JPH0542638B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a nuclear reactor fuel assembly with improved fuel economy.

[Technical Background of the Invention] A nuclear reactor fuel assembly is constructed by arranging a plurality of fuel rods in a lattice pattern. It is uniform in the axial direction. Further, the content of the burnable poison added for the purpose of suppressing the initial reactivity of the fuel is also uniform in the axial direction of the fuel assembly. Here, the fissile material content refers to at least one of uranium enrichment and plutonium enrichment, and burnable poison content refers to the content of burnable poisons in the fuel rods constituting the fuel assembly. Refers to at least one of the number of fuel rods containing burnable poison and the concentration of burnable poison in the fuel rods containing burnable poison.

However, a nuclear reactor in which such a fuel assembly is loaded into the core suffers from loss of reactivity due to the following two points. In other words, (1) A certain percentage of neutrons generated near the core surface leak out of the core and no longer contribute to the fission chain reaction.

(2) The content of burnable poisons is determined so that the fuel will burn out once one cycle is completed after it is loaded into the core, but even in that case, the content of burnable poisons in the fuel around the core or the fuel in the center of the core is determined. However, combustion does not proceed near the upper and lower ends of the fuel assembly, so burnable poison remains burned at the end of the cycle.

As a result, the reactivity of conventional fuel assemblies is reduced accordingly, which poses a problem in terms of efficient fuel utilization.

[Purpose of the invention]

The present invention has been made to solve the above problems, and it reduces the leakage of neutrons from the upper and lower surfaces of the reactor core, and at the same time reduces the amount of unburned fuel rods at the end of the cycle. This aims to reduce energy loss and improve fuel economy in nuclear reactors.

[Summary of the invention]

The present invention divides a nuclear reactor fuel assembly into three regions, an upper region, a central region, and a lower region, in the core axis direction,
The boundary between the upper region and the central region is 1/12 to 1/8 of the total length from the upper end of the reactor fuel assembly.
and the boundary between the lower region and the central region is located at a position of 1/24 to 1/12 of the total length from the lower end of the reactor fuel assembly, and the boundary between the lower region and the central region The content of fissile material is less than the content of fissile material in the central region, and the content of burnable poison in the upper region and the lower region is less than the content of burnable poison in the central region. The present invention relates to a reactor fuel assembly for a boiling water reactor. In addition, the upper and lower regions containing less fissile material contain natural combustion uranium as nuclear fuel material, waste uranium discarded in the enrichment process with a lower enrichment than natural uranium, or reprocessed spent fuel. The recovered uranium obtained from the process can be used.

The relationship between the structure of the fuel assembly as described above and improvement in fuel economy will be explained below.

First, we will discuss the point of making the content of fissile material smaller in the upper and lower regions than in the central region. In connection with reducing the leakage of neutrons from the upper and lower surfaces of the reactor core, which is the first objective of the present invention, let us consider the neutron transfer area (M ² ). The neutron movement area (M ² ) has the following relationship with the linear distance (l) that fast neutrons created by nuclear fission travel from their birth position until they are absorbed as thermal neutrons. .

^M2 =1/6 ^<l2> (<) indicates an average value. ) (1) Therefore, the neutrons leaking from the reactor core are considered to be neutrons born between the surface and a position approximately √< ² > = √6 ² inward from the reactor surface. Therefore, in order to reduce the number of neutrons escaping from the core, the number of fission in this region can be reduced by lowering the content of fissile material in this region.

The neutron transfer area M ² does not change much depending on the fissile material content or burnup of the fuel, but it changes greatly depending on the void fraction distributed from the bottom to the top of the core in boiling water reactors. do.
The void rate is approximately 0% near the bottom of the core,
The neutral dynamic area M ² there is approximately 60 cm ² . On the other hand, the void fraction near the top of the core is approximately 70%, and the neutron transfer area M ² there is approximately 140 cm ² .

Therefore, in order to reduce the leakage of neutrons from the bottom surface of the reactor core, the boundary between the lower region and the center region should be set at a distance of √ ^<2> ~√6×60~20 (cm) from the bottom surface of the core.
What is necessary is to provide it at a position more distant than that. Since the total length of a fuel assembly in a boiling water reactor is approximately 370 cm, this corresponds to 20/370 or approximately 1/18 of the total length of the fuel assembly.
On the other hand, in order to reduce the leakage of neutrons from the upper surface of the core, the boundary between the upper region and the central region should be set at a distance of √ ^<2> ~√6×140~30 (cm) or more from the upper surface of the core. . This is the total length of the fuel assembly.
0/370 corresponds to approximately 1/12.

The above provides a rough guide to the boundaries of each region in terms of reducing neutron leakage, but when the enrichment distribution is created in the axial direction of the fuel assembly, it is the neutrons that affect the fuel economy. It's not just the leak. In fact, the effective multiplication factor h _eff of the core, which is one of the indicators of fuel economy, is expressed by the neutron leakage L and the core average infinite multiplication factor ∞, as shown in the following equation (2).

h _eff =∞・(1−L) (2) This core average infinite multiplication factor will be described later, and first, the problem of improving fuel economy by reducing leakage of neutrons will be explained in detail below.

FIG. 1 shows the axial distribution of the enrichment of fissile material in a conventional fuel assembly, and the enrichment is uniformly distributed in the axial direction. The fuel assembly average enrichment is 3.00% by weight.

FIG. 2 is an axial distribution diagram of the enrichment of fissile material in the fuel assembly of the present invention. The enrichment is divided into three regions in the axial direction, and the enrichment in the upper region and the lower region is e _l wt%. , the concentration in the central part is e _h % by weight. The boundary between the upper region and the central region is located at a position t times the total length from the upper end of the fuel assembly, and the boundary between the lower region and the central region is located at a position b times the total length from the lower end of the fuel assembly. be. In this case as well, the fuel assembly average concentration is 3.00% by weight.

Figure 3 shows the fuel economy improvement rate of the fuel shown in Figure 2 relative to the fuel with uniform enrichment in the axial direction shown in Figure 1, using the boundary position value b between the lower region and the central region as a parameter. It is something. In the fuel shown in Fig. 2 used here, the boundary position value t between the upper region and the central region is set to 1/12 of the total length of the fuel assembly based on the above discussion regarding the neutron transfer area ^M2 , and the upper region and the lower region are The enrichment of the region e _l is 0.711% by weight and natural uranium. In addition, the fuel economy improvement rate can be calculated using the cycle length of the balanced reactor core loaded with fuel shown in Figure 1 as Eo (MWd/st), and the cycle length of the balanced reactor core loaded with fuel shown in Figure 2 as Eo (MWd/st). ), fuel economy improvement rate = E-Eo/Eo x 100 (%) (3).

As is clear from Fig. 3, when b takes a value between 1/24 and 1/12, that is, the boundary between the lower region and the center region is
Fuel economy is maximized when the engine is positioned between 1/12 and

The reason why there is such an optimal range of b in terms of fuel economy will be explained below based on equation (2).

FIG. 4 shows the amount of decrease in neutron leakage L of the fuel in FIG. 2 with respect to the fuel in FIG. 1 and the amount of increase in core average infinite multiplication factor ∞. The neutron leakage L decreases rapidly at first as the lower region expands, and then gradually decreases. Therefore, as stated by presenting equation (2), neutron leakage L alone cannot explain the fuel economy improvement behavior shown in Figure 3.
We must pay attention to changes in the core average infinite multiplication factor ∞.

The reason why the core average infinite multiplication factor ∞ decreases as the lower region expands will be explained below.
Here, the core average infinite multiplication factor ∞ is, when viewed in the axial direction, ∞ = (∫h∞(Z)P(Z)dZ) / (∫P(Z)dZ) (4) h∞(Z): axial Infinite multiplication factor P(Z) at position Z in the direction: Represented by the output at position Z in the axial direction.

FIG. 5 is a graph showing the relationship between infinite multiplication factor and fuel enrichment. As shown in FIG. 5, the infinite multiplication factor is an upwardly convex function with respect to the fuel enrichment. Therefore, the core average infinite multiplication factor of a fuel consisting of a high enrichment portion and a low enrichment portion is smaller than the infinite multiplication factor of a uniformly enriched fuel having the same average enrichment. This can be explained using an example as follows. Of the fuel shown in Figure 2, which consists of a high enrichment portion and a low enrichment portion, the boundary position value t between the upper region and the central region is 1/12, and the enrichment e _l of the upper region and lower region is 0.711 weight. %, and the boundary position value b between the lower region and the center region is set to 1/24, and this is fuel A, and t and e _l are the same, and b is 1/24.
8 is designated as fuel B. First, instead of the core average infinite multiplication factor ∞ defined by equation (4), we will consider the simple average infinite multiplication factor h∞° without weighting the power distribution.

h∞゜=(∫h∞(Z)dZ)/(∫dZ) (5) The enrichment levels of fuel A and fuel B in the central region are:
Calculated from the average enrichment of each fuel of 3.00% by weight, 3.33% by weight and
3.60% by weight, the h∞゜ of the fuels, fuel A, and fuel B in Figure 1 become the values shown at the positions of ○C, ○D, and ○Ho in Figure 5, respectively, and as the lower region expands, h∞゜∞゜ decreases rapidly.

On the other hand, since the output of the central region increases as the lower region expands, the weight P(Z) of the highly enriched central region increases in equation (4), and the ∞ of fuels A and B increases. moves from h∞° in FIG. 5 according to the arrow, and reaches the values indicated by the positions ○ and ○. In this way, ∞ defined by equation (4)
is not as steep as h∞° defined by Equation (5), but decreases as the lower region expands and the degree of enrichment in both the upper and lower regions and the central region widens.

As described above, in a nuclear reactor loaded with the fuel of the present invention, as the lower region expands, the neutron leakage L (Fig. 4 ○B) decreases, and the core average infinite multiplication factor ∞ (Fig. 4 ○B) As a result, the fuel economy improvement rate behaves as shown in Figure 3.

The above explanation was about the fuel economy improvement rate using the boundary position value b between the lower region and the central region as a parameter, but the same explanation was given using the boundary position value t between the upper region and the central region as a parameter. The graph shown in FIG. 6 shows the fuel economy improvement rate of the fuel shown in FIG. 2 relative to the fuel shown in FIG. 1.
Here, b is set to 1/24 based on FIG. 3, and the enrichment level e _l of both the upper and lower regions is set to 0.711% by weight, that is, natural uranium.

According to FIG. 6, the fuel economy improvement rate is maximized when t is located between 1/12 and 1/8. The reason why there is such an optimal range of t with respect to the fuel economy improvement rate can be understood from the amount of neutron leakage reduction (○) and the core average infinite multiplication factor increase (○) shown in FIG.

Next, the influence of the content of fissile material in both the upper and lower regions on improving fuel economy will be explained. As an example, a case will be described in which the boundary position value t between the above region and the central region is set to 1/12, and the boundary value b between the lower region and the central region is set to 1/24.

In FIG. 8, the horizontal axis shows the enrichment levels in both the upper and lower regions, and the vertical axis shows the fuel economy improvement rate of this fuel relative to the fuel in FIG. 1. The enrichment in the central region is determined so that the fuel assembly average enrichment is 3.00% by weight.

In Figure 9, the horizontal axis shows the enrichment in both the upper and lower regions, and the vertical axis shows the reduction in leakage of neutrons from this fuel compared to the fuel in Figure 1 and the increase in the core average infinite multiplication factor. be.

As can be seen from Figures 8-9, the lower the enrichment in both the upper and lower regions, the greater the effect of reducing neutron leakage, thereby improving fuel economy.

In these figures, the enrichment of 0.711% by weight uses natural uranium, and the enrichment of 0.3% by weight uses waste uranium discarded during the enrichment process. The concentration of ²³⁵ U in waste uranium varies depending on the enrichment method, but is usually 0.2 to 0.3% by weight. Also
Uranium with a low ²³⁵ U concentration may be recovered uranium obtained by reprocessing spent fuel;
^{The 235} U concentration is 0.8-0.9% by weight.

As mentioned above, the fuel assembly is axially divided into three regions: the upper region, the central region, and the lower region.
Fuel economy can be improved by having lower fissile material content in the upper and lower regions than in the central region. In particular, the boundary between the upper region and the central region is located between 1/12 and 1/8 of the total length from the upper end of the fuel assembly, and the boundary between the lower region and the central region is located at the lower end of the fuel assembly. By locating the fuel assembly between 1/24 and 1/12 of its total length, the effect of reducing neutron leakage from the upper and lower ends of the fuel assembly is maximized, thereby improving fuel economy. be able to. Furthermore, the lower the content of fissile material in the upper and lower regions, the less neutron leakage and the better the fuel economy.

Next, the relationship between making both the content of fissile material and the content of burnable poison smaller in the upper region and the lower region than in the central region and improving fuel economy will be explained.

In boiling water reactors, burnable poison is generally added to some of the fuel rods that make up the fuel assembly in order to suppress initial excess reactivity. The concentration of the burnable poison is determined so that the burnable poison is burned out at the end of one cycle after new fuel is loaded into the core.

Figure 10 shows the axial burnup distribution of the following conventional fuel with uniform enrichment in the axial direction: average enrichment of 3.00% by weight, 7 fuel rods containing burnable poison, and burnable poison concentration of 4.0% by weight. This is what I saw after one cycle had passed. Burning is delayed near the upper and lower ends of the core because the output is low. It will be shown below that this results in a loss of reactivity.

Figure 11 shows the infinite multiplication factor change of a conventional fuel that is uniform up and down, where ○wo and ○wa contain burnable poisons, and ○ka and ○yo do not contain burnable poisons. , and ○wo and ○ka are void rates.
In the case of 70%, ○wa and ○yo are cases where the void rate is 0%. The infinite multiplication factor of a fuel assembly containing burnable poison initially increases as combustion progresses, but then peaks at 8 to 9 GWd/st and then decreases.
In contrast, the infinite multiplication factor of a fuel assembly that does not contain burnable poisons decreases monotonically with combustion. Therefore, at the burnup in the center of the core shown in Figure 10, the burnable poisons have been burned up, but at the burnup near the top and bottom ends of the core, the burnable poisons still remain, and as a result, the reactivity decreases. are doing.

Figure 12 assumes that the void rate near the top of the core is 70% and the void rate near the bottom of the core is 0%, and the burnup distribution shown in Figure 10 and the infinite multiplication factor shown in Figure 11 are used to calculate the residual burnability of burnable poison. This represents the loss of reactivity due to

Due to unburned burnable poisons at the upper and lower ends of the new fuel at the end of the cycle, the core average infinite multiplication factor h∞ decreases by approximately 0.3% Δh, which reduces fuel economy by approximately 3%. ing.

As can be seen from Figure 12, burnable poison remains in the new fuel at the end of the cycle is present in the lower part of the core from the lower end of the fuel assembly to 1/24th of its total length, while in the upper part of the core, the remaining part of the burnable poison remains at the upper part of the fuel assembly. It is remarkable between 1/12 of its total length.

Therefore, if the burnable poison content in the upper and lower regions of the fuel assembly can be reduced, the reactivity loss due to unburned burnable poison at the end of the cycle can be reduced. This becomes possible when the content of fissile material in these regions is reduced, and favorable results can be obtained by loading without adding burnable poisons, such as when natural uranium is used.

As explained above, according to the present invention, by lowering the enrichment of fuel at the upper and lower ends of the fuel assembly and lowering the concentration of burnable poisons, leakage of neutrons from the upper and lower surfaces of the core is reduced, and at the same time, at the end of the cycle. It is possible to reduce the amount of burnable poison left unburned at the upper and lower ends of the fuel assembly, thereby improving fuel economy.

[Embodiments of the invention]

Example 1 Boundary between the upper region and the center region: 1/12 of the total length from the upper end of the fuel assembly Boundary between the lower region and the center region: 1/24 of the total length from the lower end of the fuel assembly Top Fuel in the region and lower region: Natural uranium, no burnable poison Fuel in the central region: Average enrichment 3.33% 8 fuel rods with burnable poison (burnable poison Gd ₂ O ₃ , concentration 4.0% by weight) Fuel assembly The cycle length of an equilibrium cycle created by loading fuel with an average enrichment of 3.00% by weight or more in batches of 3 or 4 is approximately
It was 8300MWd/st.

Comparative Example 1 Fuel with uniform enrichment in the axial direction: average enrichment 3.00% by weight 7 fuel rods containing burnable poison (burnable poison Gd ₂ O ₃ , concentration 4.0% by weight) Three or four batches of the above fuel were taken. The cycle length of the equilibrium cycle created by loading the
Therefore, the fuel assembly of Example 1 achieved an improvement in fuel economy of about 7% compared to the conventional fuel with uniform enrichment in the axial direction.

Example 2 Boundary between the upper region and center region: 1/12 of the total length from the upper end of the fuel assembly Boundary between the lower region and the center region: 1/24 of the total length from the lower end of the fuel assembly Top Fuel for the region and lower region: Waste uranium ( ²³⁵ U concentration 0.3% by weight) No burnable poisons Fuel for the central region: Average enrichment 3.39% by weight 8 fuel rods with burnable poisons (burnable poisons Gd ₂ O ₃ , (Concentration: 4.0% by weight) Fuel assembly average enrichment: 3.00% by weight or more The cycle length of an equilibrium cycle prepared by replacing the fuel in batches of 3 or 4 times is approximately 8450 MWd/st, which is shorter than the fuel of Comparative Example 1. This resulted in an approximately 9% improvement in fuel economy.

[Effects of the Invention] As described above, the present invention divides a reactor fuel assembly for a boiling water reactor into three regions in the axial direction, with the upper region being 1/12 to 1/8 of the total length from the upper end, and the lower region being 1/12 to 1/8 of the total length from the upper end. The region is 1/24 to 1/12 of the total length from the lower end, and the content of fissile material and burnable poison in the upper and lower regions is lower than in the central region, so that neutrons from the upper and lower ends of the fuel assembly are It is possible to make maximum use of the effect of reducing leakage, thereby improving fuel economy. Furthermore, it is possible to reduce reactivity loss due to burnable poison remaining at the upper and lower ends of the fuel assembly at the end of the cycle.

[Brief explanation of the drawing]

Fig. 1 is an axial enrichment distribution diagram of conventional fuel, Fig. 2 is an axial enrichment distribution diagram of the fuel of the present invention, and Fig. 3 is an axial enrichment distribution diagram of the fuel of the present invention.
The figure is a graph of the fuel economy improvement rate when the boundary position between the lower region and the central region is used as a parameter, and Figure 4 is a graph of the reduction in neutron leakage and the core average when the same amount as in Figure 3 is used as a parameter. A graph of the increase in infinite multiplication factor, Fig. 5 is a graph showing the relationship between infinite multiplication factor and fuel enrichment, and Fig. 6 is a graph of fuel economy when the boundary position between the upper region and the central region is used as a parameter. A graph of the improvement rate, Figure 7 is a graph of the decrease in neutron leakage and the increase in the core average infinite multiplication factor when the same amount as in Figure 6 is used as a parameter, and Figure 8 is a graph of the enrichment in the upper and lower regions. Figure 9 is a graph of the fuel economy improvement rate when parameters are used. Figure 9 is a graph of neutron leakage reduction and core average infinite multiplication rate increase when the same quantities as Figure 8 are used as parameters. Figure 10 is a graph of the increase in core average infinite multiplication factor. Fig. 1 is an axial burnup distribution diagram of the fuel shown in Fig. 1 after one cycle has passed after loading it into the core, and Fig. 11 shows the void rate of 0% and 0% for the fuel shown in Fig. 1 with and without burnable poisons. A graph showing each infinite multiplication factor at 70%, and FIG. 12 is an axial distribution diagram of reactivity loss due to unburned burnable poison at the end of the cycle when the fuel of FIG. 1 contains burnable poison.

Claims (1)

[Claims] 1. The reactor fuel assembly is placed in the upper region in the core axis direction,
The reactor fuel assembly is divided into three regions, a central region and a lower region, and the boundary between the upper region and the central region is located at a position of 1/12 to 1/8 of the total length from the upper end of the reactor fuel assembly, and The boundary between the lower region and the central region is located at a position of 1/24 to 1/12 of the total length from the lower end of the nuclear fuel assembly, and the content of fissile material in the upper region and the lower region is A boiling water type characterized in that the content of fissile material in the central region is lower than that of the fissile material, and the content of burnable poison in the upper region and the lower region is lower than the content of burnable poison in the central region. Reactor fuel assembly for nuclear reactors. 2. A patent claim in which the upper region and the lower region are loaded with natural combustion uranium, waste uranium disposed of in the enrichment process, or recovered uranium obtained by reprocessing spent fuel, and in which no burnable poison is loaded. A nuclear reactor fuel assembly according to scope 1.