JP2003167083A - Manufacturing method for fuel assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of setting the enrichment degree of plutonium to satisfy designated reactivity even if the composition ratio of uranium and plutonium in fuel material changes between the upper part and the lower part in the axial direction of the fuel assembly. <P>SOLUTION: In this manufacturing method for the fuel assembly, according to the uranium and plutonium isotope composition ratio of fuel material, the plutonium enrichment degree of a fuel rod is determined by the plutonium enrichment degree and equivalent fissile coefficient to the preset standard uranium and plutonium isotope composition ratio. As an equivalent fissile coefficient used in deciding the enrichment degree of plutonium, one of two kinds of equivalent fissile coefficients preset according to the uranium and plutonium isotope composition ratio of the fuel material is selected and used. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a boiling water reactor (hereinafter referred to as BWR), and more particularly to a fuel assembly having a fuel rod mixed with plutonium (hereinafter referred to as MOX fuel assembly). Called)).

[0002]

2. Description of the Related Art In general, as a fuel substance containing uranium and plutonium, a substance extracted from a spent fuel such as a light water reactor is used, and as a uranium material, depleted uranium (uranium-235 content generated in a uranium enrichment process is contained. rate:
0.2 wt%), natural uranium, depleted uranium extracted from spent fuel in light water reactors, or a mixture thereof. Of the above two, for plutonium fuel material, Pu-238, 239, 240, 24
Although isotopes such as 1,242 and Am-241 are mixed, the isotope composition ratio varies depending on the burnup of spent fuel, the type of nuclear reactor, and the like. In addition, Pu-241 disintegrates into Am-241 with a half-life of about 14 years.
Therefore, even in the same batch of plutonium fuel material used for producing fuel pellets for a fuel assembly, its isotope composition ratio will change within a certain range.

Therefore, as a method of setting the plutonium enrichment corresponding to the change in the composition ratio of uranium-plutonium in the fuel material in a reactor fuel assembly using a fuel material containing uranium and plutonium, for example, There is something like.

That is, assuming that the effect of Pu-239 on the core reactivity is 1.0, the relative reactivity effect of other plutonium isotopes and uranium isotopes is defined as an equivalent fisile coefficient, and plutonium isotopes and uranium isotopes are defined. The composition of each element is expressed as the sum (equivalent amount of fissile) of the composition ratio of each element multiplied by this equivalent fissile coefficient, and the reactivity of the fuel at the plutonium enrichment when the body composition ratio is a standard value. If the ratio changes from above,
The plutonium enrichment is adjusted so that the equivalent fissile amount defined above becomes constant. By doing so, the reactivity of the fuel is kept constant even when the composition of uranium / plutonium in the fuel substance changes (dynamic combustion technique No. 70, pp. 77-81).

As a general example of the design of the plutonium enrichment distribution of each fuel rod of the MOX fuel assembly, the plutonium enrichment of each fuel rod is made uniform in the axial direction, and the plutonium of the fuel rods is inside and outside the assembly. Some change the degree of enrichment (Thermal Nuclear Power Vol. 50 No. 2, Nos. 62-69)
page). This is to reduce the plutonium enrichment types of fuel rods for the purpose of streamlining the manufacturing process. Further, as the burnup of the fuel assembly is increased, the number of fuel rods is increased, and a fuel assembly structure including a partial length fuel rod in which the active fuel length is shorter than that of a normal fuel rod is provided. It is used more and more, and it is considered that MOX fuel assemblies will adopt such a fuel assembly structure in the future (Light Water Reactor Fuel Behavior Page 89 ((July 1998
Japan Nuclear Safety Research Association)).

[0006]

However, in the MOX fuel assembly having the above plutonium enrichment distribution design and fuel assembly structure, it is possible to cope with the change in the composition ratio of uranium-plutonium in the fuel material. When applying the method of setting the plutonium enrichment, there are the following problems.

That is, in the above plutonium enrichment setting method, the lower part of the assembly where the fuel substance exists at the position of the partial length fuel rod of the fuel assembly, and the fuel substance at the position of the partial length fuel rod of the fuel assembly. Plutonium enrichment is determined using the same equivalent fissile coefficient of the fuel assembly average even for the upper region of the assembly where no fuel exists. Originally, both regions have different equivalent fissile coefficients because the number of fuel rods and the like are not the same and the ratio of the area occupied by the fuel substance and the water that is the moderator is different, but both regions have plutonium isotope composition ratios. If they are the same, the plutonium enrichment in order to keep the reactivity constant in the upper part of the aggregate and the lower part of the aggregate is offset even if there is an excess or deficiency in plutonium enrichment. Enrichment can be set.
On the other hand, when the plutonium isotope composition ratio or the uranium isotope composition ratio of the fuel material is different between the lower part of the assembly and the upper part of the assembly, there is a gap in the evaluation by the same equivalent fissile coefficient of the fuel assembly average. May make it difficult to achieve the desired reactivity of the fuel assembly. When the plutonium isotope composition ratio or the uranium isotope composition ratio of the fuel material differs between the lower part of the assembly and the upper part of the assembly, for example, the production of fuel pellets filled in standard fuel rods and partial length fuel rods is performed separately. This is the case where there is a difference in the plutonium isotope composition ratio or the uranium isotope composition ratio of the fuel substance between the two.

Even in a fuel assembly which does not include a partial length fuel rod, a fuel assembly in which the ratio of the area occupied by the fuel substance and the water serving as the moderator is different in the axial upper and lower cross sections of the fuel assembly, for example, When a channel box having a different wall thickness in the axial direction as disclosed in JP-A-5-223968 and a MOX assembly not including a partial length fuel rod are combined, or the thickness of the water rod is in the axial direction. Similar problems also exist when different fuel assembly structures are used in.

In view of the above, the present invention makes it possible to satisfy a predetermined reactivity even when the composition ratio of uranium-plutonium in the fuel substance changes in the upper and lower portions of the fuel assembly in the axial direction. The purpose is to obtain a method by which plutonium enrichment can be set.

[0010]

The invention according to claim 1 is
In a fuel assembly in which a large number of fuel rods filled with a fuel substance containing uranium and plutonium are arranged in a square lattice pattern, and the ratio of the area occupied by the fuel substance and the moderator is different in each of the lower axial section and the upper axial section. The plutonium enrichment of a fuel rod filled with a fuel substance containing uranium and plutonium of the fuel rod, based on the uranium and plutonium isotope composition ratio of the fuel substance, a standard uranium and plutonium isotope composition preset In the fuel assembly manufacturing method, which is determined by the plutonium enrichment against the ratio and the equivalent fission coefficient, the equivalent fissile coefficient used for determining the plutonium enrichment is preset based on the uranium and plutonium isotope composition ratios of the fuel material. To select and use one of the two equivalent fissile coefficients And butterflies.

According to a second aspect of the present invention, there is provided a fuel assembly according to the first aspect, which includes a plurality of first fuel rods and a second fuel rod having an active fuel length shorter than that of the first fuel rods. In the fuel assembly in which the ratio of the area occupied by the fuel substance and the moderator is different in each of the lower axial section and the upper axial section, the axial section of the fuel assembly in which the fuel material exists at the position of the second fuel rod is Two types of equivalent fissile coefficients are used: a first equivalent fissile coefficient based on characteristics and a second equivalent fissile coefficient based on characteristics of an axial cross section of a fuel assembly in which no fuel substance exists at the position of the second fuel rod. However, the standard non-fissile plutonium ratio of plutonium in the standard uranium and plutonium isotope composition ratios is the standard value of the non-fissile plutonium ratio of plutonium in the fuel materials above. If it is larger, the first equivalent fissile coefficient is selected, and if the non-fissile plutonium ratio of the plutonium of the fuel material is smaller than the reference value, the second equivalent fissile coefficient is selected and the plutonium-rich coefficient is selected. It is characterized in that it is used to determine the degree of conversion.

The invention according to claim 3 is the invention according to claim 2, wherein the selection of the equivalent fissile coefficient used for determining the plutonium enrichment is performed using the first equivalent fissile coefficient, and In the case of using the second equivalent fissile coefficient of, the one having a higher plutonium enrichment as a result is used.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a longitudinal sectional view of a fuel assembly 11 according to the present invention, FIG. 2 is an enlarged transverse sectional view taken along the line AA of FIG. 1, and FIG.
2 is an enlarged cross-sectional view taken along the line BB of FIG. 1, in which standard fuel rods 12a, partial length fuel rods 12b and water rods 13 are arranged in a square lattice and are arranged at intervals in the axial direction. The plurality of spacers 14 properly maintain the gap between them. The upper and lower ends of the standard fuel rod 12a and the water rod 13 are held by an upper tie plate 15 and a lower tie plate 16, respectively. Also, the partial length fuel rod 1
2b is a fuel rod having a partial length of about 2/3 of the standard fuel rod 12a, and is held by the spacer 14 and the lower tie plate 16. The bundle of standard fuel rods 12a and partial length fuel rods 12b held by the spacer 14 is surrounded by a channel box 17, and the channel box 17 is attached to the upper tie plate 15.

The outer area of the channel box 13 is filled with non-boiling water, and the cross-shaped control rod 18 is inserted into this area. The fuel rods 12a and 12b are obtained by filling a large number of MOX fuel pellets or uranium fuel pellets with gadolinia in a cladding tube whose both ends are sealed by an upper end plug and a lower end plug, and are arranged in a gas plenum region in the cladding tube. The fuel pellets are pressed vertically by the generated spring. The MOX fuel pellet is composed of PuO ₂ which is a fuel substance and UO ₂ which is a fuel base material, and is ²³⁹ Pu, ²⁴¹ Pu and fissionable substances.
^It contains ²³⁵ U. The uranium fuel pellet containing gadolinia is composed of UO ₂ which is a fuel substance and gadolinia (Gd ₂ O ₃ ) which is a combustible poison contained therein, and contains ²³⁵ U which is a fission substance. The water rod 13 is not filled with a fuel substance, and cooling water that does not boil inside passes therethrough. Part length fuel rod 12b
In the horizontal position in FIG. 3, eight rods including corner portions are provided as the second layer from the outer layer in the fuel rod array. Fuel rods 12 in the inner area of the channel box 17
Cooling water is flowing between a, 12b and the water rod 13, and the partial length fuel rods 12b in the lower region of the assembly shown in FIG.
The position where there is is a flow path for the cooling water in the upper region of the assembly of FIG.

Fuel rods 12a constituting the fuel assembly 11,
As shown in FIGS. 2 and 3, 12b indicates fuel rod numbers P1 to P1.
There are five common types of P4 and G. These fuel rod numbers are arranged in the cross section of the fuel assembly in the channel box 17, as shown in FIGS. The square grid fuel rod array is 9 rows × 9 columns. Fuel rod P1
~ P4 is a MOX fuel rod containing no gadolinia,
Fuel rod number G is a uranium fuel rod containing gadolinia. Here, the MOX fuel content is uniform in the axial direction,
2 and 3, the fuel rods at the same positions have the same content, and the magnitude relationship is P1, P2, P3, from the one with the higher Pu content.
It is P4.

The essential part of the method for manufacturing the fuel assembly of this embodiment is a method for setting the plutonium enrichment of the plutonium fuel substance contained in the MOX fuel rod. This is explained below.

The basic idea used for setting the plutonium enrichment is to use the known equivalent fissile method. This method defines the effect of Pu-239 on the core reactivity as 1.0, and defines the relative reactivity effect of other plutonium isotopes and uranium isotopes as the equivalent fissile coefficient, and determines the plutonium and uranium isotopes. The compositional ratio of each element is expressed as the sum of the compositional ratio of each element multiplied by this equivalent fissile coefficient (equivalent fissile amount) when the compositional ratio of is a standard value. When is changed from above, the plutonium enrichment is adjusted so that the equivalent fissile amount defined above becomes constant.

Conventionally, one kind of aggregate average is used as the equivalent fissile coefficient, but in the present invention, two kinds of equivalent fissile coefficients evaluated in the aggregate lower region and the aggregate upper region are used. , By using this properly depending on the plutonium isotope composition ratio of the fuel material, it is possible to satisfy the target reactivity even if the plutonium isotope composition ratio in the assembly is not uniform and the distribution is different in the axial direction. It is something to do.

FIG. 4 is a diagram showing an example of plutonium enrichment adjustment with respect to changes in the composition ratio of plutonium when the equivalent fissile coefficients of the aggregate average, the aggregate lower region, and the aggregate upper region are used. The solid line shows the case of using the average equivalent fissile coefficient, the alternate long and short dash line shows the case of using the equivalent fissile coefficient of the lower part of the aggregate, and the dotted line shows the case of using the equivalent fissile coefficient of the upper part of the aggregate.

Compared to the upper region of the assembly, the lower region of the assembly has a large number of fuel rods and therefore a large amount of fuel substance (Vf) and, conversely, a small amount of water (Vm) as a moderator. The ratio Vm / Vf, which is the ratio, is small. This Vm / Vf is closely related to the neutron spectrum in the fuel assembly,
When / Vf becomes smaller, the amount of water having the effect of the neutron moderator becomes relatively small, so that the neutron spectrum is hardened (the ratio of neutrons having relatively high energy is large).
To do. The equivalent fissile coefficient represents the relative reactivity effect of other plutonium isotopes and uranium isotopes, with the effect of Pu-239 on the core reactivity being set to 1.0. Pu-240 and Pu-242, which are fissile materials, have the property of absorbing neutrons in the resonance region of relatively high energy,
The negative effect becomes larger because the neutron absorption effect increases in the state where the neutron spectrum is hard, since the reactivity decreases as the amount of the substance increases (the equivalent Physile coefficient is negative). Value increases). Thus, especially in the lower region of the assembly and the upper region of the assembly having different Vm / Vf, Pu-240 and Pu-
The reactivity value of non-fissile materials such as 242 is different, Vm
The non-fissile Pu in the lower region of the aggregate with a small / Vf
It is more sensitive to changes in the ratio of, and more plutonium enrichment adjustments are needed to keep the reactivity constant. In FIG. 4, when the equivalent fissile coefficient in the lower part of the assembly (dashed line) is used, the plutonium enrichment with respect to the increase in the non-fissile Pu ratio is higher than that in the upper part of the assembly (dotted line). This is the reason why the slope of the degree adjustment amount increase becomes large.

In the present invention, paying attention to the above characteristics, the equivalent fissile coefficient to be applied as follows is properly used according to the proportion of non-fissile Pu in the fuel material.

That is, (1) When the proportion of non-fissile Pu is smaller than the standard composition ratio, the equivalent Fissay coefficient (dotted line) of the upper region of the assembly is used. (2) When the ratio of non-fissionable Pu is larger than the standard composition ratio, the equivalent Physics coefficient (dashed line) of the lower region of the assembly is used.

According to FIG. 4, this application method can be applied to any Pu
Even in the case of composition, this means choosing the equivalent fissile coefficient with the higher resulting plutonium enrichment.

On the other hand, the following patterns can be considered as distributions in which the plutonium isotope composition ratios in the aggregate are different. (A) The non-fissionable Pu ratio is large in the lower region of the aggregate and is high in the upper region of the aggregate. When the ratio of non-fissionable Pu is small (b), the ratio of non-fissionable Pu in the lower region of the assembly is small, and the ratio of non-fissionable Pu in the upper region of the assembly is small (a), the example of the present invention is applied. Therefore, since the equivalent fissile coefficient corresponding to each region is used, the plutonium enrichment adjustment amount is necessary and sufficient to satisfy the target reactivity. In the case of the pattern of (b), by applying the example of the present invention, the equivalent fissile coefficient that makes the plutonium enrichment higher is selected in any of the regions than in the case of using the equivalent fissile coefficient of each region. As a result, the average reactivity of the aggregate is also above the target.

In the end, with either pattern, it is possible to set the plutonium enrichment so that the reactivity becomes equal to or higher than the target value.

On the other hand, when the equivalent average Phishie coefficient of the aggregate is used, in the case of the pattern (a), the plutonium enrichment is higher in any region than in the case of using the equivalent Physile factor of each region. Since the equivalent fissile coefficient is selected to be low, the plutonium enrichment is set so that the reactivity does not satisfy the target value.

The second embodiment of the present invention will be described below. This is to change the criterion for the proper use of the equivalent fissile coefficient from the ratio of the non-fissile Pu of the fuel material in the first embodiment to the resulting pluto-plutonium enrichment, and the applicable method is as follows. .

As equivalent fissile coefficients, one for the lower region of the assembly and one for the upper region of the assembly are prepared, and the plutonium enrichment is calculated based on the composition ratio of the fuel substance using both of them, and the higher plutonium enrichment is obtained. You may make it select the equivalent fissile coefficient which estimates.

According to this standard, the plutonium enrichment degree can be set such that the reactivity becomes equal to or higher than the target value even when the plutonium isotope composition ratio in the aggregate is distributed in the axial direction by the same operation as in the above embodiment. Is. According to this standard, although the ratio of plutonium to non-fissile plutonium is slightly higher than the standard composition ratio, the reactivity value of plutonium as a whole is higher than the standard composition ratio due to the difference in the isotopic composition ratio of fissile plutonium. Even if there is a slight change in the isotope composition ratio such that the enrichment ratio is high and thus the plutonium enrichment becomes smaller, the selection criterion of the equivalent fissile coefficient becomes clearer than that in the above embodiment.

As described above, the embodiment of the present invention has eight partial length fuel rods whose length is about 2/3 of the standard fuel rod.
Although the fuel assembly of rows × 9 columns has been described as an example, the operation of the present invention is effective regardless of the length, the number, and the arrangement of the partial length fuel rods. Many 10 lines ×
It is also applicable to fuel assemblies such as 10 rows.

Even in a fuel assembly that does not include a partial length fuel rod, a fuel assembly in which the ratio of the area occupied by the fuel substance and the water that is the moderator is different in the axial upper and lower cross sections of the fuel assembly, for example, For a combination of a channel box having a different wall thickness in the axial direction and a MOX assembly that does not include a partial length fuel rod, or a MOX fuel assembly using a fuel assembly structure in which the thickness of the water rod is different in the axial direction. Also, the present invention can be applied.

[0033]

As described above, according to the present invention,
The plutonium enrichment of MOX fuel assemblies is different from that of conventional MOX fuel assemblies in which the ratio of the area occupied by the fuel substance and the water serving as the moderator is different in the axial upper and lower cross sections of the fuel assembly due to the fact that it has partial length fuel rods. It can be set by the technique of the equivalent fissile method, and even when the plutonium isotope composition ratio has a distribution in the axial direction, it is easy to adjust the plutonium enrichment necessary to satisfy the target reactivity or higher. Can be realized.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a fuel assembly according to the present invention.

FIG. 2 is a cross-sectional view taken along the line AA of FIG.

FIG. 3 is a cross-sectional view taken along the line BB of FIG.

FIG. 4 is a diagram showing a relationship between a Pu composition and an aggregate average plutonium enrichment adjustment amount in the first example of the present invention.

[Explanation of symbols]

11 Fuel assembly 12a Standard fuel rod 12b Part length fuel rod 13 Water rod 14 Spacer 15 Upper tie plate 16 Lower tie plate 17 channel box 18 Cross control rod

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Sadayuki Izutsu             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan (72) Inventor Yasushi Hirano             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan (72) Inventor Shingo Fujimaki             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan (72) Inventor Manabu Yoshida             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan

Claims (3)

[Claims] 1. A large number of fuel rods filled with a fuel substance containing uranium and plutonium are arranged in a square lattice, and the ratio of the area occupied by the fuel substance and the moderator in each of the axially lower and axially upper cross-sections. The plutonium enrichment of fuel rods filled with the fuel material containing uranium and plutonium in different fuel assemblies was determined based on the standard uranium and plutonium isotope composition ratios of the fuel material. In the method of manufacturing a fuel assembly, which is determined by the plutonium enrichment ratio and the equivalent fission coefficient with respect to the body composition ratio, the equivalent fissile coefficient used for the plutonium enrichment determination is based on the uranium and plutonium isotope composition ratios of the fuel material in advance. Select and use one of the two equivalent fissile coefficients you have set. Method for producing a fuel assembly according to claim. 2. A fuel material is provided in each of the axially lower and axially upper cross sections of the fuel assembly by including a plurality of first fuel rods and a second fuel rod having an active fuel length shorter than that of the first fuel rods. In the fuel assemblies in which the ratio of the area occupied by the moderator is different, the first equivalent fissile coefficient based on the characteristics of the axial cross section of the fuel assembly in which the fuel substance exists at the position of the second fuel rod, and the second equivalent Plutonium at standard uranium and plutonium isotope composition ratios using two equivalent fissile coefficients, the second equivalent fissile coefficient, which is based on the characteristics of the axial cross section of the fuel assembly in which the fuel substance is not present at the fuel rod position. If the non-fissile plutonium ratio of the fuel material is larger than the standard value with reference to the non-fissile plutonium ratio of And selecting the second equivalent fissile coefficient for use in determining the plutonium enrichment, if the non-fissile plutonium ratio of plutonium in the fuel material is less than a reference value. 1. The method for manufacturing the fuel assembly according to 1. 3. The selection of the equivalent fissile coefficient used for determining the plutonium enrichment depends on the case of using the first equivalent fissile coefficient and the case of using the second equivalent fissile coefficient. 3. The method for producing a fuel assembly according to claim 2, wherein the one having a higher plutonium enrichment is used.