JPH04265896A - Fuel assembly - Google Patents

Abstract

PURPOSE:To enable an amount of plutonium charging amount per MOX fuel assembly to be increased without losing a thermal soundness. CONSTITUTION:All fuel rods MOX fuel assembly which mixes and charges the MOX fuel assembly which is constituted by mixing uranium and plutonium as a fissionable substance and a uranium fuel assembly which is constituted by charging uranium in a light water reactor are used as an MOX fuel rod 11 except a fuel rod with flammable toxic substance 12. An infinite magnification rate of the MOX fuel assembly is reduced as compared with an infinite magnification rate of the uramium fuel assembly in a large portion of a burnup period excluding an initial period of combustion, thus obtaining a reactor core with a high operation allowance. Also, by using those than a fuel rod with gadolinia as the MOX fuel rod 11, an amount of charged plutonium per MOX fuel assembly is incresed as compared with that of an island type.

Description

[Detailed description of the invention]

[Object of the invention]

0002

[Industrial Application Field] The present invention relates to a fuel assembly loaded in the core of a light water reactor, and particularly relates to a fuel assembly loaded in the core of a light water reactor.
X) It relates to a fuel assembly that can increase the amount of plutonium loaded per fuel assembly as much as possible.

0003

[Prior Art] In general, in a light water reactor such as a boiling water reactor, a fuel assembly is assembled by bundling fuel rods, which are made by burning uranium oxide into fuel pellets and packing them into fuel cladding tubes, into a lattice shape using spacers. Loading the body.

FIG. 12 shows a conventional uranium fuel assembly of this type. In the figure, reference numeral a is a channel box of the uranium fuel assembly, and inside this channel box a are uranium fuel rods b and The fuel rod c containing gadolinia is
They are arranged in 8 rows and 8 columns, and a water rod d is arranged in the center.

The nuclear fuel of the uranium fuel rod b is enriched uranium obtained by enriching uranium-235 from natural uranium, and several types of nuclear fuel with different concentrations (weight ratios of U-235) are used. In FIG. 12, there are symbols 1 and 1 in the fuel rod.
Four types of uranium fuel rods b labeled 2, 3, and 4 are used.

[0006] In addition, gadolinia-containing fuel rod c is formed by mixing gadolinium oxide (gadolinia: Gd2O3) with uranium oxide, and has a large neutron absorption cross section, so it is possible to control the initial reactivity. It is used to In FIG. 12, the symbol G in the fuel rod
Ten gadolinia-containing fuel rods C are used.

Table 1 shows the uranium fuel assembly (
Each fuel rod b in (also simply referred to as uranium fuel)
U-235 enrichment (weight percent: w/o) of c, G
d2 O3 concentration (weight percent: w/o) and number of bottles are shown.

[0008]

[Table 1]

By the way, uranium fuel is loaded into the core of a nuclear reactor and becomes used after operating for a predetermined period of time, and this spent fuel is cooled for several years and then reprocessed. Uranium and plutonium recovered during reprocessing can be recycled as nuclear fuel for light water reactors, thereby saving uranium resources. Recycling the plutonium in light water reactors is called pluthermal, and it has been practiced early in Europe and the United States. In pluthermal, plutonium is mixed with uranium oxide as a base material in the form of oxide, forming mixed oxide (MOX).
As such, it is baked into pellets and packed into a fuel cladding tube to form MOX fuel rods, and these fuel rods are bundled to form a MOX fuel assembly (hereinafter abbreviated as MOX fuel). This fuel assembly is loaded into the light water reactor again and used as nuclear fuel.

Plutonium used as a fuel has the isotopes shown in Table 2.

[0011]

[Table 2]

[0012] In Table 2, σγ is 2200m
The (n, γ) reaction cross section for neutrons /s, Iγ, is
The effective resonance integral of the (n, γ) reaction, σf, is 2200 m/
The (n, f) reaction cross section of s for neutrons, If, is (
n, f) is the effective resonance integral of the reaction. Isotopic elements marked with * in Table 2 are fissile elements. In Table 2, although Am-241 is not a plutonium isotope element, it is included in the group of plutonium isotope elements because it is produced by beta decay of Pu-241 with a half-life of 147 years. Unlike the case of uranium fuel, the isotope ratio of plutonium fuel cannot be fixed because it is recovered through reprocessing, but plutonium obtained through typical uranium reprocessing As an example, the isotope element ratio (%) of Pu-238:Pu-239:Pu-240:P
u-241:Pu-242:Am-241=1.5:5
9.5:26.4:7.6:3.8:1.2. Therefore, the fissile material of plutonium Puf (Pu
-239 and Pu-241) is 67.1%.

When designing MOX fuels, it is customary to adopt the same hard assembly (channel box) geometry as the previously approved uranium fuels. However, the hard shape of the fuel assembly is designed to optimize the water-to-fuel volume ratio of water, which is the moderator, and uranium, which is the fuel, so that the uranium fuel has the highest reactivity. In general, the neutron absorption cross section of plutonium isotope elements (
The sum of the neutron capture cross section σγ and the fission cross section σf) is
Table 2 shows that it is several times larger than that of uranium isotopes.
As shown. Therefore, as long as the same hard aggregate shape as that of uranium fuel is adopted, the energy spectrum of neutrons in MOX fuel will be harder than that of uranium fuel (slightly biased toward the high speed side). This poses three difficulties in MOX fuel design.

The first difficulty is that in order to design a MOX fuel assembly with the same reactivity as uranium fuel, the amount of total fissile material (U-235 and Puf) must be greater than that of uranium fuel. There is a need.

The second difficulty is that most of the reactivity control ability (reactivity value) of gadolinium, which is used for reactivity control, is significant in the thermal neutron region. , the reactivity value decreases. Therefore, M with the same reactivity as in Figure 12
When producing OX fuel, it is necessary to increase the number of fuel rods containing gadolinia compared to uranium fuel. Since the output of gadolinia-containing fuel rods is extremely low in the initial stage of combustion, the number of fuel rods that bear the burden of output is substantially reduced. Therefore, the output per other fuel rod increases, and the margin for operation to maintain the thermal integrity of the fuel rod decreases. The local power peaking coefficient is used as an index indicating the power burden ratio of the fuel rods.

If all fuel rods bear the same output power, the local power peaking coefficient of any fuel rod is 1.
It is 0. FIG. 13 shows the local power distribution at the initial stage of combustion of the uranium fuel assembly shown in FIG. 12. The underlined output coefficients are for fuel rods containing gadolinia and are all 0.4.
It is a strong value. The maximum local output (peaking coefficient) is *
It appears at the marked position and its value is 1,238. With MOX fuel, the local power peaking coefficient will generally increase more than with uranium fuel as the number of fuel rods bearing the power burden is reduced.

The third difficulty is that, as shown in FIG. 14, there is less moderator in the central part of the fuel assembly than in the peripheral part.
The neutron spectrum becomes harder. As a result, if all the fuel rods have the same uranium enrichment, the fuel rods in the periphery will have a larger output peak than the central part, which affects the thermal integrity of the fuel. undesirable.

In order to improve the thermal integrity of the fuel, that is, to make the neutron spectrum as uniform as possible in the cross section of the fuel assembly, a water rod 4 is installed in the center of the fuel assembly as shown in FIG. The uranium enrichment level is lower in the periphery than in the center. FIG. 13 shows that the thermal neutron flux is flattened by arranging the water rod 4 in the central portion.

However, in the hard shape of the fuel assembly that is optimally designed for uranium fuel, if MOX fuel is loaded instead of uranium fuel, the difference in the neutron spectra between the peripheral part and the central part will be further reduced. The power peaking is so significant that even if the distribution of the amount of fissile material in each MOX fuel rod is optimized, the power peaking will be more severe than with uranium fuel. To avoid this, it is possible to adopt an "island type" design in which only the peripheral fuel rods are uranium rods, or to increase the number of MOX fuel rods with different ratios of plutonium to fissile material Puf. It has been taken.

Note that, as a requirement for fuel molding, gadolinium and plutonium must not be mixed. This is not only because there is no past track record, but also to avoid an increase in the cost of the fuel molding production line.

Furthermore, as described above, the proportion of fissile plutonium Puf used in the MOX fuel varies depending on the burnup of the original uranium fuel, the cooling period, etc. That is, the adjustment of Puf is more costly than the adjustment of uranium enrichment. Therefore, in MOX fuel design, it is desirable that the number of types of MOX fuel rods with different Puf ratios be as small as possible. This is a requirement that is inconsistent with the third difficulty.

Incidentally, in order to improve the fuel economy of fuel assemblies, the burn-up of fuel assemblies is gradually becoming higher. In order to achieve high burnup, it is necessary to increase the content of fissile material. This means that if the conventional fuel hard shape is designed to increase the burnup, the neutron spectrum will become hard and the output peaking will become high. Therefore, although it was mentioned earlier that pluthermal has been implemented in Europe and the United States from an early stage, it can be said that designing MOX fuel is more difficult now than it was then. Furthermore, considering the safety of transporting and handling plutonium, it is more economical to load plutonium in one fuel assembly as much as possible. In this sense, an "all-MOX type" in which all fuel rods are loaded with plutonium is more desirable than an "island type".

[0023]

[Problem to be solved by the invention] In the conventional technology, MO
We have been designing MOX fuel under the condition that the reactivity of X fuel is the same as that of uranium fuel. For this reason, if the fuel hardware was not changed, it was necessary to use an "island type" or increase the number of types of fuel rods. Additionally, when changing the fuel hardware, water rods were added to increase the water-to-fuel volume ratio. For this reason, the number of MOX fuel rods per fuel assembly decreased, causing a contradiction in that the amount of plutonium loaded decreased and at the same time, the number of fuel rods bearing the power output decreased.

The present invention has been made in consideration of the above-mentioned circumstances, and it is possible to increase the amount of plutonium loaded per fuel assembly without reducing the number of MOX fuel rods per fuel assembly. The purpose is to provide a fuel assembly that can have operational flexibility. [Structure of the invention]

[0025]

[Means for Solving the Problems] The present invention provides a mixed oxide fuel assembly made of a mixture of uranium and plutonium as fissile materials, and a uranium fuel loaded with uranium. In a light water reactor loaded with a mixed oxide fuel assembly, most of the fuel rods of the mixed oxide fuel assembly are composed of mixed oxide fuel rods, excluding fuel rods containing burnable poison, and the mixed oxide fuel The infinite multiplication factor of the uranium fuel assembly is made lower than the infinite multiplication factor of the uranium fuel assembly during most of the combustion period excluding the initial combustion period.

[0026]

[Operation] In the fuel assembly according to the present invention, most of the fuel rods of the MOX fuel assembly are composed of MOX fuel rods, excluding fuel rods containing burnable poison such as gadolinia, and the MOX fuel assembly The infinite multiplication factor of the uranium fuel assembly is made lower than that of the uranium fuel assembly during most of the combustion period except for the initial combustion period.

By the way, there is a gross peaking coefficient as a measure of thermal constraint conditions in core design. In order to reduce the gross peaking coefficient in a light water reactor core loaded with a mixture of MOX fuel assemblies and uranium fuel assemblies, the infinite multiplication factor of the MOX fuel assemblies, which causes a large local power peaking coefficient, can be reduced. .

In the present invention, since the infinite multiplication factor of the MOX fuel assembly is lower than the infinite multiplication factor of the uranium fuel assembly, it is possible to design a reactor core that satisfies the thermal constraints with a margin. becomes.

On the other hand, if the infinite multiplication factor of the MOX fuel assembly is lowered, the amount of plutonium loaded will decrease, but most of the fuel rods in the MOX fuel assembly, excluding fuel rods containing burnable poisons such as gadolinia, Since it is composed of fuel rods, the amount of plutonium loaded per fuel assembly can be increased compared to the island type.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a fuel assembly according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows MOX fuel (MOX
This MOX fuel rod is an example in which fuel rods are arranged in a grid of 8 rows and 8 columns. In the figure,
Reference numeral 10 designates a channel box for MOX fuel. Inside this channel box 10, a MOX fuel rod 11 having tail uranium as a base material and a fuel rod 30 containing a burnable poison such as gadolinia are arranged. A water rod 13 is arranged.

Four types of MOX fuel rods 11 are used as shown in the fuel rods shown in FIG. As shown by the symbol G in one fuel rod, 16 are used.

Table 3 shows U-2 of each of the fuel rods 11 and 12.
35 enrichment, Puf enrichment, total fissile material content rate, gadolinia (Gd2O3) concentration, and number of particles.

[0034]

[Table 3]

The MOX fuel shown in FIG. 1 is designed to have an average core burnup of 22 GWd/t at the end of the cycle, and has an average burnup of 32 GWd/t after 15 months of operation.
t is achievable. The MOX fuel is designed to be mixed and loaded with the uranium fuel shown in FIG. For this reason, the number of gadolinia-containing fuel rods 12 is 16 in the case of uranium fuel, whereas it is 10 in the case of uranium fuel. Eight of the 16 rods are arranged to surround the large-diameter water rod 13 at the center. This is to improve the control value of gadolinia by placing it as a burnable poison in areas with as much water as possible.

Further, the uranium enrichment of the gadolinia-containing fuel rod 12 is 3.2% in uranium fuel, but in this embodiment it is set to 4.5%, which is higher than that. By increasing the uranium enrichment, the output burden ratio of the gadolinia-filled fuel rods 12 becomes slightly higher than that of uranium fuel, and the local power peaking at the early stage of combustion due to the increase in the number of gadolinia-filled fuel rods 12 is reduced. This is because it works in the direction of improving the deterioration, albeit slightly.

FIG. 2 shows the local power distribution at the initial stage of combustion of the MOX fuel shown in FIG. 1, and the maximum value is 1.23.
It is 3. When compared with the local power distribution of uranium fuel in FIG. 13, it can be seen that the power of the fuel rod containing gadolinia has increased. For uranium fuel, the average core burnup at the end of the cycle is 25 GWd/t, and the average burnup at extraction is 38 GWd/t, which is higher than MOX fuel. However, this combination is the most feasible since it results in a higher burnup of the fuel.

Next, the operation of this embodiment will be explained.

FIG. 3 shows the relationship between the core average infinite multiplication factor of the MOX fuel assembly (MOX fuel) and the proportion of fissile material at the end of the cycle, using the average core burnup at the end of the cycle as a parameter. be. The dashed line is for uranium fuel, and the solid line is for all MOX fuels targeted by the present invention. MOX fuel is designed to have the same hard shape as uranium fuel. The core is critical at an effective multiplication factor of 1.0, below which the cycle cannot continue operating. Since the leakage of neutrons from the core is approximately 5% Δk, the average infinite multiplication factor of the core at the end of the cycle is 1.05. The core average infinite multiplication factor at the end of this cycle is represented by a dashed-dotted line.

[0040] The average core burnup at the end of the cycle is 25 GWd.
/t of fuel is designed with uranium fuel, U-23
This figure shows that the required fissile material fraction for MOX fuel is approximately 4.6 wt% (point B), whereas the required enrichment for MOX fuel is approximately 3.8 wt% (point R). It represents. That is, as described above, since MOX fuel has a hard neutron spectrum, fuels with the same reactivity can only be obtained by loading more fissile material than uranium fuel. Moreover, the slope of MOX fuel is less steep than that of uranium fuel, meaning that it is less sensitive than uranium fuel to changes in fissile material loading.

By the way, the uranium fuel shown in FIG. 12 is designed so that the core average burnup at the end of the cycle is 25 GWd/t, and its local power peaking coefficient is 1.238 at the beginning of combustion. is as shown in FIG. On the other hand, MO with the same reactivity
The local power peaking coefficient of X fuel is 1.246, which is higher than that of uranium fuel.

FIG. 4 compares the combustion dependence of the local power peaking coefficient of uranium fuel and MOX fuel having the same reactivity.

As is clear from FIG. 4, there is a characteristic difference in local power peaking between uranium fuel and MOX fuel. That is, in the case of uranium fuel, there is an almost monotonous decrease, whereas in the case of MOX fuel, there is no monotonous decrease even in the latter half of combustion. The reason is that, due to the hardness of the neutron spectrum, the conversion from U-238 to Pu-239 proceeds well in the MOX fuel rods in the central part of the fuel assembly, which begin to output power in the latter half. however,
Even if the local power peaking coefficient is large, it will not be a thermal constraint if the power of the fuel assembly is small throughout the core.

FIG. 5 shows a comparison of combustion changes of uranium fuel and MOX fuel at infinite multiplication factors.

As is clear from FIG. 5, the infinite multiplication factor of MOX fuel is higher than that of uranium fuel during a considerable period in the early stage of combustion (first loading cycle) and in the late stage of combustion. ing. In particular, when MOX fuel is mixed with uranium fuel and loaded into the reactor core, as can be seen from Figure 5, the infinite multiplication factor of MOX fuel is higher than that of uranium fuel at the initial stage of combustion, so the local power peaking coefficient of MOX fuel is need to be lower than that of uranium fuel.

The reason for this will be explained below. The gross peaking coefficient is used as a guideline for thermal limiting conditions in core design. The gross peaking coefficient is the maximum value of the product of the core's radial peaking coefficient, axial peaking coefficient, and local power peaking coefficient. The radial peaking coefficient is determined primarily by the average infinite multiplication factor of the fuel assembly. Therefore, in order to reduce the gross peaking coefficient in a reactor core loaded with a mixture of MOX fuel and uranium fuel, it is sufficient to reduce the infinite multiplication factor of the MOX fuel, which causes a large local power peaking coefficient. As a guideline, FIG. 6 shows the combustion change of the product of the infinite multiplication factor and the local output peaking coefficient. From now on, the value of MOX fuel will be larger than that of uranium fuel during the entire period of nuclear fuel combustion, making it difficult for the thermal design of the reactor core.

[0047] Therefore, a MOX fuel was designed in which the proportion of fissile material was lowered by 0.5% on average in the fuel assembly. As a result, the value of the local power peaking coefficient at the early stage of combustion is shown in Figure 7.
As shown in Figure 2, it has decreased to 1.233, which is lower than that of uranium fuel. The values of the infinite multiplication factor and the local power peaking coefficient also decrease overall, as shown in FIGS. 4 and 5, respectively. Instead, the average burnup of the reactor core is small at 22 GWd/t, but since all fuel rods are MOX fuel rods except for the fuel rods containing gadolinia as a burnable poison, the loading ratio of plutonium is
It is still larger than the island type. Since the infinite multiplication factor is smaller than that of uranium fuel during most of the combustion period, except at the beginning of combustion, the product of the infinite multiplication factor and the local power peaking factor is This increases the combustion period, which has a value comparable to that of uranium fuel, and is designed to comfortably meet the thermal constraints of the reactor core.

[0048] In Fig. 7, to what extent the weight proportion of fissile material in MOX fuel should be lowered is that it must be at least below point C;
The smaller the value, the easier the core characteristics will be. However, if you lower it too much, the amount of plutonium loaded per fuel assembly will become the same as that of an island type.
This limits the limit to which it can be lowered. Point A is just one example, and if MOX fuel closer to point C can be designed without problems, the plutonium loading ratio can be maximized.

[0049] Therefore, it is possible to increase the amount of plutonium loaded per MOX fuel assembly as much as possible to configure a reactor core with a sufficiently high operating margin without impairing thermal performance even when mixed with uranium fuel. is possible. The advantages of lowering the plutonium enrichment in terms of core characteristics are effective not only during operation but also in improving the reactor shutdown margin during reactor shutdown. Generally, in a core where the amount of change in the infinite multiplication factor of the assembly (cold-hot swing) when changing from operating to cold shutdown is positive and large, the margin for reactor shutdown is tight. In the case of this embodiment, the initial combustion value of cold hot swing -0.
2%Δk, M before lowering plutonium enrichment
This is improved over the value of OX fuel +0.1% Δk.

FIG. 8 shows an example in which the MOX fuel according to the present invention is applied to a fuel assembly in which fuel rods are arranged in 9 rows and 9 columns. In the figure, numeral 10 is a channel box. Inside the channel box 10, MOX
Fuel 11 and fuel rods 12 containing gadolinia as a burnable poison
A large diameter water rod 13 is arranged in the center.
Two are placed.

Four types of the MOX fuel rods 11 are used, as shown in the fuel rods in FIG. Two types are used, as shown by the symbols G1 and G2 attached to the rods.

Table 4 shows U-2 of each of the fuel rods 11 and 12.
35 enrichment, Puf enrichment, total fissile material weight percentage, gadolinia (Gd2O3) concentration, and number of particles.

[0053]

[Table 4]

The MOX fuel shown in FIG. 8 exhibits good core characteristics when mixed and loaded with uranium fuel having an average burnup of 45 GWd/t.

FIG. 9 shows the combustion changes of the MOX fuel and the uranium fuel shown in FIG. 8 at an infinite multiplication factor, and the infinite multiplication factor of the MOX fuel is , which is lower than the infinite multiplication factor of uranium fuel.

FIG. 10 shows the combustion changes in the local power peaking coefficients of the MOX fuel and the uranium fuel shown in FIG. 8, and during the first cycle of loading,
The cost of MOX fuel is lower than that of uranium fuel. As a result, the product of the local power peaking coefficient and the infinite multiplication factor is also lower for MOX fuel than for uranium fuel, as shown in FIG.

In FIG. 8, even if the two gadolinia-containing fuel rods 12 marked G1 facing the two water rods 13 are removed and one water rod with a larger diameter is placed, Similar effects can be expected.

[0058]

As explained above, according to the present invention, the amount of plutonium loaded per MOX fuel assembly can be increased as much as possible without impairing thermal integrity.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing one embodiment of a MOX fuel assembly according to the present invention.

FIG. 2 is an explanatory diagram showing the local power distribution in the early stage of combustion of the MOX fuel assembly in FIG. 1;

FIG. 3 is a graph showing the relationship between the fissile material ratio and the core average infinite multiplication factor in the MOX fuel of FIG. 1 in comparison with uranium fuel and MOX fuel with the same reactivity.

FIG. 4 is a graph showing combustion changes in local power peaking coefficient for the MOX fuel of FIG. 1 in comparison with uranium fuel and MOX fuel with the same reactivity.

FIG. 5 is a graph showing the combustion change of infinite multiplication factor in the MOX fuel of FIG. 1 in comparison with uranium fuel and MOX fuel with the same reactivity.

FIG. 6 is a graph showing the relationship between the product of the local power peaking coefficient and the infinite multiplication factor and burnup in the MOX fuel of FIG. 1 in comparison with uranium fuel and MOX fuel with the same reactivity.

FIG. 7 is a graph showing the relationship between the fissile material ratio and the local power peaking coefficient in the MOX fuel of FIG. 1 in comparison with uranium fuel and MOX fuel with the same reactivity.

FIG. 8 is a cross-sectional view showing another embodiment of the MOX fuel assembly according to the present invention.

FIG. 9 is a graph showing the combustion change of the infinite multiplication factor in the MOX fuel of FIG. 8 in comparison with uranium fuel.

FIG. 10 is a graph showing the combustion change in local power peaking coefficient in the MOX fuel of FIG. 8 in comparison with uranium fuel.

FIG. 11 is a graph showing the combustion change in the product of the local power peaking coefficient and the infinite multiplication factor in the MOX fuel of FIG. 8 in comparison with the uranium fuel.

FIG. 12 is a cross-sectional view showing a conventional uranium fuel assembly.

FIG. 13 is an explanatory diagram showing the local power distribution in the early stage of combustion of a uranium fuel assembly.

FIG. 14 is a graph showing the radial distribution of neutron flux in a typical fuel.

[Explanation of symbols]

10 Channel box 11 MOX fuel rod 12 Fuel rod containing galidonia (fuel rod containing burnable poison) 13 Water rod

Claims (1)

[Claims] 1. In a light water reactor in which a mixed oxide fuel assembly made of a mixture of uranium and plutonium as fissile materials and a uranium fuel assembly made of uranium are mixedly loaded, the mixed oxide fuel The majority of the fuel rods in the assembly are made up of mixed oxide fuel rods, excluding fuel rods containing burnable poison, and the infinite multiplication factor of this mixed oxide fuel assembly is calculated as follows: A fuel assembly characterized in that the multiplication factor is lower than the infinite multiplication factor of the uranium fuel assembly in most parts.