JPH05164874A - Light water reactor core - Google Patents

Abstract

PURPOSE:To improve the void coefficient effectively without reducing the reactivity, in a light water reactor core which has a high conversion ratio. CONSTITUTION:A light water reactor core 15B composed of fuel aggregates made by bundling numerous fuel rods which consist of an uranium and plutonium mixture fuel, in which the water-to-fuel volume ratio is less than 0.4, is composed of fuel aggregates 16 and 17 whose fissonable plutonium enrichments in the loading are different each other. And when the reactor core is divided into two areas making the numbers of the fuel aggregates equal in the diameter direction from the center of the core, the number of low enrichment fuel aggregates 16 loaded at the inner area are more than those in the outer area, in the calculation. And when the reactor core is divided into three areas making the numbers of fuel aggregates equal in the diameter direction from the center, the average value of the burn-up of the high enrichment fuel aggregates 17 loaded at the innermost area is higher than that in the immediately outer area, in the calculation.

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 core such as a light water cooled light water moderator reactor having a high conversion ratio from a fuel parent material to a fissile material, and particularly to uranium and plutonium having a large safety margin with an improved void coefficient. The present invention relates to a LWR core loaded with a fuel assembly filled with a mixed fuel.

[0002]

2. Description of the Related Art In a nuclear reactor core, fission products such as uranium-235 are consumed by fission reaction, while neutron absorption of uranium-238 produces new fissile materials such as plutonium-239. To be done. The ratio of the production rate of fissile material and the consumption rate of fissile material at the time of taking out spent fuel assemblies is called the conversion ratio. This conversion ratio is about 0.5 for ordinary light water reactors (light water cooled light water moderator reactor). Is. Therefore, it is considered to increase the conversion ratio in order to save the uranium resource as an energy source.

In Japanese Unexamined Patent Publication No. 1-227993, a plurality of fuel rods filled with a nuclear fuel material are densely arranged so that an effective fuel-to-fuel volume ratio of water to fuel rods is 0.4 or less. The conversion ratio of 1.0 is achieved by loading a large number of the fuel assemblies into the core of a boiling water reactor in which the coolant flows.
It is proposed to raise it to the neighborhood. That is, according to the fuel assembly, almost the same amount of fissionable substances as plutonium-239 and uranium-235 that have been consumed are obtained as plutonium-239. Therefore, enrich this fissionable plutonium with natural uranium, recovered uranium obtained by reprocessing spent fuel, depleted uranium obtained by enrichment work, slightly enriched uranium, or a mixture thereof, and It can be used as a plutonium mixed fuel and loaded into a nuclear reactor for combustion. This burning again yields approximately the same amount of fissile material as the fissile material that was consumed. Using this fissile plutonium, fuel such as natural uranium is enriched again to form a uranium-plutonium mixed fuel, which is loaded into a nuclear reactor and burned. By repeating such a cycle, plutonium can be effectively used and uranium resources can be saved.

Generally, the loading method of the uranium-plutonium mixed fuel affects the core characteristics of the nuclear reactor. That is, when the effective water-to-fuel volume ratio of the core is around 2.0 and the neutron spectrum is soft as in a normal boiling water reactor, the void coefficient of the uranium-plutonium mixed fuel, that is, the void fraction of the moderator The change in reactivity due to the change has a more negative value than the void coefficient of the enriched uranium fuel, but the absolute value tends to become smaller when the fuel is densely arranged and the volume ratio of water to fuel is decreased to increase the conversion ratio. There is a positive value in the core having an effective water-to-fuel volume ratio of 0.4 described in JP-A-1-227993.

The output coefficient is used as an index for the safety during abnormal transient changes and accidents in a nuclear reactor. This output coefficient is a quantity indicating the rate of change in reactivity when the unit output changes, and the void coefficient. Is expressed as an output coefficient which is the sum of the Doppler coefficient indicating the change in reactivity due to temperature change. In the core described in the above publication, the void coefficient is positive, but the power coefficient is negative, and there is no safety problem. However, reducing the positive value of the void coefficient or setting it to a negative value has the effect of increasing the margin of reactor safety, and is desirable.

As one means for reducing the positive value or the negative value of the void coefficient, the change in the amount of neutron leakage from the core is reduced by decreasing the ratio of the diameter to the height of the core. It is known how to make it bigger. The void coefficient of the core is a combination of changes in the infinite multiplication factor of neutrons and changes in the amount of neutron leakage due to changes in the void rate.Therefore, by reducing the ratio of core diameter to height as described above, neutron leakage is reduced. By constructing an easy core, even if the void ratio changes and the number of neutrons increases, the amount of leaked neutrons also increases and the change of the infinite multiplication factor is suppressed. In other words, it has the effect of reducing the positive value or making it negative with respect to the void coefficient. However, there is a problem that the amount of neutron leakage in a steady state increases and the reactivity decreases because the core is a core in which neutrons easily leak.

In Japanese Patent Application No. 2-061013, the weight ratio of fissile plutonium to heavy metal (hereinafter simply referred to as enrichment degree) in each of two regions that divide the axial total length of the active fuel length portion of the fuel assembly into two equal parts. The above problem is attempted to be solved by making the average value of the above (1) may be higher in the region located on the upstream side of the coolant flow, that is, on the lower side of the fuel assembly than on the downstream side. With this method, the void coefficient can be reduced without lowering the reactivity. However, this only deals with the enrichment distribution in the axial direction of the core, and does not mention the structure of the core for reducing the void coefficient, that is, the method of loading the fuel assembly.

[0008]

As described above, in the core described in Japanese Patent Laid-Open No. 1-227993, the conversion ratio is 1.
It can be increased to near 0, and the void coefficient is positive but the output coefficient is negative, which is safe for safety. However, as described above, it is desirable to make the positive value of the void coefficient smaller or to make it a negative value because it has the effect of increasing the margin of reactor safety.

According to the method of increasing the change in the amount of leakage of neutrons from the core by decreasing the ratio of the diameter to the height of the core, the positive value of the void coefficient can be reduced or made negative. However, there is a problem that the amount of neutron leakage in the steady state increases and the reactivity decreases because the core is prone to neutron leakage.

Further, in the core described in Japanese Patent Application No. 2-061013, the void coefficient can be reduced without lowering the reactivity, but only the enrichment distribution in the axial direction of the core is targeted. No mention is made of the structure of the core for reducing the void coefficient, that is, the method of loading the fuel assembly.

In view of the above problems, an object of the present invention is to provide a light water reactor core which effectively improves the void coefficient while minimizing the decrease in reactivity by appropriately loading the fuel assemblies. is there.

[0012]

In order to achieve the above object, according to a first concept of the present invention, a water-fuel mixture is constituted by a fuel assembly in which a large number of fuel rods made of a mixed fuel of uranium and plutonium are bundled. A light water reactor core having a volume ratio of 0.4 or less is composed of at least two types of fuel assemblies having different weight ratios of fissionable plutonium to heavy metal when loaded.

[0013] Preferably, at least one of the fuel assemblies has the weight ratio of 3% to 5% by weight.

In order to achieve the above object, according to the second concept of the present invention, in the above light water reactor core, the number of fuel assemblies is set to be equal in the radial direction from the center of the core.
When divided into two regions and calculated, the number of bodies loaded in the region inside the two regions of the fuel assemblies with the lower weight ratio is loaded in the region outside the two regions. It is configured so that it is larger than the number of bodies.

Preferably, the fuel assembly having the lower weight ratio is uniformly loaded adjacent to the fuel assembly having the higher weight ratio.

In order to achieve the above object, according to a third concept of the present invention, in the above light water reactor core, three regions are arranged so that the number of fuel assemblies is equal in the radial direction from the center of the core. Of the fuel assemblies having the higher weight ratio, the average value of the burnup of the fuel assemblies having the higher weight ratio, which are loaded in the innermost regions of the three regions. Is higher than the average value of the burnup of the fuel assembly having the high weight ratio in the region immediately outside thereof.

Preferably, the fuel assembly having the lower weight ratio is uniformly loaded adjacent to the fuel assembly having the higher weight ratio.

[0018] Preferably, the fuel assembly having the lower weight ratio is radially adjacent to the fuel assembly having the higher weight ratio and is annularly loaded.

Preferably, the two types of fuel assemblies are fuel assemblies loaded at the same time.

[0020]

[Function] In general, the infinite multiplication factor of uranium and plutonium mixed fuels increases almost proportionally with the increase of the fissile plutonium enrichment, so the infinite multiplication factor of the region constituted by the fuels of different enrichment is It becomes equal to the infinite multiplication factor of the region constituted by the fuel of uniform enrichment equal to the average value of the different enrichments. On the other hand, the void coefficient of the uranium and plutonium mixed fuel increases as the fissile plutonium enrichment increases, but is not in a proportional relationship, and the increase of the void coefficient slows as the enrichment increases. Therefore, the void coefficient of the region constituted by the fuels of different enrichment having the same average enrichment is lower than the void coefficient of the region constituted by the fuel of uniform enrichment. The first concept of the present invention is based on this principle, and by constructing a light water reactor core with at least two types of fuel assemblies having different fissile plutonium enrichments, the void coefficient can be reduced without lowering the reactivity. Can be lowered.

As described above, when at least two kinds of different enrichments are selected, it is clear that it is desirable for the difference in the enrichments to be as large as possible in order to reduce the void coefficient. As the fuel consumption increases, the fuel share with a higher enrichment (hereinafter referred to as the high-enrichment fuel assembly) increases the output share, which adversely affects the core by lowering the conversion ratio or decreasing the thermal margin. .. In the present invention, the fissile plutonium enrichment is from 3% by weight to 5%.
Since at least one kind of fuel assembly whose weight% is used is used, an appropriate enrichment difference can be provided.

Even if the structure of the fuel assembly of the light water reactor core is the same, the reactivity of the reactor core changes due to the change of the neutron flux distribution in the core. That is, when the void fraction of the moderator changes, the neutron flux becomes low in the fuel with a high infinite multiplication factor, and the neutron flux becomes high in the fuel with a low infinite multiplication factor. The second concept of the present invention is, in addition to the operation of the first concept,
By utilizing this principle, the void coefficient is further reduced. That is, according to the second concept of the present invention, when the light water reactor core is divided into two regions so that the number of fuel assemblies is equal in the radial direction from the center, the fissile plutonium enrichment is lower. In the fuel assembly (hereinafter, referred to as low enrichment fuel assembly), the number of bodies loaded in the area inside the two areas is larger than the number of bodies loaded in the area outside the two areas. Constitute. Normally, when the void fraction of the moderator increases, the amount of neutron leakage from the core outer periphery increases, so the neutron flux at the core outer periphery decreases relatively, and the neutron flux at the core inner periphery relatively increases. To do. Therefore, in the present invention, the neutron flux of the fuel assembly having a low infinite multiplication factor (low enrichment) arranged in the region inside the core is relatively increased, and the neutron flux is arranged in the region outside the core. The neutron flux of fuel with a high infinite multiplication factor (high enrichment) is reduced, which suppresses an increase in the reactivity of the core when the void fraction increases. That is, the void coefficient becomes low. However, when the void ratio increases, the infinite multiplication factor of each fuel increases with high enrichment fuel,
Since low enrichment fuel will decrease, if only low enrichment fuel assemblies are put in the inner region, the neutron flux there will decrease. Therefore, in the present invention, a highly enriched fuel assembly is also placed in the inner region to prevent this, which substantially reduces the void coefficient. It should be noted that the prevention of neutron flux decrease in this inner region is more effectively achieved by uniformly loading the low-enrichment fuel assembly adjacent to the high-enrichment fuel assembly. .

In the uranium-plutonium mixed fuel in which the effective water-to-fuel volume ratio is 0.4 or less so that the conversion ratio is around 1.0, the void coefficient becomes uniform as the burnup increases. To increase. In the third concept of the present invention, in addition to the functions of the first and second concepts, the neutron flux in the low enrichment fuel assembly in the inner region is further increased based on this principle, and the void coefficient is increased. It is to lower it further. That is, according to the third concept of the present invention, when the calculation is performed by dividing into three regions so that the number of fuel assemblies is equal in the radial direction from the center, three of the high enrichment fuel assemblies are calculated. Since the average value of the burnup of those loaded in the innermost region of the region is made higher than the average value of the burnup of those loaded in the outer region, when the void fraction increases, the neutron flux in the outermost region becomes In addition to the relative decrease due to neutron leakage, the void coefficient in the innermost peripheral region becomes higher than the void coefficient in the region immediately outside thereof, which causes the neutron flux in the innermost peripheral region to increase relatively. Thus, the void coefficient is further effectively reduced by increasing the neutron flux in the inner region of the core where the low-enrichment fuel assembly having a low infinite multiplication factor exists. In this case, by loading the low-enrichment fuel assemblies uniformly or annularly adjacent to the high-enrichment fuel assemblies, the neutrons in the inner region are similar to the effect of the second concept. The bundle increase is achieved more effectively.

[0024]

EXAMPLE A light water reactor core according to an example of the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional view of a fuel assembly that constitutes a core according to this embodiment, and FIG. 2 is a cross-sectional view of a light water reactor core configured thereby.

Since the fuel assembly of this embodiment has a conversion ratio of about 1.0, it contains a plurality of fuel rods densely arranged so that the effective water-to-fuel volume ratio is 0.4 or less. It is loaded in a boiling water type light water cooled light water moderator reactor (light water reactor). That is, in FIG. 1, the fuel assembly 11 constituting the core of the present embodiment has a hexagonal cross section, and is arranged in a hexagonal close-packed lattice with the channel box 12.
It is composed of 51 fuel rods 13 and 18 control rod guide tubes 14. The outer shape of the fuel rod 13 is 11.8 mm,
The fuel rod spacing is 1.3 mm and the geometrical water to fuel volume ratio within the fuel assembly is 0.5. In this arrangement, for example, when the void ratio is 20%, the effective water-fuel volume ratio is about 0.4, and when the void ratio is 55%, the water-fuel volume ratio is 0.24.

In FIG. 2, the light water reactor core 15 of this embodiment is shown.
Is constructed by using the fuel assembly 11. That is,
The light water reactor core 15 is composed of two types of fuel assemblies 16 and 17 having different weight ratios of fissile plutonium to heavy metals during loading, that is, enrichment degrees. One fuel assembly hatched (low enrichment) Degree fuel assembly) 16 has an enrichment of 391 at 4 wt% (hereinafter wt% is referred to as wt%).
Body, the other fuel assembly (highly enriched fuel assembly) 17 has an enrichment of 9.5 wt% and 390 bodies, 781 bodies in total are arranged in a hexagonal lattice in the LWR core 15. The average enrichment is 6.75 wt%.

In this embodiment, the principle that the void coefficient can be lowered without reducing the reactivity will be described. FIG. 3 shows an infinite multiplication factor with respect to the enrichment of the uranium and plutonium mixed fuel when the effective water-to-fuel volume ratio is kept constant at a small value (0.24 as an example) for the purpose of high conversion ratio, and The relationship between void coefficients is shown. As shown by the solid line in FIG. 3, the infinite multiplication factor increases almost proportionally with the increase in the enrichment ratio, and the infinite multiplication factor of the region constituted by the fuels with different enrichment ratios is It becomes equal to the infinite multiplication factor of the region constituted by the fuel of uniform enrichment equal to the average value. On the other hand, the void coefficient of the mixed fuel of uranium and plutonium increases as the enrichment increases as shown by the broken line in FIG. 3, but is not in a proportional relationship, and the increase of the void coefficient with the enrichment increases. Get loose. Therefore, the void coefficient of the region constituted by the fuels of different enrichment having the same average enrichment is lower than the void coefficient of the region constituted by the fuel of uniform enrichment. Since the infinite multiplication factor at that time is the same as described above, when two types of fuel assemblies having different enrichments are used, the void coefficient can be lowered without decreasing the reactivity.

For the above reasons, the light water reactor core 1 of this embodiment is used.
Also in consideration of No. 5, by loading two types of fuel assemblies 16 and 17 having different enrichments, the void coefficient of the core can be reduced without lowering the reactivity. Here, when selecting two different enrichments, it is clear that it is desirable for the difference in enrichment to be as large as possible in order to reduce the void coefficient, but if the difference is too large, the high enrichment will be high. Since the output sharing in the fuel assembly is increased, the conversion ratio is reduced and the thermal margin is reduced, which adversely affects the core. Therefore, it is desirable to make an appropriate enrichment difference, and as a low enrichment fuel assembly, 3w
It is appropriate to have an enrichment in the range of t% to 5 wt%, and for this reason low enrichment fuel assemblies 16 in this example.
The enrichment ratio of is set to 4 wt%.

The following results were obtained when the light water reactor core 15 according to the present embodiment had a core specification as shown in FIG. As an example for comparison with this example, the enrichment degree is 6.75.
A light water reactor core composed of one wt% fuel assembly was used. The light water reactor core for comparison has the same conditions as those of the present embodiment except that the enrichment of the aggregate is one.

The void coefficient in this embodiment is 0.03% compared with the core composed of one type of fuel assembly.
ΔK / K /% void decreased, and reactivity was 0.9% Δ
K / K increased. That is, the void coefficient of the reactor core could be improved (reduced) without lowering the reactivity. The reactivity was rather increased by the two types of fuel assemblies. The reason for this is that the highly enriched fuel assembly 17 with high reactivity was used.
Enriched fuel assembly 1 with low neutron flux reactivity
It is considered that this is because the reactivity is higher than the neutron flux in No. 6 and therefore the reactivity of the entire core is increased.

According to this embodiment, the light water reactor core is constituted by at least two kinds of fuel assemblies 16 and 17 having different enrichments, so that the void coefficient of the reactor core is improved without lowering the reactivity ( Can be reduced).

A light water reactor core according to another embodiment of the present invention will be described with reference to FIG. FIG. 5 is a cross-sectional view of the light water reactor core 15A according to the present embodiment. 5, the number of fuel assemblies and the structure of the fuel assemblies are the same as those in the embodiment shown in FIG. This light water reactor core 15A is composed of two types of fuel assemblies 16 and 17 having different enrichment degrees, and one enriched fuel assembly (low enrichment fuel assembly) 16 has an enrichment of 4 wt. %, And the enrichment degree of the other fuel assembly (high enrichment fuel assembly) 17 is 9.
It is 619 at 5 wt%. In addition, this light water reactor core 5A
Is an inner region composed of a low enrichment fuel assembly 16 having an enrichment of 4 wt% and a high enrichment fuel assembly 17 having an enrichment of 9.5 wt%, and an enrichment of 9.5 wt. % Of the fuel assemblies 17 in the inner region, the number of fuel assemblies in the inner region is 70% of the total number of fuel assemblies in the inner and outer cores. And
Moreover, as shown in the drawing, the low enrichment fuel assemblies 16 and the high enrichment fuel assemblies 17 are loaded in a uniform arrangement. The average enrichment in the core is 7.2 wt%. Fuel assembly 1
When 6 and 17 are arranged in this way, when the core is divided into two regions in the radial direction from the center so that the number of fuel assemblies is equal, the low enrichment fuel assemblies in the inner region are calculated. The number of 16 bodies is larger than that of the high enrichment fuel assemblies 16 in the outer region.

In the light water reactor core 5A, if the void ratio of the moderator increases, the amount of neutron leakage in the radial direction increases.
The neutron flux in the outer region decreases relatively, and the neutron flux in the inner region increases relatively. Since 4 wt% of the fuel assembly 16 having a low infinite multiplication factor is arranged in the inner region, the infinite multiplication factor in the inner region is lower than the infinite multiplication factor in the outer region. For this reason, the neutron flux relatively increases in the inner region where the infinite multiplication factor is relatively low when the void fraction increases, so that there is an effect of suppressing the increase in reactivity. However, when the void ratio increases, the infinite multiplication factor of each fuel increases with high-enrichment fuel and decreases with low-enrichment fuel, so only the low-enrichment fuel assembly 16 is put in the inner region. Then, the neutron flux there will drop. Therefore, in this embodiment, the highly enriched fuel assembly 17 is also arranged in the inner region to prevent this, and thereby the void coefficient is substantially reduced. still,
The prevention of neutron flux reduction in this inner region is more effectively achieved by uniformly loading the low-enrichment fuel assembly 16 adjacent to the high-enrichment fuel assembly 17. ..

In the light water reactor core 5A according to this embodiment,
The void coefficient was 0.12% ΔK / K /% void smaller than that of the above-described comparative example, that is, the core composed of one type of fuel assembly (however, the enrichment was 7.2 wt%). .. As described above, by loading the low-enrichment fuel assemblies 16 in an appropriate arrangement, as compared with the case of loading in a uniform arrangement as in the embodiment shown in FIG. 2, in the embodiment of FIG. In addition to the action, the above action is obtained, so that the effect of reducing the void coefficient becomes large.

As described above, according to the embodiment, the LWR core is constituted by at least two kinds of fuel assemblies 16 and 17 having different enrichments, and the low enrichment fuel assemblies 16 are appropriately arranged. As a result, the effect of appropriately arranging the low enrichment fuel assembly 16 is added to the effect of the embodiment shown in FIG. 2, and the effect of reducing the void coefficient is further enhanced.

A light water reactor core according to still another embodiment of the present invention will be described with reference to FIG. FIG. 6 is a cross-sectional view of the light water reactor core 15B according to the present embodiment. In FIG. 6, the number of fuel assemblies and the structure of the fuel assemblies are the same as those in the embodiment shown in FIG. 2, and the numbers in the fuel assemblies show the number of batches staying in the reactor. This light water reactor core 15B
Is composed of two types of fuel assemblies 16 and 17 having different enrichments, and one enriched fuel assembly (low enrichment fuel assembly) 16 has a richness of 4 wt% and 162 bodies. ,
The enrichment of the other fuel assembly (highly enriched fuel assembly) 17 is 9.5 wt%, which is 619. Further, similar to the embodiment shown in FIG. 5, the light water reactor core 15B has an inner region composed of the low-enrichment fuel assembly 16 and the high-enrichment fuel assembly 17, and the high-enrichment fuel assembly 17 The outer region is composed of only the outer region, and the number of fuel assemblies in the inner region is 70% of the total number of fuel assemblies in the inner and outer cores.
Then, as shown in the figure, the low enrichment fuel assemblies 16 and the high enrichment fuel assemblies 17 are loaded in a uniform arrangement. Among them, the highly enriched fuel assembly 17 has a number of in-reactor batches of 6, 5, 4, from the center of the LWR core toward the radially outer side.
It changes in the order of 3, 2, 1, 7, 8, 9, 10 and, focusing on the inner region, the burnup gradually increases toward the center of the LWR core. Low enrichment fuel assembly 16
On the contrary, they are arranged from the center of the core toward the outside in order of increasing number of staying batches. When the high-enrichment fuel assemblies 17 are arranged in this way, when the core is divided into three regions in the radial direction from the center so that the number of fuel assemblies included in each region becomes equal, the innermost The average value of the burnup of the highly enriched fuel assembly 17 in the region is higher than the average value of the burnup of the highly enriched fuel assembly 17 in the region immediately outside thereof.

In the uranium-plutonium mixed fuel in which the effective water-to-fuel volume ratio is 0.4 or less so that the conversion ratio is around 1.0, the void coefficient becomes uniform as the burnup increases. To increase. Therefore, the void coefficient of the high-enrichment fuel assembly 17 in the inner region composed of the low-enrichment fuel assembly 16 and the high-enrichment fuel assembly 17 is composed of only the high-enrichment fuel assembly 17. It becomes higher than the void coefficient of the highly enriched fuel assembly 17 in the outer region, and when the void ratio increases, the neutron flux in the inner region relatively increases. Although the void coefficient of the outer region composed only of the high-enrichment fuel assemblies 17 is higher than the void coefficient of the inner region composed of the low-enrichment fuel assemblies 16 and the high-enrichment fuel assemblies 17, , When the void fraction increases, the neutron flux decreases relatively due to neutron leakage. in this way,
When the void ratio increases, the neutron flux increases in the inner region where many low-enrichment fuel assemblies 16 having a low infinite multiplication factor are arranged, so that the increase in reactivity is further suppressed. That is, the void coefficient is further lowered.

In order to further increase the neutron flux in the inner region, it is effective to reduce the flow rate in the low enrichment fuel assembly 16 by reducing the orifice diameter. As a result, the amount of change in the void ratio in the low-enrichment fuel assembly 16 is smaller than that in the other assemblies, and the effect of reducing the neutron flux in the low-enrichment fuel assembly 16 having a negative void coefficient is suppressed. You can

In the light water reactor core 15B according to the present embodiment, the void coefficient is 0 as compared with the above-mentioned comparative example, that is, the core constituted by one kind of fuel assembly (however, the enrichment is 7.2 wt%). .18% ΔK / K /% void became smaller. In this way, by considering the burnup of the high-enrichment fuel assembly and loading it in an appropriate arrangement, as shown in FIG.
The above effect is added to the effect of the embodiment shown in FIG. 5, and the effect of reducing the void coefficient is increased.

According to the present embodiment, the LWR core is composed of at least two types of fuel assemblies having different enrichments, and the low enrichment fuel assemblies are properly arranged in the first region, that is, uniform. 5 and by loading in an appropriate arrangement in consideration of the burnup of the highly enriched fuel assembly, that is, an arrangement in which the burnup increases toward the center of the LWR core in the inner region. The void coefficient can be further improved (reduced) as compared with the embodiment shown in FIG.

A light water reactor core according to still another embodiment of the present invention will be described with reference to FIG. FIG. 7 is a cross-sectional view of the light water reactor core 15C according to the present embodiment. The configuration of the light water reactor core 15C in FIG. 7 is the same as the configuration of the light water reactor core 15B shown in FIG. 6 except that the low enrichment fuel assemblies 16 are loaded annularly. If the low-enrichment fuel assemblies 16 are loaded with a large number of them, the void coefficient in that region becomes negative, so the neutron flux decreases when the void ratio increases, and this is a factor that increases the reactivity of the core. Becomes Therefore, the low enrichment fuel assemblies 16 are appropriately arranged. The embodiment shown in FIG. 6 is one such example, and this embodiment shows another example in this respect. That is, the high-enrichment fuel assembly 17 having a high void coefficient, that is, a relatively high burnup, faces the low-enrichment fuel assembly 16 in the radial direction, and the low-enrichment fuel assembly 16 is annularly positioned. The same purpose can be achieved by arranging as described above.

In the light water reactor core 5C according to this embodiment,
The void coefficient was 0.17% ΔK / K /% void smaller than that of the above-described comparative example, that is, the core composed of one type of fuel assembly (however, the enrichment was 7.2 wt%). .. In this way, by considering the burnup of the highly enriched fuel assembly 17 and loading the fuel assembly 17 in an appropriate arrangement, the same effect as that of the embodiment shown in FIG. 6 can be obtained.

According to the present embodiment, the LWR core is constituted by at least two kinds of fuel assemblies having different enrichments, and the low enrichment fuel assemblies are appropriately arranged, that is, annularly loaded in the inner region. Further, by loading in a proper arrangement in consideration of the burnup of the highly enriched fuel assembly, that is, an arrangement in which the burnup increases in the radial direction from the center of the LWR core in the inner region, as shown in FIG. The same effect as the embodiment shown can be obtained.

[0044]

According to the present invention, since a light water reactor core is constituted by at least two types of fuel assemblies having different weight ratios of fissile plutonium to heavy metal during loading, it is effective while minimizing the decrease in reactivity. The void coefficient can be improved.

In addition to the above structure, when the core is divided into two regions in the radial direction from the center so that the number of fuel assemblies is equal, the low enrichment fuel in the inner region is calculated. Since the fuel assemblies are arranged so that the number of assemblies is larger than the number of highly enriched fuel assemblies in the outer region, the effect of improving the void coefficient is further increased while minimizing the decrease in reactivity. can do.

Furthermore, since the low-enrichment fuel assemblies are uniformly loaded so as to be adjacent to the high-enrichment fuel assemblies, the reactivity of the core increases due to a decrease in the neutron flux when the void fraction increases. Can be prevented, and the void coefficient can be substantially reduced.

In addition to the above structure, when the core is divided into three regions in the radial direction from the center so that the number of fuel assemblies included in each region becomes equal, the innermost region is calculated. Since the fuel assembly is arranged so that the average burnup value of a high-enrichment fuel assembly is higher than the average burnup value of the highly-enriched fuel assembly in the region immediately outside of it, the reactivity decreases. It is possible to further increase the effect of improving the void coefficient while minimizing.

Further, since the low-enrichment fuel assemblies are uniformly loaded so as to be adjacent to the high-enrichment fuel assemblies, or are loaded so as to be adjacent to each other in the radial direction and have an annular shape, the void fraction is increased. At the time of increase, it is possible to prevent the reactivity of the core from increasing due to the decrease of neutron flux, and it is possible to substantially reduce the void coefficient.

Therefore, it is possible to expand the self-controllability margin of a nuclear reactor having a light water reactor core with a high conversion ratio and improve the safety.

[Brief description of drawings]

FIG. 1 is a sectional view of a fuel assembly of a light water reactor core according to an embodiment of the present invention.

FIG. 2 is a sectional view of a light water reactor core according to an embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the infinite multiplication factor and the void coefficient with respect to the weight ratio of uranium and plutonium mixed fuel to heavy metal.

FIG. 4 is a diagram showing core specifications of a light water reactor core according to the present invention.

FIG. 5 is a sectional view of a light water reactor core according to another embodiment of the present invention.

FIG. 6 is a sectional view of a light water reactor core according to still another embodiment of the present invention.

FIG. 7 is a sectional view of a LWR core according to still another embodiment of the present invention.

[Explanation of symbols]

 11 Fuel Assembly 12 Channel Box 13 Uranium-Plutonium Mixed Oxide Fuel Rod 14 Control Rod Guide Tube 15, 15A, 15B, 15C Core 16 Low Enrichment Fuel Assembly 17 High Enrichment Fuel Assembly

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenzo Takeda 1168 Moriyama-cho, Hitachi, Hitachi Ibaraki Energy Research Institute

Claims (14)

[Claims] 1. A fissile plutonium for heavy metal at the time of loading in a light water reactor core composed of a fuel assembly in which a large number of fuel rods made of a mixed fuel of uranium and plutonium are bundled and having a water to fuel volume ratio of 0.4 or less. A light water reactor core comprising at least two types of fuel assemblies having different weight ratios. 2. The light water reactor core according to claim 1, wherein at least one of the fuel assemblies has a weight ratio of 3% by weight to 5% by weight. 3. When the core is divided into two regions such that the number of fuel assemblies is equal in the radial direction from the center, the two of the fuel assemblies having the lower weight ratio are The light water reactor core according to claim 1 or 2, wherein the number of bodies loaded in an area inside the area is larger than the number of bodies loaded in an area outside of the two areas. 4. The light water reactor core according to claim 3, wherein the fuel assembly having the lower weight ratio is uniformly loaded adjacent to the fuel assembly having the higher weight ratio. .. 5. When calculating by dividing the core into three regions such that the number of fuel assemblies in the radial direction from the center is equal, among the fuel assemblies having the higher weight ratio,
The average value of the burnup of the fuel assembly having the higher weight ratio loaded in the innermost area of the three regions is the average of the burnup of the fuel assembly having the higher weight ratio in the area immediately outside thereof. The LWR core according to claim 4, wherein the LWR is higher than the value. 6. The light water reactor core according to claim 5, wherein the fuel assembly having the lower weight ratio is uniformly loaded adjacent to the fuel assembly having the higher weight ratio. .. 7. The fuel assembly having the lower weight ratio is radially adjacent to the fuel assembly having the higher weight ratio and is annularly loaded. Light water reactor core. 8. The light water reactor core according to claim 1, wherein the two types of fuel assemblies are fuel assemblies loaded at the same time. 9. A fissile plutonium weight ratio to a heavy metal is different in a light water reactor core composed of a fuel assembly in which a large number of fuel rods made of a mixed fuel of uranium and plutonium are bundled and having a water to fuel volume ratio of 0.4 or less. A light water reactor core comprising at least two types of fuel assemblies uniformly and uniformly loaded. 10. A light water reactor core comprising a fuel assembly in which a large number of fuel rods composed of uranium and plutonium mixed fuel are bundled, and a water-to-fuel volume ratio is 0.4 or less,
At least two types of fuel assemblies having different fissionable plutonium weight ratios with respect to heavy metals are provided, the inner region of the core is constituted by the two types of fuel assemblies, and the outer region is one of the two types of fuel assemblies. A light water reactor core characterized in that it is composed only of fuel assemblies having a higher weight ratio. 11. The number of fuel assemblies forming the inner region accounts for 70% of the number of fuel assemblies forming the entire core including the inner region and the outer region. Light water reactor core described. 12. The light water reactor core according to claim 10, wherein, in the inner region, the fuel assembly having the higher weight ratio is loaded so that the burnup becomes higher toward the center of the core. .. 13. The fuel assembly having the lower weight ratio is
The light water reactor core according to any one of claims 10 to 12, wherein the fuel assembly having the higher weight ratio is uniformly loaded adjacent to the fuel assembly. 14. The fuel assembly having the lower weight ratio is radially adjacent to the fuel assembly having the higher weight ratio and is annularly loaded in the inner region. The light water reactor core according to any one of claims 10 to 12.