JPS60262090A - Nuclear reactor - 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] [Field of Application of the Invention] The present invention relates to a nuclear reactor, and is particularly suitable for loading uranium-plutonium mixed oxide fuel and uranium fuel to reduce the number of fuel replacement units. Regarding nuclear reactors.

[Background of the invention]

JP 58-207155 After a nuclear reactor has been operating for about one year, periodic inspections are carried out, and at that time about 1/4 to 1/4 of the fuel assemblies loaded in the reactor core are inspected.
3 is being replaced with new fuel. Refueling is aimed at minimizing the ratio of maximum operating fuel output to core average fuel output (radial peaking), maximizing fuel economy, and refueling. A number of in-core loading patterns are possible even with the same number of refueling bodies, depending on different concepts such as those aimed at minimizing work.

A fuel loading pattern aimed at minimizing fuel exchange operations is disclosed in Japanese Patent Laid-Open No. 58-207155, and will be explained with reference to FIG.

This figure shows the loading pattern of the fuel assemblies in the reactor at the time of the cycle.1 Each small square represents one fuel assembly. In the figure, 1 indicates a fuel assembly in the first cycle of stay in the reactor, and 2 to 6 below indicate fuel assemblies in second to sixth cycles of stay in the reactor, respectively.

In the fuel loading pattern shown in this figure, the fuel loaded during the first cycle of stay in the reactor does not move from that position until it is finally taken out of the reactor. In other words, in the area inside the core where the code in the figure is not surrounded by a 0 frame, stay in the core is 1.
Each fuel does not move from the same position from the 4th cycle to the 4th cycle, and after staying in the 4th cycle, the fuel in the 5th cycle is taken out of the furnace. In addition, in the area around the core where the code in the figure is surrounded by a circle, each fuel does not move from the same position from the first cycle to the sixth cycle when it stays in the reactor, and after the 7th cycle after staying in the reactor for 6 cycles, the fuel stays outside the reactor. It is taken out to. The above-mentioned loading pattern means that during actual fuel replacement work during periodic inspections, new fuel is only loaded at the position after the fuel has been taken out of the reactor, and other fuels cannot move inside the reactor core. This is the optimal fuel loading pattern from the perspective of minimizing fuel exchange work, and has the advantage of shortening periodic inspection periods and improving plant utilization.

2 In the inner core area where the code in the figure is not surrounded by a circle, the fuel that has reached the fifth cycle after staying for four cycles is taken out of the reactor, and in the core outer area where the code in the figure is surrounded by a circle. The reason why the fuel is taken out of the reactor at the seventh cycle after staying for six cycles will be explained below.

Figure 3 shows a reactor core that adopts the fuel loading pattern shown in Figure 2 and is divided into five concentric areas in the radial direction, with a uranium enrichment of approximately 3.3.
The average output per cycle in each region in the equilibrium cycle after loading new fuel and operating for several cycles is:
This is a dispute expressed as a relative output value normalized with the average core output as 1.0.

In FIG. 3, regions 1 to 4 correspond to the inner regions where the symbols are not surrounded by circles in FIG. 2, and region 5 corresponds to the outer peripheral regions where the symbols are surrounded by circles in FIG.

Since the output of the outer core region is only about 50 times the average core power, the fuel loaded in the outer core region is
When the fuel is taken out of the reactor in the 5th cycle after staying in the core for 4 cycles, the take-out burnup is only about 50 times as much as in the case where it stays in the core inner region for 4 cycles and taken out in the 5th cycle. This...I...P7゜Yoyoyaa... Yoeeeeoo! In order to increase the amount, in the inner region of the core it stays for 4 cycles and is taken out in the 5th cycle, whereas in the outer core region it stays for 6 cycles and taken out in the 7th cycle.

On the other hand, in order to effectively utilize plutonium and save uranium, the application of uranium-plutonium mixed oxide fuel (hereinafter referred to as MOX fuel) for light water reactors is being considered.

Figure 5 shows the uranium and island type MOX fuel for BWR.
This shows an example of plutonium distribution. Fuel rods numbered 1 to 4 are uranium fuel rods, and the smaller the number, the higher the weight percentage of 235U. W indicates a water rod, G indicates a fuel rod made of uranium fuel rod and gadolinia, Pi + PR indicates a MOX fuel rod made by mixing plutonium oxide with natural uranium, and P
l is P.

Contains more plutonium than In the island type MOX fuel assembly shown in this example, 230
The U content is 2.20 Wlo, P n f
The enrichment is 1.04W10, which is P2O, and the enrichment is 1.53W10°, and the fissile material weight percentage is 3.24W10. As shown in this example, in an island-type MOX fuel assembly, the fuel rods adjacent to the control rods (the fuel rods on the outer periphery of the control rods are not made of plutonium, but instead are used as uranium fuel), and the control rod value is equivalent to that of the uranium fuel assembly. It is characterized by having a void coefficient.

On the other hand, Fig. 6 shows a design example of a discrete MOX fuel assembly for BWR, and the fuel rods indicated by numbers P, ~P are MOX fuel rods in which natural uranium is mixed with p, o, The smaller the number, the more plutonium it contains. W indicates a water rod, and G indicates a gadolinia-containing MoX fuel rod. In the type MoX fuel assembly,!
A″U content is 0.68W10 P++f enrichment is 2.58W10 P60. Enrichment is 3.78W10 Fissile material weight percentage is 3.26W10.

Discrete MoX fuel assemblies are characterized in that plutonium is mixed with uranium fuel and dispersed throughout all the fuel rods, thereby ensuring a much larger plutonium loading.

However, when loading these MoX fuels into the reactor core, no consideration has been given to reducing the number of fuel replacement units by a loading method that takes advantage of the characteristics of MoX fuels.

[Purpose of the invention]

, The purpose of the present invention is to apply MoX fuel to a reactor core that has an optimal fuel loading pattern from the viewpoint of minimizing the fuel exchange work described above, and to develop a fuel loading method that takes advantage of the characteristics of this MoX fuel. The objective is to provide a nuclear reactor core that reduces the number of fuel replacement units, makes effective use of plutonium, and saves uranium.

[Summary of the invention]

FIG. 4 shows changes in the infinite multiplication factor due to combustion of MoX fuel and uranium dioxide fuel. In FIG. 4, 11 is uranium dioxide fuel, 12 is natural uranium-based MoX fuel,
13 shows the change in the infinite multiplication factor accompanying the combustion of MoX fuel with depleted uranium pace.

In the core shown in FIG. 2, the burnup at which the uranium dioxide fuel assemblies in the peripheral region are removed is approximately 23 GWd/l, and the infinite multiplication factor at the time of removal is approximately 1.00. On the other hand, Mo
The burn-up at which the infinite multiplication factor at the time of extraction of X fuel is approximately 1.00 K, which is the same as that of uranium dioxide fuel, is approximately 26 GWd, as shown in Figure 4.
/l, MoX fuel can be kept in the core longer than uranium dioxide fuel. In the reactor shown in Figure 2, the incremental burnup of the fuel loaded in the outer peripheral region during one cycle is approximately 40 Wd/C.
Therefore, MoX fuel can be loaded in the core for one cycle longer than uranium dioxide fuel.

Furthermore, since MoX fuel contains uranium and plutonium, which has a large thermal neutron absorption cross section, when loaded into a normal boiling water reactor (hereinafter referred to as BWR), the thermal neutron flux is insufficient. Therefore, it is necessary to increase the moderator. Since water, which is a moderator, exists in the outer core region as well as in the core inner region, loading MoX fuel in the core outer region has the same effect as loading MoX fuel in the core inner region. You can take advantage of it.

Therefore, the loaded fuel does not move until it is finally taken out of the reactor (referred to as a no shirt full core).
In this case, loading MoX fuel into the outer peripheral region of the core is the best way to take advantage of the characteristics of MoX fuel. Based on this idea, examples will be described below.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. 1st
The fuel loading pattern shown in the figure is almost the same as the fuel loading pattern in Figure 2. In the figure, numeral 1 is the fuel assembly in the first cycle of stay in the reactor, and hereinafter, numerals 2 to 7 are the fuel assembly staying in the reactor, respectively. The fuel assembly of the 2nd to 7th cycles is shown. The loaded fuel does not move from its position until it is finally removed. That is, the symbol 15 in FIG.
In the area not enclosed by the frame, during the period of 4 cycles, and
7 in the area surrounding the code 14 with an O frame.
During the cycle, fuel is burned at the same location and then removed from the furnace.

In Fig. 1, the areas inside the core whose symbols are not circled are:
Fuel with an initial uranium enrichment of about 3.3% by weight is loaded, and the area around the core surrounded by a circle is a natural uranium-based MoX whose infinite multiplication factor accompanying combustion is shown in Figure 4.
Load fuel. In the core shown in Figure 2, the burnup at which uranium dioxide fuel is extracted from the surrounding area is approximately 23 GWd.
/l, and the infinite multiplication factor at the time of extraction is about i,oo.

On the other hand, the infinite multiplication factor of MoX fuel when extracted is about 1.0, which is the same as that of uranium dioxide fuel. The burnup that becomes Oo is approximately 2 as shown in Figure 4.
It is 6GWd/l. For this reason, the uranium dioxide fuel loaded in the outer peripheral region of the core, which is surrounded by the symbol ○, had an incremental burnup of about 40 Wd/l during one cycle, so it was removed at the seventh cycle after staying for six cycles. , MOX fuel based on natural uranium can be taken out in the seventh cycle of eight cycles after staying in the reactor core for seven cycles. As a result, the number of fuel replacement bodies loaded in the outer peripheral area of the core can be reduced to 1.
Approximately 2 lines per cycle can be reduced.

Next, an example will be described in which, instead of MOX fuel based on natural uranium, MOX fuel based on depleted uranium, whose infinite multiplication factor accompanying combustion is shown in FIG. 4, is loaded.

In this case as well, as in the case of loading the natural uranium-based MOX fuel, the number of replacement fuels loaded in the outer peripheral region of the core can be reduced by about 2 per cycle.

Furthermore, since depleted uranium is used instead of natural uranium,
It is even more effective in saving uranium.

[Effects of the Invention] According to the present invention, in a shirtless core, by loading MOX fuel in the outer peripheral region of the core, the number of refueling bodies can be reduced by about 2, and the effective use of plutonium can be reduced. Utilization and uranium conservation can be achieved.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a fuel loading pattern according to an embodiment of the invention, Fig. 2 is a diagram showing a fuel loading pattern of a conventional nuclear reactor, and Fig. 3 is a diagram showing each radial region of the conventional nuclear reactor shown in Fig. 2. Figure 4 is a diagram showing the change in infinite multiplication factor due to combustion of uranium dioxide fuel and MOX fuel, and Figure 5 is a diagram showing the relative output value of uranium dioxide fuel and MOX fuel assembly for boiling water reactors.
FIG. 6 is a diagram showing an example of uranium/plutonium distribution in a WMOX fuel assembly for a boiling water reactor. 11... Change in infinite multiplication factor due to combustion of uranium dioxide fuel, 12... Change in infinite multiplication factor due to combustion of MOX fuel based on natural uranium, 13... Change in infinite multiplication factor due to combustion of MOX fuel based on depleted uranium. Change of infinite multiplication factor associated with A8.14
,! P1. Mj! l#-t' M OX! Rd(!!?
['j ah' region, 15...A region where uranium dioxide fuel is loaded in the core inner region. Agent Patent Attorney Takahashi F! A6 Husband 2nd area Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. In a nuclear reactor core loaded with a large number of fuel elements, fuel assemblies and combinations, the core is divided into an inner region and an outer peripheral region, and the loaded fuel elements are kept in the same position in the core for a predetermined number of cycles. A nuclear reactor loaded according to a fuel loading pattern that remains in place, and characterized in that the inner region is loaded with uranium dioxide fuel and the outer region is loaded with uranium-plutonium mixed oxide fuel.