JPS6314790B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nuclear reactor, and particularly to a boiling water nuclear reactor that can improve fuel economy without impairing core shutdown margin. Generally, in a boiling water reactor, reactor control rods installed in the reactor are moved in and out of the reactor core by a control rod drive device.
In particular, when shutting down a nuclear reactor, the drive device inserts all control rods into the reactor core to the maximum extent possible. At that time, if one control rod with the highest reactivity value among the control rods in the core cannot be inserted into the reactor core due to a failure of the control rod drive device or some other cause, or Even if one control rod with the highest reactivity value in the reactor core is accidentally pulled out from a reactor shutdown state in which all control rods are inserted to the maximum, the effective neutron multiplication factor in the reactor core remains at least 1. The reactor core must be maintained in a subcritical state. This is required by core design. The effective neutron multiplication factor in the core when all other control rods are inserted to the maximum in the core except for the control rod with the highest reactivity value in the core.
Keff, and ΔK=1−Keff is defined as the reactor shutdown margin. Considering the safety of the reactor, the larger the reactor shutdown margin ΔK, the higher the safety. The fuel assemblies loaded in the reactor core are kept within the reactor shutdown margin ΔK at any point during the unit combustion period (referred to as a cycle) during which the fuel assemblies are operated without fuel replacement.
Actual core design is carried out to ensure 0.01 (1%) or more. In conventional nuclear reactors, in order to satisfy this reactor shutdown margin, burnable poisons such as gadolinia (Gd ₂ O ₃ ) (hereinafter referred to as gadolinia) are added to a few to 10 of the fuel rods that make up the fuel assembly. It is mixed into some fuel rods. In a core (first cycle) in which all of the fuel assemblies loaded into the core are new fuel assemblies, the core is composed of one type of fuel assembly or two types of fuel assemblies. The core configuration is such that the amount of gadolinia mixed in in the fuel assemblies loaded in the outermost row of the core is smaller than the amount of gadolinia mixed in in the other fuel assemblies. FIGS. 1 and 2 schematically show a horizontal cross section of a typical conventional core configuration described above.
Figure 1 shows type A fuel assemblies (hereinafter abbreviated as fuel A) arranged throughout the core, in which the amount of gadolinia contained in the fuel assemblies loaded into the core is the same, and Figure 2 shows the same amount of gadolinia mixed in the fuel assemblies loaded into the core. Type B fuel assemblies (hereinafter referred to as fuel B) containing less gadolinia than fuel A are arranged in the outermost row of the core, and fuel A is arranged in other areas. In addition, in FIG. 1, FIG. 2, and other figures to be explained later, the symbol A indicates fuel A, and the symbol B indicates fuel B.
, and the symbol C indicates a control rod. Figure 3 shows the combustion change of typical fuel A and fuel B at infinite multiplication factor during reactor operation.
Fuel A and Fuel B are fuel assemblies containing the same weight of uranium. Until gadolinia burns out, as shown in Figure 2, by arranging fuel B, which has a larger infinite multiplication factor than fuel A, in the outermost row of the core, the output of the fuel assembly at that location is promoted. It is possible to improve the neutron economy by efficiently burning the fuel assemblies in the most peripheral row, and to minimize the deterioration of reactor shutdown margin since the fuel B arrangement location has the largest leakage of neutrons. Can be done. On the other hand, if fuel B is not limited to only the outermost row of the core but is arranged toward the inside of the core where neutron leakage is smaller, the neutron economy will further improve, but on the other hand, the extent to which the reactor shutdown margin will deteriorate is getting bigger. In other words, the improvement in fuel economy due to improvement in neutron economy and the margin for reactor shutdown are in a contradictory relationship. The present invention was made in view of the current situation, and its purpose is to provide a nuclear reactor that can suppress the deterioration of reactor shutdown margin to a practical extent and improve fuel economy. . In the present invention, fuel B is loaded in the most peripheral row of the core, and every other fuel is loaded in a row one step inside the most peripheral row, and fuel A is loaded in the remaining portion, and fuel B and fuel The number of gadolinia-containing fuel rods of Fuel B is equal to or one less than that of Fuel B, and the gadolinia concentration mixed over the entire length of the gadolinia-containing fuel rod of Fuel B is set to 1.5 to 2.0 weight percent (W/O). This is what I did. The present invention will be explained below with reference to the drawings. FIG. 4 shows a horizontal cross section of a fuel assembly of fuel A and fuel B having the nuclear properties shown in FIG. It is the same as fuel B.
Symbols G ₁ and G ₂ are fuel rods mixed with gadolinia, and the gadolinia concentration of fuel rod G ₁ is 5.0 W/O, and the gadolinia concentration of fuel rod G ₂ is 2.0/W/O. Further, the symbol W indicates a water rod. Figure 5 shows fuel A and fuel B shown in Figure 4.
The rate of improvement in fuel economy and the rate of deterioration in reactor shutdown margin when arranging fuel B in the core are expressed using the loading rate of fuel B into the core as a variable, and the black circle graph indicates the rate of improvement in fuel economy. In addition, the white circle graph shows the deterioration rate of the reactor shutdown margin. Here, the deterioration rate of reactor shutdown margin is defined below.
Let the reactor shutdown margin in the core configuration of Figure 5 A be Δk°, and the reactor shutdown margins B to G be expressed as Δk. Deterioration rate of reactor shutdown margin = (Δk° - Δk) × 100. On the other hand, the definition of fuel economy improvement rate is as follows. Let the cycle N length in the core configuration of Figure 5A be l ⁰ (megawatt day/ton),
When the cycle length of (L) to (G) is expressed as l, it is defined as: fuel economy improvement rate=l- _l0 / ^l0 ×100. In FIG. 5, symbols A and B indicate the first
The results are shown for the core configurations shown in Figs. show. Furthermore, the rate of deterioration of the reactor shutdown margin in FIG. 5 shows the result at the point in time when the reactor shutdown margin becomes the worst in the entire cycle. As is clear from Figure 5, the deterioration rate of the reactor shutdown margin for code A is about 0.1 to 0.2% for codes B and C, whereas the reactor shutdown margin deteriorates rapidly for codes 2 to G. . This means that the control rod with the maximum reactivity value in the reactor core is the control rod located at the center of the reactor core in the case of codes B and C, as in the case of code A, whereas the control rod with the maximum reactivity value in the case of codes D to G is the control rod that has the maximum reactivity value. This is because control rods with The number of fuels B arranged in the third row from the outermost periphery of the core is different between the core configuration in FIG. 8 corresponding to code C and the core configuration in FIG. 9 corresponding to code D. Comparing the reactivity values of the control rods in contact with fuel B, the fuel B is adjacent in the case of code 2, whereas it is not adjacent in the case of code c, so they are different. On the other hand, the improvement rate for fuel economy with sign A is:
It slows down as the symbol goes from RO to G. this is,
As shown in Figure 3, the infinite multiplication factors of fuels A and B are the same after gadolinia burns, so even though the loading rate of fuel B increases from code RO to code G toward the center of the core. This is because at the end of the cycle, the gadolinia in fuel A near the center of the reactor has been burned out and has the same infinite growth rate as fuel B. Therefore, the rate of improvement in fuel economy is slower than the rate of deterioration in reactor shutdown margin. Based on the above, in the present invention, the loading rate of fuel B is set as shown in symbol C. In other words, by loading fuel B in the most peripheral row of the core, loading every other fuel in the row one step inside the most peripheral row, and loading fuel A in the remaining center, the rate of deterioration of the reactor shutdown margin can be reduced. The aim is to improve fuel economy over conventional core configurations while minimizing fuel economy. Next, the amount of gadolinia mixed in fuel B will be explained.

【table】

[Table] Table 1 shows the results obtained when the gadolinia concentration of fuel B and the number of fuel rods mixed with gadolinia are changed when the arrangement of fuel B in the core is as shown in FIG. 6. This table summarizes the rate of improvement in fuel economy and the rate of deterioration in reactor shutdown margin for the conventional core configuration shown in . However, in Table 1, fuel A
The number of fuel rods mixed with gadolinia is 3,
These are the results of the effect of fuel B when its concentration was 5.0 W/O. As is clear from Table 1, the rate of improvement in fuel economy is much higher than the maximum rate of improvement of 2.6% (approximately 3 % or more) Fuel B to obtain the effect
It is necessary to reduce the concentration of gadolinia mixed into the fuel to 2.0W/O or less, and if it becomes 1.0W/O or less, there will be little improvement in fuel economy compared to the case of 2.0W/O gadolinia. It can be seen that only the deterioration rate of reactor shutdown margin increases. From the above results, it is preferable that the concentration of gadolinia mixed in fuel B be in the range of 1.5 to 2.0 W/O, and that the number of fuel rods containing gadolinia be equal to or one less than that of fuel A. As explained above, according to the present invention, it is possible to minimize the deterioration of reactor shutdown margin and improve fuel economy.

[Brief explanation of the drawing]

Figures 1 and 2 are horizontal cross-sectional views showing conventional core configurations, Figure 3 is a graph showing typical changes in infinite multiplication factors for Fuel A and Fuel B, and Figure 4 is for Fuel A and Fuel B. FIG. 5 is a horizontal cross-sectional view showing the configuration of fuel B, and FIG. FIG. 10 is a horizontal sectional view of the reactor core showing an example of the method of loading fuel B. A... Fuel A, B... Fuel B, C... Control rod, W... Water rod.

Claims (1)

[Claims] 1. In a nuclear reactor in which fuel rods containing fissile material and burnable poison are bundled into a fuel assembly, and a large number of these fuel assemblies are arranged to constitute a reactor core, a relative Two types of fuel assemblies are used: Type A fuel, which has a large amount of burnable poison mixed in, and Type B fuel, which has a relatively small amount of burnable poison mixed in, and the Type B fuel is loaded in the outermost row of the core. and 1 of the most peripheral column
Load every other row on the inside of the stage, type A fuel in the remaining part, and make the number of fuel rods containing burnable poison equal to type B fuel and type A fuel, or type B fuel. A nuclear reactor characterized in that the number of fuel rods containing burnable poisons of type B fuel is reduced by one, and the concentration of burnable poisons mixed in the fuel rods containing burnable poisons of type B fuel is lower than that of type A fuel.