JPS60165583A - Fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly used in a boiling water nuclear reactor.

[Technical background of the invention and its key points]

A conventional example of a fuel assembly for a boiling water nuclear reactor will be explained with reference to FIGS. 1 to 6. FIG. 1 is a perspective view showing a fuel assembly partially in section. The fuel assembly 1 shown in the figure is a bundle in which a large number of elongated cylindrical fuel rods 2 are bound together, and the upper part of the bundle is connected by an upper tie plate 3, and the lower part is connected by a lower tie plate 4. In this bundle, the fuel rods 2 are maintained at equal intervals by spacers 5. In addition to the fuel rods 2, two quarter ronds (not shown) are incorporated in the bundle. The outer periphery of this bundle is surrounded by a channel box 6, and this channel box 6 is joined to the upper tie plate 3 at the upper part and to the lower tie plate 4 at the lower part.

FIG. 2 shows a cross-sectional view taken along the line A-A of the fuel assembly, in which 156 pieces of fuel s2, 6 pieces of burnable poison-containing fuel 8, and 2 water rods 7 are arranged in 8 rows 8. This is an example of a square arrangement in columns. The fuel rod 2 is a Zircaloy fuel cladding tube 2.
Uranium dioxide (UO21) enriched with uranium-235 in a
Uranium dioxide pellet 2
A plurality of b are loaded in the axial direction, and upper and lower ends a-(
(not shown). The enrichment degree of uranium dioxide in the axial direction of each fuel rod 2 is kept constant.

As described above, this fuel assembly 1 includes two water rods 7 (numbered W in the figure) and six fuel rods containing burnable poison (hereinafter referred to as Gd rods) in order to average the output.
8 is included. These two water rods 7 are arranged diagonally toward the center and are made of Zircaloy tubes so that the coolant flows from the bottom to the top. The Gd rods 8 are arranged at two equal openings from the outermost circumference of the squarely arranged fuel s2. This Gd rod 8 is made of gadolinia-containing pellets, which are made by mixing uranium dioxide with a burnable poison such as gadolinia (Gd, O,) at a concentration of several percent by weight in a Zircaloy fuel cladding tube and baking the mixture into a pellet shape. It is formed by loading a plurality of them in the axial direction. The enrichment degree of uranium dioxide and the amount of gadolinia mixed in the axial direction of each Gd rod 8 are kept constant. Note that the diameters of each fuel rod 2 and Gd rod 8 are the same. The fuel assembly 1 constructed as described above is installed in the reactor core along the grating 9 having a cross-shaped cross section.

The function of the Gd rod 8 will be explained below based on FIGS. 3 to 5. Generally, in light water reactors, fuel is replaced once per batch using a batch replacement method in which fuel is partially replaced.The fuel with a high burnup is removed from the reactor and new fuel is loaded into the reactor core in its place. .

If there were no restrictions on fuel consumption, increasing the fuel enrichment would increase the core reactivity and reduce the refueling frequency or
It is possible to improve burnup by reducing the number of fuel changes. However, the excess reactivity of the reactor core must be within a range that can be compensated for by the control rods 9 due to restrictions on the amount of waste. This control ability, that is, reactor shutdown margin, must be ensured at all times, and therefore an upper limit is set for the surplus reactivity. In the case of such a batch replacement method, burnable poison (
A Gd rod 8 containing fuel mixed with burner-pull poison is used.

Here, Figure 3 shows a graph of surplus reactivity in the core of a boiling water reactor in a low-temperature state depending on the presence or absence of burnable poison, with surplus reactivity on the vertical axis and cycle burnup on the horizontal axis. .

Further, FIG. 4 shows a surplus reactivity diagram when the low temperature state in FIG. 3 is changed to the rated operating state. As shown in Figures 3 and 4, the cycle burnup of both partners is 3000MWD/T.
Under JU, a decrease in reactivity due to burnable poisons is observed, and furthermore, the surplus reactivity decreases by the amount of reactivity decrease from the low temperature state to the rated operating state. Therefore, in the rated operating state, the excess reactivity is compensated for by inserting about 20 to 0% of the control rods 9 of the total control s9. The amount of control rod insertion is approximately proportional to the excess reactivity. In order to maintain the reactor at criticality under rated operating conditions, the amount of control rods inserted is sometimes adjusted according to changes in the burnup of excess reactivity.

Here, Fig. 5(a) shows the surplus reactivity curve depending on the height of the surplus reactivity of the reactor core, and Fig. 5(b) shows the control rod insertion amount adjustment timing corresponding to this. An example of a core with low surplus reactivity (B) is shown in FIG. 5(C). As shown in FIG. 5(a), in a core with a high surplus reactivity (A), the change in burnup of surplus reactivity is large, so the number of pattern adjustments is extremely large as shown in FIG. 5(b). If butter/adjustment increases in this way, it becomes necessary to reduce the output during pattern adjustment, which increases the number of output reductions and reduces the operating rate of the plant. On the other hand, in a core with low surplus reactivity (B), the burnup change in surplus reactivity is small as shown in Figure 5 (a), so the number of pattern adjustments is increased as shown in Figure 5 (C). The reduction in the plant operating rate is smaller than that of a core with a high surplus reactivity (A). As described above, a reactor core in which the change in burnup due to surplus reactivity is small is superior in terms of plant availability and operability. Therefore, under conditions such as fuel enrichment, fuel exchange rate, and cycle burn-up, the amount of reduction in reactivity due to burnable poisons should be adjusted so that the change in burn-up of surplus reactivity is small while maintaining a certain amount of surplus reactivity. I have to adjust. Therefore, at present, Gd rod 8
This was done by adjusting the number of trees. Here, a conventional adjustment example will be explained with reference to FIG. In the figure, in addition to the conventional six Gd rods 8 arranged at the second circumference from the outermost circumference, a seventh-grained Gd rod 8 is provided at the center of the fuel assembly 1. However, in this adjustment example, the surplus reactivity may be too small or too low, and there is a limit to designing a fuel with optimal surplus reactivity characteristics under the above conditions by simply adjusting the number of Gd rods 8. .

That is, in the conventional fuel assembly 1 in which the diameter of the ordinary fuel rod 2 and the diameter of the Gd rod 8 are the same, the amount of reactivity reduction for one burnable poison fuel rod is quite large, so However, sufficient optimization could not be achieved.

[Purpose of the invention]

The present invention has been made in view of the above points, and an object of the present invention is to provide a fuel assembly that can obtain core characteristics in which changes in surplus reactivity due to fuel combustion are small.

[Summary of the invention]

In the present invention, a plurality of fuel rods are arranged in a channel box having a substantially square cross section. are arranged at equal intervals,
and a fuel assembly for a boiling water reactor, in which water rods are arranged in the central portion, and one or more of the fuel rods is a burnable poison fuel rod, and the burnable poison fuel rod is arranged in the fuel assembly. The fuel assembly is characterized in that the radius of curvature of one or more of the fuel rods is different from the diameter of the fuel rod.

In addition, the present invention reduces the number of control rod pattern adjustments by reducing the change in burnup due to excess reactivity.
This is a fuel assembly that can improve the operating rate of nuclear reactors.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 7 to 10. Here, the same parts as in the conventional example are denoted by the same reference numerals, and the explanation of the structure will be omitted. One embodiment of the present invention is
As shown in FIG. 7, in the conventional six Gd rods 8,
The diameter of the two rods labeled G1 is Gd rod 1o, which has a diameter 125 times that of the ordinary fuel rod 2. The Gd rod 8, 1o
is loaded with uranium dioxide mixed with several percent by weight of gadolinia, baked and hardened into pellets.

Next, the operation of this embodiment will be explained based on FIGS. 8 and 9. Here, FIG. 8 is a surplus reactivity diagram comparing the present invention and a conventional example, with surplus reactivity plotted on the vertical axis and cycle burnup plotted on the horizontal axis. In the same figure, the broken line C shows the surplus reactivity in the fuel assembly 1 without Gd rod, the solid line shows the surplus reactivity in the conventional fuel assembly 1 shown in FIG. 2, and the solid line E shows the surplus reactivity in the conventional fuel assembly 1 shown in FIG. The solid line F shows the surplus reactivity in the fuel assembly J of the conventional adjustment example shown in FIG. 7, and the solid line F shows the surplus reactivity in the fuel assembly 1 of the embodiment of the present invention shown in FIG. As shown in the figure, in the curve of the conventional fuel assembly 1, the change in burnup of surplus reactivity changes greatly in the curve H.

That is, there may be cases where the change in burnup of surplus reactivity is not optimized just by selecting the number of Gd rods. On the other hand, in the case of one embodiment of the present invention, as shown by curve F in the figure, the change in burnup of excess reactivity is small and is optimal.

If the content of gadolinia in the uranium oxide pellet is about several percent by weight, the mean free path of thermal neutrons in the Gd rod 8 is 100 to 200 microns, so the Gd
Since neutron absorption by the rod 8 mainly occurs on the surface of the fuel rod, the reactivity effect ΔKGd of the Gd rod 8 is
If they have the same diameter, they are approximately proportional to their number. That is, ΔKGd: CX yrd If the diameters are different, the book=1, where di is the diameter of each Gd rod.

Based on this evaluation, ΔKGd in the fuel assembly 1 of the present invention shown in FIG. 7 and KGd in the fuel assembly shown in the conventional example shown in FIGS. became like
The present invention shows intermediate values between the conventional example and the conventional adjustment example,
Changes in burnup due to excess reactivity are reduced.

Table 1 Here, FIG. 9 shows pattern adjustment points based on the surplus reactivity of the fuel assembly, with cycle burnup plotted on the horizontal axis. Also,
FIG. 9(a) shows an example of the present invention, FIG. 9(b) shows a conventional example, and FIG. 9(cl shows a conventional adjustment example. As shown in the figure, the cycle burnup is In the case of 3000 MWD/T or less, the number of pattern adjustments of the control rods 9 is required less in the conventional adjustment example than in the conventional example. In addition, Table 2 shows the operation rate loss (121, lower EFPDL), which is expressed in terms of rated output days, based on the number of pattern adjustment points. It can be seen that the operating rate loss is reduced by 17 to 50%.

Table 2 Since the fuel assembly of the present invention is configured as described above,
Changes in surplus reactivity due to fuel combustion can be reduced, and furthermore, the number of control rod pattern adjustments can be reduced, so the operating rate of the nuclear power plant γL can be improved. Furthermore, since the number of control rod patterns FAN can be reduced, the burden on the operators of nuclear power generation plants can be reduced.

In the fuel assembly of the present invention, -(Gd
The use of six rods 8 instead of seven has made the genitals better than before, so the number of Gd rods 8 can be reduced, and furthermore, the power distribution of the fuel assembly 1 can be made even more flat. can. Further, according to the present invention, the fuel rod wire power density of the reactor core can be reduced, so the operating rate can be improved and the safety of the nuclear reactor can be improved.

Further, another embodiment of the present invention will be described based on FIG. 10. As shown in the figure, in another embodiment of the present invention, the Gd rod 8 is arranged at the same position as in the fuel assembly 1 of the conventional adjustment example. However, among the Gd rods 8 arranged, two in the second row from the outermost circumference and one in the center of the fuel assembly 1, a total of three, are connected to the diameter of the other fuel rods 2. Gd rod 11 indicated by symbol 02 in the figure is 067 times
I have to. ΔKQd of this fuel assembly 1 is estimated to be 65Cπd according to equation (2), and shows a reactivity effect similar to that of the embodiment of the present invention shown in FIG.

〔Effect of the invention〕

In the fuel assembly of the present invention, two of the six Gd rods arranged on the second circumference from the outermost circumference of the fuel assembly are 1.25 times larger than ordinary fuel rods, or the maximum Two of the six Gd rods placed on the second lap from the outer periphery and one placed in the center of the fuel assembly are 0.67 times larger than normal fuel rods, which reduces the fuel consumption. It is possible to obtain core characteristics with little change in surplus reactivity due to combustion.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway perspective view showing a conventional fuel assembly. Figure 2 is a sectional view taken along the line A-A in Figure 1, Figure 3 is a conventional surplus reactivity diagram in the reactor's low-temperature state, and Figure 4 is the conventional surplus reactivity diagram in the rated state of the reactor. Figure 5 is an explanatory diagram of the reactor in its rated state, Figure 5(a) is the surplus reactivity diagram in the high and low surplus reactivity states of the reactor, and Figure 5C(b) is the diagram of the reactor in the rated state. Fig. 5(c) is an explanatory diagram of pattern adjustment points in a state of high surplus reactivity of the reactor, Fig. 5(c) is an illustration of pattern adjustment points in a state of low surplus reactivity of the reactor, and Fig. 6 is a cross section of a fuel assembly showing a conventional adjustment example. Figure m1 and Figure 7 are cross-sectional views of a fuel assembly showing one embodiment of the present invention.
The figure is a surplus reactivity diagram comparing the conventional example and the present invention in the rated operating state of the reactor, Figure 9 is an explanatory diagram of pattern adjustment points based on surplus reactivity characteristics comparing the conventional example and the present invention. 9(a) is an explanatory diagram of pattern adjustment points in the present invention, FIG. 9 (bl is an explanatory diagram of pattern adjustment points in the conventional example, FIG. 9(C) is an explanatory diagram of pattern adjustment points in the conventional adjustment example, The figure is a cross-sectional view of a fuel assembly showing another embodiment of the present invention. 1...Fuel assembly 2...Fuel rod 6...Channel box 7...Water rod 8,
10.11...Representative for fuel rods containing burnable poison Kensuke Chika, patent attorney (and 1 other person) Figure 1 Figure 2 ■ Figure 3 Figure 4 Figure 5 (O-2 Buifru) Degree (Hsu/Anno (C) Figure 10)

Claims (3)

[Claims] (1) A plurality of fuel rods are bundled and arranged at approximately equal intervals in a channel box whose cross section is approximately square, and a water rond is arranged in the center, and one or more of the fuel rods is flammable. In a boiling water reactor fuel assembly which is a poisonous fuel rod, the diameter of one or more of the burnable poisonous fuel rods arranged in the fuel assembly is different from the diameter of the fuel rod. Featured fuel assembly. (2) Claim 1, characterized in that the two burnable poison fuel rods arranged in the fuel assembly are thicker than the diameter of the fuel rod and 1.25 times or less the diameter of the fuel rod. fuel assembly. (3) The three burnable poison-containing fuel rods arranged in the fuel assembly are thinner than the fuel rods and are 0.67 times or more larger than the fuel rods. Health fee collection.