JPS6038674B2 - fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel assembly used in a boiling water nuclear reactor (B "VR"), and particularly to a fuel assembly loaded into a reactor core that performs load following operation.

As the proportion of nuclear power generation in the total amount of power generation increases, instead of so-called standard load operation in which nuclear reactors are operated at a constant load, load following operation that takes into account day and night load demands is required.

In conventional BWRs, in order to increase the thermal margin during base low load operation, approximately 1/1/2 of the effective fuel length from the bottom end of the fuel assembly is
The power distribution is flattened by using a fuel assembly with a high gadolinia concentration in the second part or a fuel assembly with a low enrichment in that part. An example of the former is Samurai Seki 54-1.
Publication No. 9093, and an example of the latter is Chikai Sho 53-401.
There are inventions described in Publication No. 81 and No. 401, and with these inventions, the operating limit value of the maximum linear power density (13. W/night for an 8×8 fuel assembly) during basic low load operation can be improved. It is now possible to operate with a margin of 15% or more. However, in operations that involve output changes such as load following operations,
Envelope output is the limiting value rather than operating limiting value such as maximum linear power density. Envelope output is the preconditioned output before the start of output change operation.
(11 KW/ft for fuel in the early stage of combustion, W/ft for fuel in advanced combustion). BWR
In order to maintain fuel integrity, the reactor is operated by adjusting the core output so that the output during load following operation is equal to or less than the envelope output. FIG. 1 shows an example of a load request, in which time (h) and output (%) are plotted on the machine axis and vertical axis, respectively, and t2 and t3 are respectively plotted.
It shows daytime, nighttime, and daytime/nighttime switching times, and this load request is for 100% output for 12 hours during the daytime and 60% output for 8 hours at night.

In BWR, core coolant flow rate (hereinafter referred to as core flow rate)
The core power can be changed by increasing or decreasing . FIG. 2 shows the distribution of the infinite multiplication factor (K) of the fuel assembly described in the above-mentioned Japanese Patent Application Laid-Open No. 53-40188. There are many axial isotopic layers, and the enrichment of parts A and B is 2, respectively.
．． Its infinite multiplication factors (K) at 1% by weight and 1.9% by weight are 1.1232 and 1.0857, respectively.

Figure 3 shows an example of operation in which a nuclear reactor with a power density of 50 KW loaded with fuel assemblies having an infinite multiplication factor distribution as shown in Figure 2 is controlled by the core flow rate to achieve the load requirement described above. The horizontal axis is time (h), and the vertical axis is output,
The core flow rate (both in %) and △P (KW/ft) are taken, and D, E and F are the power, core flow rate and △, respectively.
It shows P. In other words, in the fuel assembly used in this operation example, the enrichment from the bottom end of the fuel to 11/24th of the effective fuel length is lower than that at the top, and the infinite multiplication factor (K) distribution shown in Figure 2 is However, the amount of excess (△P) from the envelope output during load following operation becomes the largest approximately 6 hours after returning to 100% output, and in this operation example, it is approximately 0. View W/ft. Therefore, from the viewpoint of maintaining fuel integrity, if the plant is operated with a lower high output return level so as to reduce the excess amount to zero, the plant utilization rate will decrease. The present invention eliminates such conventional drawbacks and
The purpose of this fuel assembly is to provide a fuel assembly that can maintain fuel integrity during load following operation and improve plant utilization, and is used in boiling water reactors. Approximately 1 of the effective fuel length from its lower end
There is a dividing point of infinite multiplication factor in the area between /8 and 7/24, and the average infinite multiplication factor of the area below this dividing point is smaller than the average infinite multiplication factor of the area directly above this dividing point. It is characterized by the fact that

Figure 4 shows the power distribution at the time when the envelope excess amount is maximum in the operation example shown in Figure 3.
/ft), G and day indicate the output after preconditioning and the output after returning to high output, respectively, and 1
Hedgehog, showing value.

The power distribution at the time when the envelope excess amount is maximum is skewed downward from the furnace compared to the preconditioned power distribution, and the maximum envelope excess amount is at a point about 1/8 of the effective fuel length from the lower end of the fuel. It is occurring in This is primarily due to the reduction in core flow rate to compensate for the increased reactivity due to xenon combustion. That is,
When the core flow rate decreases, the void generation point moves, causing the power distribution to skew downwards in the core. Figure 5 shows the ratio of the power distribution P(x) in the axial direction of the fuel assembly when the core flow rate decreases to the power distribution Po(x) at the rated flow rate, that is, f(x) = P(x)/ Po(x). ．．
．． ．． ．． ．． '1) is shown.

Here, x is the axial position, and P and Po are standardized so that the output of the fuel assembly is equal. The axis of the figure and the vertical axis are the direction of the ball, f (=P/Po), respectively.
is taken, and J, K and L each have a flow rate of 90%.
, 80% and 70% cases are shown. At the point when the amount of excess from the envelope output is at its maximum, the influence of the xenon distribution on the amount of excess from the envelope is small, and there is almost no variation in the power distribution in the core radial direction during preconditioning and load following operation.

In particular, since the difference in the radial power distribution during high output return and preconditioning is less than 1%, it is assumed that the output of the fuel assembly is equal when the output level returns to the same as that during preconditioning. good. Therefore, the deviation ΔP'(
x) (' is added to distinguish that the threshold value is not taken into account) can be expressed as follows from the {1} formula. △P(x)
~Po(x). (f(x)-1).・・・． ...(2}f
(x) depends on the void coefficient of the fuel, etc., and the distribution above the void generation point changes depending on the fuel, but the maximum value below the fuel changes by about 2%.

Therefore, in order to reduce the excess amount from the preconditioning output, the output Po' of the part where f(x) exceeds 1 must be
It is necessary to reduce (x). The portion where f(x) exceeds 1 is the portion from the lower end of the fuel assembly to approximately 7/24 of the effective fuel length, as shown in FIG. Therefore, in order to reduce the output ratio of the above part, divide the effective fuel length from about 7/24 to about 1/8, where f takes a large value, and divide the average infinite multiplication factor below the dividing point. It is sufficient to make it smaller than the average infinite multiplication factor above the point, and by doing so, the objective can be achieved.

Furthermore, if we flatten the power distribution by increasing the infinite multiplication factor of the threshold below the reactor core, the part with the power below the threshold value, we can reduce the power of the part with the power above the threshold value. I can do it.

Hereinafter, the present invention will be explained in detail with reference to Examples. FIG. 6 shows, as an example, an embodiment in which the present invention is applied to the fuel assembly described in the aforementioned Tokukan Sho 53-40188 publication. The key and vertical axes of the figure are, respectively,
The axial position of K is taken, and the enrichment of parts A, B and C is 2.1% by weight, 1.9% by weight and 1.8% by weight.
It is. That is, in this fuel assembly, the enrichment from the lower end of the fuel to 1/24 of the effective fuel length is 2.1% by weight, and the enrichment is 1/24% by weight.
24~2/24 concentration 1.9% by weight, 2'24~1
/4 concentration is 1.8% by weight, therefore, the infinite multiplication factor (K) is 1.1232 in part A, 1.0857 in part B, axial position 1/C at the bottom of the border, The average of parts B and A is 1.0814. In other words, when compared under a constant void ratio, the average value of the infinite multiplication factors of these parts is equal to It is horizontally formed so that it is smaller than the magnification. FIG. 7 shows an example in which load follow-up operation was performed based on the load request shown in FIG. 1 in a nuclear reactor with a power density of 50 kW/1 loaded with a fuel assembly having such a configuration.

Time (h), output (%), and △P (kW/ft) are plotted on the mechanical axis and vertical axis of this figure, respectively, where M is the output, and N and 0 are the inventive and conventional ones, respectively. △P of the case
It shows. From this figure, when the fuel assembly of the example is used, the maximum excess amount from the envelope output is 0.
7kW/ft, which is approximately 20% lower than the conventional case. FIG. 8a shows a fuel assembly in which the present invention is applied to a fuel assembly having no enrichment distribution in the axial direction.

Figure 8b shows a uniform concentration fuel assembly shown for comparison; in each figure, the horizontal and vertical axes indicate the axial position of K, and the axial positions of A, B, and C shows a concentration of 2.1% by weight, 1.9% by weight, and 1.8% by weight, respectively, so its infinite multiplication factor (of K) is 1.0857 in part B, and The average of C, B, and A sections below the directional position 1/mountain border is 1.814. The envelope excess amount in this embodiment is 1.0 water W/ft, whereas in the conventional case shown in FIG. 8b, it is 1.4 water W/ft, so the envelope excess amount is about 25. It has been reduced by 9%. In the above embodiment, the infinite multiplication factor distribution was achieved by changing the concentration, but the same effect can be obtained by changing the concentration of gadolinia.

As explained above, by using the fuel assembly described in the example, it is possible to reduce the excess amount from the envelope output, and even when reducing the core output from the viewpoint of maintaining fuel integrity, the amount of decrease can be reduced. can.

Therefore, the plant utilization rate is improved, and the economic effect is large. As described above, the fuel assembly of the present invention maintains fuel integrity during load following operation and makes it possible to improve the plant utilization rate, so it has great industrial effects.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of a load request in load following operation, Fig. 2 is a diagram showing an infinite multiplication factor distribution of a conventional fuel assembly, and Fig. 3 is a diagram showing the fuel assembly of Fig. 2. Figure 4 is a diagram showing the characteristics of an example of load following operation using FIG. 6 is a diagram showing the ratio of the output distribution to the output distribution at rated flow rate, and FIG. 7 is a diagram showing the infinite multiplication factor distribution of an embodiment of the fuel assembly of the present invention.
This figure is a diagram showing the characteristics of an example of load following operation using the fuel assembly of FIG. 6, and FIG. 8a is a diagram showing the infinite multiplication factor distribution of another example of the fuel assembly of the present invention. , and FIG. 8b is a diagram showing the infinite multiplication factor distribution of a fuel assembly with uniform enrichment for comparison. Figure 1 Figure 2 Figure 3 *Figure 4 Figure 5 Figure 6 Figure 8 Figure 8

Claims (1)

[Claims] 1. In a fuel assembly used in a boiling water reactor,
Approximately 1/8 and 7/2 of the effective fuel length from the lower end of the fuel assembly
It is characterized by having a dividing point with an infinite multiplication factor in the area between 4 and 4, and the average infinite multiplication factor of the area below the dividing point being smaller than the average infinite multiplication factor of the area directly above the dividing point. fuel assembly. 2. The fuel according to claim 1, wherein the region below the dividing point is divided into two or more partial regions, and the infinite multiplication factor of each portion toward the lower end of the fuel assembly increases toward the lower end. Aggregation. 3. The fuel assembly according to claim 1, wherein the infinite multiplication factor in the region below the dividing point is constant.