross section of a fuel assembly,
(B) is a diagram showing plutonium enrichment distribution in the axial direction of the fuel rod.

FIG. 3A is a configuration diagram showing a cross section of another fuel assembly, and FIG. 3B is a diagram showing an enrichment / plutonium enrichment distribution in the axial direction of each fuel rod.

4A is a configuration diagram showing a cross section of still another fuel assembly, and FIG. 4B is a diagram showing enrichment, plutonium enrichment, and gadolinia concentration distribution in the axial direction of each fuel rod.

FIG. 5 is a view showing a combustion change of local peaking in the embodiment.

FIG. 6 is a diagram showing a combustion change at an infinite multiplication factor in the embodiment.

FIG. 7 is a schematic sectional view showing a boiling water reactor which is a conventional light water reactor.

FIG. 8 is a configuration diagram showing a cross section of a cell of a conventional light water reactor core.

FIG. 9 is an explanatory diagram showing a configuration of a core in a boiling water reactor.

FIG. 10 is a perspective view showing a fuel assembly in a boiling water reactor.

11A is a configuration diagram showing a cross section of a fuel assembly, FIG.
(B) is a diagram showing the axial enrichment, plutonium enrichment, and gadolinia concentration distribution of each fuel rod.

FIG. 12 is an explanatory diagram showing a fuel loading pattern of a conventional light water reactor core.

FIG. 13 is a characteristic diagram showing a thermal margin of the core when the first group of ΜOX fuel assemblies are loaded.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Reactor pressure vessel 12 Reactor core 13 Fuel assembly 14 Cooling water 15 Steam separator 16 Steam dryer 17 Main steam outlet nozzle 18 Feedwater nozzle 19 Unit lattice (cell) 20 Control rod 21 Neutron source 22 Incore monitor 23 Control cell 24 Fuel pellet 25 Fuel rod 26 Water rod 27 Channel 28 Upper tie plate 29 Lower tie plate 30 Spacer 31 Channel fastener 32 {O} fuel assembly 33 Uranium fuel assembly

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-238292 (JP, A) JP-A-60-262090 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 5/00 G21C 5/12

Claims (5)

(57) [Claims] 1. In a light water reactor core loaded with an MOΧ fuel assembly using a uranium-plutonium mixed oxide, the MO が fuel assembly has three or less types of plutonium-enriched MOX fuel rods. MOΧ fuel assemblies of the first group and plutonium enrichment type of MOX fuel rods are 4
A second group of MOX fuel assemblies of at least one type, wherein the number of loadings of the first group of MOX fuel assemblies is less than the number of loadings of the second group of MOX fuel assemblies; 1
The group of MOX fuel assemblies includes at least one outermost region of the core and at least one region inside one layer of the outermost periphery.
A light water reactor core characterized by loading during a cycle. 2. The MOX fuel assembly according to claim 1, wherein all of the MOXs constituting the first group of MOX fuel assemblies are provided.
A light water reactor core, wherein the plutonium enrichment of the fuel rod is the same as any of the MOΧ fuel rods constituting the MOX fuel assembly of the second group. 3. A light-water reactor core according to claim 1, wherein a uranium fuel assembly using uranium dioxide is loaded, and a first group of M having only one type of uranium fuel rod.
The enrichment of the uranium fuel rods constituting the OΧ fuel assembly is determined by the enrichment of one of the uranium fuel rods constituting the second group MOΧ fuel assembly or the fuel of one of the uranium fuel assemblies.
A light water reactor core having the same enrichment as a rod . 4. The light water reactor core according to claim 1, wherein the first and second groups of MOX fuel assemblies each include a fuel rod containing gadolinia, and the first group of MOX fuel assemblies contain gadolinia. A light water reactor core, wherein the number of fuel rods is smaller than the number of gadolinia-containing fuel rods in the second group of MOX fuel assemblies. 5. The light water reactor core according to claim 1, wherein the sum of the weight of plutonium and the weight of americium divided by the sum of the weight of uranium, the weight of plutonium and the weight of americium is the same as that of the first group. of MOX fuel assembly
MOX fuel of the MOX fuel rods and MOX fuel assemblies of the second group
The ratio of the contents of americium 241 and plutonium 241 in the plutonium enrichment of the rod is the MOX of the first group.
Those of MOX fuel rods of the fuel assembly is in the second group of fuel of the united
A light water reactor core characterized by being larger than a MOX fuel rod .