JPS6312271B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel assembly for a boiling water reactor, and particularly to a fuel assembly that continues to operate while partially replacing the fuel assembly with new fuel every cycle. The present invention relates to a fuel assembly in which the enrichment between unit fuel assemblies is limited so that the transition to an equilibrium cycle is made quick and the thermal characteristics during the transition period are stable and good.

In a boiling water reactor, four fuel assemblies with different enrichments are combined to form a set fuel assembly, and the fuel assembly with the lowest reactivity is replaced with a new fuel assembly every cycle. Attempts have been made to rapidly transition to an equilibrium cycle by continuing operation while maintaining the balance. The equilibrium cycle here refers to an operation cycle (hereinafter referred to as the "first cycle") in which operation continues while partially exchanging fuel and using an initially loaded fuel assembly (hereinafter referred to as the "first cycle", after which the fuel is partially exchanged). This is a cycle in which the fuel components in the entire reactor have reached a nearly constant state after a considerable period of time has passed since the previous cycle and the next cycle. A stable cycle with no change in thermal characteristics.

In a nuclear reactor using such a fuel assembly,
At the end of each cycle, the reactor is stopped and the unit fuel assembly with the lowest reactivity is replaced with a new fuel assembly, and the reactor continues to operate, but the thermal characteristics of each cycle If the burn-up is not achieved or the target burn-up is not achieved,
This is not desirable from the safety point of view of the reactor, and also results in a reactor with poor performance economically. In other words, the target reactor must have thermal characteristics and cycle burnup obtained in the intermediate cycle (hereinafter referred to as the "transition cycle") during the transition from the first cycle to the equilibrium cycle that are as good as those in the equilibrium cycle. Preferably, it is stable or quickly converges to these values. However, in conventional fuel assemblies, the correlation between unit fuel assemblies necessary to provide such conditions was not fully understood, so the transition to the equilibrium cycle took a long time, and the output during the transition cycle decreased. The stability was also not satisfactory.

It is an object of the present invention to provide a fuel assembly capable of providing stable characteristics with little cycle-to-cycle variation in terms of thermal properties during the transition cycle and cycle-acquired burn-up.

According to the research conducted by the present inventors, this purpose is to increase the number of unit fuel assemblies (hereinafter referred to as the first to fourth fuel assemblies in descending order of enrichment) in the assembled fuel assemblies to be initially loaded. It has been found that this can be achieved by satisfying specific conditions regarding the degree of enrichment. That is, the fuel assembly of the present invention is a fuel assembly in which four types of fuel assemblies having different enrichments (i.e., first to fourth fuel assemblies in descending order of enrichment) are arranged in a lattice pattern. , the uranium enrichment of the first fuel assembly is 0.71 to 1.0% (wt%; the same shall apply hereinafter unless otherwise specified), and the ratio of the enrichment of the second fuel assembly to the third fuel assembly is 0.65 to 1.0%.
0.72 range, and is characterized in that the enrichment of the fourth fuel assembly is the same as the enrichment of the fuel assemblies to be used after the cycle following the operation cycle in which the above set of fuel assemblies is loaded. .

Hereinafter, the present invention will be explained in more detail with reference to the drawings as necessary.

FIG. 1 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention, taken at right angles to the longitudinal direction.

That is, in FIG. 1, the assembled fuel assembly 10 is
A first fuel assembly 11, a second fuel assembly 12, a third fuel assembly 13, and a fourth fuel assembly 14 whose enrichment levels gradually increase are combined in a lattice shape as shown in the figure, and a control rod 15 is inserted between them. It has a structure in which .

Enrichment of the first fuel assembly 11 (isotope U-
235 weight percent) is greater than or equal to 0.71%, i.e., greater than or equal to the U-235 concentration in natural uranium and less than or equal to 1.0%. The reason why the value is set at 0.71% or more is that to use U-235 weight percent less than 0.71%, depleted uranium would be used, which has rarely been used in commercial reactors, and that this first Since the output of the fuel assembly is too low, other fuel assemblies 12,
This is because the outputs of 13 and 14 tend to become too high. Moreover, when it exceeds 1.0%, this is because the output of the first fuel assembly in the first cycle tends to increase relatively rapidly compared to the output at 0.71% to 1.0%. (See FIG. 8) Furthermore, the enrichment ratio of the second fuel assembly and the third fuel assembly is in the range of 0.65 to 0.72. If this ratio is less than 0.65, it means that the difference in enrichment between the first and second fuel assemblies and the third and fourth fuel assemblies is large, thereby causing There is a strong tendency for the variation in the cycle-obtained burnup to increase (Figures 3 and 5).

If it exceeds 0.72, the thermal characteristics during the transition cycle tend to become unstable as shown in FIG.

Furthermore, the enrichment of the fourth fuel assembly is the same as that of the fuel assemblies used from the second cycle onwards. In this embodiment, the fourth fuel assembly is used as a subsequent replacement fuel assembly, and the design parameters other than the enrichment (the distribution area of the enrichment) are also the same.

The essential nature of the above conditions will be explained below using design data shown in FIGS. 2 to 9. In obtaining the results shown in these drawings, the average enrichment of the above-mentioned fuel assemblies was assumed to be approximately 2.0%, the enrichment of the fourth fuel assembly (new fuel) was approximately 3.0%, and the The enrichment ratio of the three fuel assemblies is represented by the parameter x.

Figure 2 shows the maximum linear power density (unit:
KW/ft = kilowatt/30cm) is taken on the vertical axis. In addition, Figure 3 shows the burnup obtained in each cycle of the transition cycle (unit: MWD/ST = megawatt/day/US ton) under the same conditions, with x on the horizontal axis and the vertical axis. It is ivy. From FIG. 2 and FIG. 3, it can be seen that the range of x in which the thermal characteristics of the transition cycle converge well and the burnup obtained in the cycle is stable is in the range of 0.65 to 0.70.

Similarly, in Figures 4 and 5, when the enrichment of the first fuel assembly is 0.92%, x is taken on the horizontal axis, and the vertical axis represents the maximum linear power density and the cycle acquired burnup at the end of each cycle. , and here also x
Good convergence of thermal properties and stability of cycle-acquired burn-up are obtained when the range is 0.65 to 0.72.

Next, assuming that the enrichment level of the first fuel assembly is 1.10%,
Similarly, Figures 6 and 7 show the relationship between x, maximum linear power density, and cycle burnup.
It is a diagram. Looking at Figures 6 and 7, it can be seen that the cycle-to-cycle fluctuations in the maximum linear power density and cycle-acquired burnup during the transition cycle are large and unstable;
I can't find a good range for x.

Furthermore, Fig. 8 plots the maximum linear power density in the cycle where the maximum linear power density is the largest in the transition cycle, with x being 0.68 and the enrichment of the first fuel assembly being plotted on the horizontal axis. It can be seen that the enrichment of the first fuel assembly is preferably in the range of 0.71 to 1.0% in order to maintain the maximum linear power density at a good value during the transition cycle.

Therefore, from the results shown in Figures 2 to 8 above, the enrichment of the first fuel assembly is 0.71 to 1.0%, and the enrichment ratio x of the second and third fuel assemblies is 0.65. It can be seen that the range of ~0.72 has an essential effect for stabilizing the maximum linear power density and cycle acquired burnup during the transition cycle.

Next, FIG. 9 shows the state of convergence to the equilibrium cycle under the conditions of the first fuel assembly enrichment of 0.92% and x=0.705, which are values within the range of the present invention. That is, FIG. 9 shows the cycle acquisition burnup on the horizontal axis and the maximum linear power density on the vertical axis under the above conditions. If we look at the change in the maximum linear power density for each cycle shown in FIG. 9, we can see that it quickly converges to an equilibrium cycle as it passes through the second and third cycles. In particular, it can be seen that the cycle has almost converged to an equilibrium cycle in the fourth cycle, and that the equilibrium cycle has been sufficiently reached in the fifth cycle.

In determining the concentration of each unit fuel assembly in the fuel assembly of the present invention, it is preferable to do so as follows.

First, based on the annual heat generation requirement of the replacement core and the initial loading core, we will calculate the enrichment of the replacement fuel and the average of the initial loading core when the fuel in the replacement equilibrium core is replaced by approximately 1/4. The degree of enrichment is determined.

Thereby, the fuel assembly of the fourth fuel assembly is determined, and then the enrichment level of the first fuel assembly is approximately estimated in accordance with the request for the amount of heat generated in the replacement core. When the required amount of heat generated by the initial loading core is extremely large,
Close to 1.0% by weight, and 0.71 when the required amount of heat generated is not too large compared to the required amount of replacement core.
Choose a concentration close to % by weight. After that, the second
The third enrichment is divided into the average enrichment of the initially loaded core, the enrichment of the first and fourth fuel assemblies, and 0.65x
Select some of the 0.72 conditions, perform the assembly design, and then decide based on the core characteristics. After looking at the core characteristics determined in this way, we make some further modifications and make a final decision.

As described above, according to the fuel assembly of the present invention, by suppressing the enrichment of the first fuel assembly and the enrichment ratio of the second fuel assembly and the third fuel assembly within a certain range, It is possible to cause a pseudo-equilibrium cycle to appear from the cycle, to speed up the convergence to the equilibrium cycle, and to keep the maximum linear power density and cycle-acquired burnup during the transition cycle in a stable and favorable range. Another advantage is that the core characteristics are stable during each cycle, making it easier to plan operations.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of one embodiment of the fuel assembly of the present invention, and FIGS. 2 to 8 are graphs for explaining the basis of the limiting conditions of the present invention regarding the enrichment of the fuel assembly. FIG. 9 is a graph showing the state of convergence of the maximum linear power density within the limit conditions of the present invention. 1 to 5... Maximum linear power density at the end of the first to fifth cycles, respectively, 10... Group fuel assembly, 11 to
14...first to fourth fuel assemblies, a, b, respectively
...the upper and lower limits of the maximum linear power density, c, d...the upper and lower limits of the cycle-obtained burn-up, , ...the range of x that provides the optimal thermal characteristics and the cycle-obtainable burn-up, (x=
0.65 to 0.72),... Range of enrichment of the first fuel assembly that provides stable thermal properties (0.71 to 1.0%).

Claims (1)

[Claims] 1 A fuel assembly consisting of four types of fuel assemblies each having different enrichment levels arranged in a lattice pattern, where the uranium enrichment level in the fuel assembly with the lowest enrichment level is 0.71.
~1.0% by weight, and the enrichment ratio of the second and third fuel assemblies in descending order of enrichment is set to 0.65.
~0.72, and the enrichment of the fourth fuel assembly is
A assembled fuel assembly for a boiling water reactor, characterized in that the enrichment level is the same as that of a fuel assembly used after the cycle following the operation cycle in which the assembled fuel assembly is loaded.