JP3325462B2 - Manufacturing method of fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a fuel assembly containing nuclear fuel materials such as uranium and plutonium. More specifically, the present invention relates to a method for producing a fuel assembly in which an effective multiplication factor at the end of a reactor operation cycle is substantially constant. The present invention relates to a method for producing a new fuel assembly by adjusting the content of nuclear fuel material.

[0002]

2. Description of the Related Art In a fast reactor, uranium-plutonium mixed oxide fuel (MOX fuel) is used. This plutonium was recovered by reprocessing spent fuel of the nuclear reactor. Such plutonium is supplied to the reactor type (light water reactor, gas reactor,
It is characterized in that its isotope composition ratio fluctuates depending on the burn-up of fuel, the burn-up of the fuel, and the cooling period (the period from when it is taken out of the furnace to when it is newly loaded into the fast reactor). Moreover, since the plutonium isotopes have different nuclear properties (fission properties, neutron absorption properties, etc.), the nuclear properties of the reactor necessarily correspond to the plutonium composition.

In order to operate a nuclear reactor for a predetermined combustion period, it is necessary to ensure a reactivity for making the core critical at the end of an operation cycle. However, each nuclear fuel material in the new fuel assembly undergoes various composition changes due to neutron irradiation in the reactor, etc., so if nuclear fuel materials with different compositions are used, even if the nuclear value of the new fuel assembly is the same Even at the end of the driving cycle, however, the core values are generally not equal. Therefore, in order to obtain a predetermined reactivity at the end of combustion, every time the plutonium isotope composition ratio is different,
Plutonium enrichment needs to be determined.

[0004] Considering the differences in the nuclear value of plutonium with different compositions, a technique for adjusting the plutonium enrichment so that fuels of various plutonium isotope compositions have equal reactivity values in the furnace has been developed using the technique of " Equivalent Fissail Method "
A method called is known. This is Pu-239
The relative reactivity effect of other plutonium isotopes (including Am-241) and uranium isotope is defined with the effect on the core reactivity of 1.0 as 1.0 (this is referred to as "equivalent fissile coefficient"), In this method, the overall effect on the degree is expressed as the sum of the composition ratio of each element multiplied by the equivalent fissile coefficient. Therefore, the concept of equivalent fissile enrichment is introduced so that the plutonium enrichment of fuel can be described in a unified manner even if the plutonium isotope composition ratio is different.

First, an equivalent fissile coefficient [F] is determined for each nuclide in the fuel. The equivalent fissile coefficient describes the ratio of the reactivity value of Pu-239 to the reactivity value of uranium and plutonium isotopes in the form of a matrix. Neutron energy 1 group, 1 point reactor approximation,
Each element F _i is defined by the following equation. F _i = (νσ _f −σ _a ) _i / (νσ _f −σ _a ) ₂₃₉ where F _i : equivalent fissile coefficient of nuclide i ν: number of neutrons generated per fission σ _f : microfission cross section σ _a : The micro absorption cross section. Since plutonium isotope equivalent Fissile coefficients [F] in fast reactors have values other than 0, all plutonium isotopes have significant reactivity value. Therefore, when producing a fuel assembly for a fast reactor, plutonium isotope composition is an important parameter for setting the enrichment.

Next, using the equivalent fissile coefficient defined in this way, the equivalent fissile enrichment (E ₂₃₉ )
Is defined by the following equation. E ₂₃₉ = εΣα _i F _i + (1−ε) Σβ _j F _j where ε: plutonium enrichment α _i : composition ratio of plutonium isotope i β _j : composition ratio of uranium isotope j F _i : plutonium Equivalent fissail coefficient of isotope i F _j : Equivalent fissail coefficient of uranium isotope j Using this equation, the standard equivalent fissile enrichment is determined from the plutonium enrichment and the equivalent fusile coefficient at the reference composition. . For the plutonium actually used, the plutonium enrichment required for the composition is estimated (back-calculated) from the equivalent fissile coefficient and the equivalent fissile enrichment determined above.

[0007] By handling in this manner, even when fuels with different plutonium enrichment are loaded,
The reactivity of the reactor core can be kept constant.

[0008]

In the core design, the plutonium enrichment is set so that a predetermined number of operating days can be obtained assuming a certain plutonium composition. However, the method of decreasing the effective multiplication factor accompanying combustion differs depending on the plutonium composition, so even if the plutonium composition is different, if the initial plutonium enrichment is fixed, it is not always possible to secure a predetermined number of operating days Absent. In actual reactor operation, the value of excess reactivity at the beginning of the operation cycle is also important from the viewpoint of reactor shutdown margin, but from the viewpoint of securing the number of operation days, maintaining the core reactivity at the end of the operation cycle is important. It is. As a countermeasure, there is a method of redesigning each time using the available plutonium composition, a method of giving plenty of plutonium enrichment at the time of design, a method of increasing or decreasing the number of replacements of new fuel, adjusting the fuel loading position A method can be considered. However, these countermeasures have various problems such as being not economical or requiring complicated trial and error work based on experience.

An object of the present invention is to provide a new fuel assembly in consideration of a difference in nuclear value of nuclear fuel materials such as uranium and plutonium used as a new fuel assembly and a change in nuclear value due to a change in composition during combustion. It is an object of the present invention to provide a method for manufacturing a fuel assembly in which the content of these nuclear fuel materials in a fuel assembly can be appropriately set, thereby suppressing a change in nuclear value at the end of an operation cycle.

[0010]

SUMMARY OF THE INVENTION The present invention provides a method for filling a part or all of a plurality of fuel rods with a nuclear fuel material containing uranium and plutonium according to the isotope composition ratio of the nuclear fuel material to be filled. This is a method of manufacturing a fuel assembly using a plurality of fuel rods obtained by changing the content of a substance. In the present invention, the equivalent fissile coefficient [F] of the reactivity preservation method, which is a coefficient representing the reactivity value equivalent to plutonium-239, and the solution of a combustion equation, which is a differential equation representing a time change during combustion of each nuclear fuel material, are obtained. Based on the reactivity
By multiplying by a combustion matrix [M] composed by extracting only nuclides having a large influence on the nuclide , the equivalent fissile coefficient of the combustion assurance method defined by the following equation [B] = [F] · [M] (3) [B] is obtained, and the content rate of each nuclear fuel substance is determined using the equivalent fissile coefficient [B] of the combustion assurance method so that the effective multiplication factor at the end of the operation cycle becomes substantially constant.
The "equivalent fissile coefficient" of the "combustion assurance method" clearly indicates that the effective multiplication factor _keff at the end of the combustion cycle is guaranteed to be a constant value of 1 or more (for example, approximately 1.01). Used for On the other hand, the equivalent fissile coefficient of the "reactivity preservation method" is the equivalent fissile coefficient that the reactivity is preserved when a new fuel is used, which has been used in the past, and is equivalent to the current "combustion assurance method". This term is used to distinguish it from the Fissail coefficient. In the above,
[B], [F], and [M] indicate that they are matrices.

If the composition of the fuel after combustion is obtained, it is possible to determine the reactivity value of the fuel at that time by using the equivalent fissile coefficient [F]. Therefore, no matter what composition of plutonium is used, if the reactivity value at this time can be kept constant, the reactivity at the end of the operation cycle can be kept constant.

By the way, the change due to the burning of nuclear fuel material is as follows.
It can be expressed by the following differential equation. _{dN i / dt = -λ i N} i - (σ a) i φN i + λ i-1 N i-1 + (σ c) i + 1 φN i + 1 ... (4) where, N _i: nuclide i Atomic number density λ: decay constant φ: neutron flux in the reactor σ _c : micro-capture cross section Therefore, it is considered to identify the isotope composition of the fuel after combustion using the combustion matrix [M] based on the solution of the above equation (4) in the fuel of each nuclear fuel substance. By using the isotope composition and the equivalent Fissail coefficient obtained here, the reactivity value of the fuel after combustion can be obtained. Specifically, the equivalent fissile coefficient [F] of the reactivity preservation method is
A new coefficient [B] multiplied by the combustion matrix [M ] is introduced. This coefficient is called an equivalent fissile coefficient of the combustion assurance method. [B ₁ (t)... B _n (t)] = [F (0)] · [M (t)] where n is the number of nuclides, [B (t)] is a matrix of 1 row and n columns, [F (0)] is a matrix of 1 row and n columns, and [M (t)] is n
It is a matrix with n rows and n columns . Note that B (t) and M (t) indicate that B and M are functions of the combustion time, and
(0) indicates an initial value (a value when loaded as a new fuel). The coefficient [B] obtained here
Is used as F in the expression (2), it is necessary to divide each element in [B] by the value of Pu-239 in the same [B].

The value of each element B _i of the above matrix takes into account the effects of changes to other nuclides during burning of that nuclide. Therefore, by using the equivalent fissile coefficient of this combustion assurance method, it is possible to identify the composition of the fuel loaded as new fuel into the nuclear reactor after combustion, and to obtain the reactivity value at that time. By this method, the plutonium enrichment of the fuel for which a predetermined number of operating days can be obtained can be obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A fuel composition [W _st ] after burning for t hours of a fuel assembly, which is a reference whose plutonium enrichment has been adjusted so that a predetermined number of operation days can be secured, is assumed.
This [W _st ] can be obtained by a detailed nuclear design. The equivalent fissile coefficient [F] of the reactivity preservation method
Thus, the equivalent fissile enrichment [E _t ] of this fuel composition is calculated by the following equation. [E _t ] = [F] · [W _st ] For an arbitrary composition [W _t ], if the following expression is satisfied at time t, a nuclear value equal to [W _st ] will be obtained. [E _t ] = [F] · [W _t ] On the other hand, the following relationship exists between this composition and the composition [W ₀ ] at the beginning of combustion. [W _t ] = [M _t ] · [W ₀ ] where [M _t ] is a combustion matrix. Therefore, if the following formula is satisfied for the composition of the new fuel, the same nuclear value will be obtained at the time t. [E _t ] = [F] · [M _t ] · [W ₀ ] = [B _t ] · [W ₀ ] The equivalent fissile coefficient [B _t ] of the combustion assurance system introduced in the present invention is a function of time. By setting this time t appropriately, the intended purpose can be achieved. That is,
When the time t is the end of combustion, the fluctuation of the effective multiplication factor at the end of the operation cycle can be extremely reduced.

The combustion matrix [M] is prepared as follows. The nuclides in the fuel change into a huge number of nuclides during operation of the reactor due to neutron irradiation and the like. [M] needs to include information on the generation of these nuclides as an element. However, it is difficult to handle them all. In addition, considering the contribution to the reactivity based on the amount of the nuclide after the change and the size of the cross-sectional area, it is not always necessary to handle all the nuclides. A matrix is created by extracting only nuclides that have a large effect on reactivity. Therefore, we simulated the irradiation conditions of neutrons in the reactor and investigated how each nuclide in the fuel changes due to nuclear reactions with neutrons, etc., using nuclear calculation codes. From the results, it is understood that about 15 nuclides should be considered as the combustion matrix [M].

A comparison of the calculation results of the reactivity value between the case where the equivalent fissile coefficient of the reactivity preservation method is used for the fast reactor (prior art) and the case of using the equivalent fissile coefficient of the combustion guarantee method is shown in U-238. , Pu-23
9, it was found that the effects of Pu-240 and Pu-241 were significantly different. Pu-23 for U-238
9, the generation of Pu-239 is Pu-240, the generation of Pu-240 is Pu-241,
The generation of Pu-242 mainly contributes to Pu-241.

[0017]

[Embodiment] The present invention method (equivalent fissile method of combustion assurance method) is applied to a case where Pu having various isotope composition ratios is used as a fuel for the first loading core of a large fast reactor equivalent to 600,000 kWe. The results obtained will be described. An example of the combustion matrix [M] is shown below.

[0018]

(Equation 1)

Here, 0: ²³⁵ U, 1: ²³⁶ U, 2: ²³⁸ U, 3: ²³⁸ Pu, 4: ²³⁹ Pu, 5: ²⁴⁰ Pu, 6: ²⁴¹ Pu, 7: ²⁴² Pu, 8: ²⁴¹ Am , 9: ²⁴² mAm, a: ²⁴³ Am, b: ²⁴² Cm, c: ²⁴³ Cm, d: ²⁴⁴ Cm, e: ²⁴⁵ Cm, f: FP- ²³⁵ U, g: FP- ²³⁸ U, h: FP- ²³⁹ Pu, i: FP- ²⁴¹ Pu.

Each element of the combustion matrix [M] is specifically described as follows. ^{_{m 00 = a (235 U)}} m 10 = b (235 U, 236 U) m f0 = c (235 U) m 11 = a (236 U) m g1 = c (236 U) m 22 = a (238 U ) M ₄₂ = b ( ²³⁸ U, ²³⁹ Pu) m ₅₂ = d ( ²³⁸ U, ²³⁹ Pu, ²⁴⁰ Pu) m ₆₂ = e ( ²³⁸ U, ²³⁹ Pu, ²⁴⁰ Pu, ²⁴¹ Pu) _{mg 2} = c ( ²³⁸ U) ) M ₃₃ = a ( ²³⁸ Pu) m ₄₃ = b ( ²³⁸ Pu, ²³⁹ Pu) m _h3 = c ( ²³⁸ Pu) m ₄₄ = a ( ²³⁹ Pu) m ₅₄ = b ( ²³⁹ Pu, ²⁴⁰ Pu) m ₆₄ = f ( ²³⁹ Pu, ²⁴⁰ Pu, ²⁴¹ Pu) m _h4 = c ( ²³⁹ Pu) m ₅₅ = a ( ²⁴⁰ Pu) m ₆₅ = g ( ²⁴⁰ Pu, ²⁴¹ Pu) m ₈₅ = h ( ²⁴⁰ Pu, ²⁴¹ Pu, ²⁴¹⁾ Am) m _i5 = c ( ²⁴⁰ Pu) m ₃₆ = i ( ²⁴¹ Pu, ²⁴¹ Am, ²⁴² Cm, ²³⁸ Pu) m ₆₆ = j ( ²⁴¹ Pu) m ₇₆ = k ( ²⁴¹ Pu, ²⁴² Pu, ²⁴¹ Am) m ₈₆ = l ^{(241 _{u, 241 Am) m 96 =}} m (241 Pu, 241 Am, 242m Am) m a6 = n (241 Pu, 241 Am, 242m Am, 243 Am) m b6 = o (241 Pu, 241 Am, 242 Cm) m _i6 = p ( ²⁴¹ Pu) m ₇₇ = a ( ²⁴² Pu) m _a7 = b ( ²⁴² Pu, ²⁴³ Am) _md7 = d ( ²⁴² Pu, ²⁴³ Am, ²⁴⁴ Cm) _mc7 = q ( ²⁴² Pu, ²⁴³ Am, ²⁴⁴ Cm, ²⁴⁵ Cm) _{mi 7} = c ( ²⁴² Pu) m ₃₈ = r ( ²⁴¹ Am, ²⁴² Cm, ²³⁸ Pu) m ₄₈ = s ( ²⁴¹ Am, ²⁴² Cm, ²³⁸ Pu, ²³⁹ Pu) m ₈₈ = a ( ²⁴¹ Am) m ₉₈ = t ( ²⁴¹ Am, ²⁴² mAm) _mb8 = u ( ²⁴¹ Am, ²⁴² Cm) _mc8 = v ( ²⁴¹ Am, ²⁴² Cm, ²⁴³ Cm) _mi8 = c ( ²⁴¹ Am) m ^{_{99 = a (242m Am) m}} i9 = c (242m Am) m aa = a (243 Am) m ia = c (243 Am) m bb = j (242 Cm) m ib = p _{^{242 Cm) m cc = a (}} 243 Cm) m ic = c (243 Cm) m dd = a (244 Cm) m id = c (244 Cm) m ee = a (245 Cm) m ie = c (245 Cm )

Here, the above equations are as follows.

(Equation 2)

(Equation 3)

(Equation 4)

(Equation 5)

(Equation 6)

(Equation 7)

(Equation 8)

(Equation 9)

(Equation 10)

[Equation 11]

(Equation 12)

(Equation 13)

[Equation 14]

(Equation 15)

(Equation 16)

[Equation 17]

(Equation 18)

[Equation 19]

(Equation 20)

(Equation 21)

(Equation 22)

(Equation 23)

Σ _a : micro absorption cross section σ _c : micro capture cross section σ _f : micro fission cross section λ: decay constant φ: neutron flux t: combustion period

(Equation 24)

(Equation 25) It is.

[0023] shows a calculation example of an equivalent Fissairu coefficients F _i reactivity saving scheme when 1 to 600,000 kWe equivalent large fast reactor table.

[Table 1]

[0024] shows a calculation example of an equivalent Fissairu coefficients B _i of combustion guarantee scheme for large fast reactor 600,000 kWe equivalent to Table 2.

[Table 2]

Table 3 shows the plutonium used in this study.
Shown in Plutonium shall be taken out from the light-water reactor UO ₂ and MOX spent fuel, they burn about 3-45000 MWd / t, plutonium taken from outside the furnace cycle time (LWR used fast reactor Used in the range of 0 to 15 years.

[0026]

[Table 3]

For Cases 1 to 10 shown in Table 3, the plutonium enrichment was calculated by using the equivalent fissile method of the combustion assurance method, and the combustion calculation was performed using the plutonium enrichment by a nuclear calculation code. The change of the effective multiplication factor was examined. FIG. 1 shows a change in the effective multiplication factor accompanying the combustion.
As shown in the figure, the effective multiplication factors at the end of the combustion cycle coincided within a range of about 0.03% Δk. However, in the first loading core using the equivalent fissile coefficient of the conventional reactivity preservation method, the variation range of the effective multiplication factor at the end of the operation cycle is about 1.
It was in the range of 37% Δk.

[0028]

According to the method of the present invention, the difference in nuclear value of nuclear fuel materials such as uranium and plutonium used as a new fuel assembly and the change in nuclear value due to a change in composition during combustion are considered. Since the content rates of these nuclear fuel substances in the new fuel assembly can be appropriately set, a fuel assembly in which fluctuations in nuclear value are suppressed can be manufactured. Thus, the core loaded with the fuel assembly can have an almost constant effective doubling rate at the end of the operation cycle.

According to the method of the present invention, even when the number of days of cycle operation of the fast reactor increases, the predetermined number of days of operation can be managed more accurately. Further, in the future, plutonium supplied as fuel for fast reactors will be diversified, such as increasing the burnup of light water reactors and using MOX fuel by light water reactors. Can respond to.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in an effective multiplication factor with respect to the number of combustion days using an equivalent fissile coefficient of a combustion assurance method.

Continuation of the front page (56) References Nobuo Nakae and three others "Plutonium enrichment management using the equivalent fissile method", JASDF, No. 70, published in June 1989, p. 77-81 Toshikazu Takeda, "Reactor Physics" Second Edition, January 1995, p. 360-374 (58) Field surveyed (Int. Cl. ⁷ , DB name) G21C 3/30, 17/00, 17/06

Claims (1)

(57) [Claims] When filling a part or all of a plurality of fuel rods with a nuclear fuel material containing uranium and plutonium, the content of the nuclear fuel material is changed according to the isotope composition ratio of the nuclear fuel material to be filled. A method for manufacturing a fuel assembly by using a plurality of fuel rods obtained by the method, comprising: an equivalent fissile coefficient [F] of a reactivity preservation method, which is a coefficient representing a reactivity value equivalent to plutonium-239;
A nucleus that has a large effect on reactivity based on the solution of the combustion equation, which is a differential equation representing the time change during combustion of each nuclear fuel material
By multiplying by a combustion matrix [M] constituted by taking out only the seeds, an equivalent fissile coefficient [B] of a combustion assurance method defined by the following equation [B] = [F] · [M] is obtained. A method for producing a fuel assembly, characterized in that the content ratio of each nuclear fuel substance is determined so that the effective multiplication factor at the end of an operation cycle is substantially constant using the equivalent fissile coefficient [B] of the guarantee system.