JPS61292590A - 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 using heavy water and light water as neutron moderators, and particularly to a nuclear reactor with improved fuel economy compared to conventional reactors.

[Background of the Invention] A nuclear reactor that generates electricity using the heat generated from the nuclear fission reaction between neutrons and atomic nuclei such as uranium and plutonium requires fuel containing fissile material, a coolant for removing heat,
It consists of a moderator that slows down neutrons and structural materials that make up the reactor. Light water is used as a moderator or coolant.

These include heavy water, gas, and sodium, and are respectively called light water reactors, double heavy water reactors, gas reactors, and liquid metal cooled reactors. Light water reactors and heavy water reactors both use light water and heavy water, which have strong neutron moderating abilities, so neutrons produced with high-speed energy through nuclear fission lose energy through collision with moderator nuclei and become low-energy. They exist in the reactor core in the form of thermal neutrons. However, the moderator-to-fuel ratio in the JI subreactor has a significant impact on core characteristics. Additionally, core characteristics vary greatly depending on whether light water or heavy water is used as a moderator.

Figure 2 shows changes in the infinite multiplication factor and fuel conversion ratio (fissile material produced/fissile material reduced), with moderator/fuel on the horizontal axis, for nuclear reactors with light water and heavy water as moderators. The following was shown. In light water reactors, the infinite multiplication factor is larger than in heavy water reactors, and tends to increase as the moderator-to-fuel ratio increases.

However, the conversion ratio is conversely smaller than in heavy water reactors and decreases as the moderator-to-fuel ratio increases. BW, a light water reactor
In R and PWR, the ratio of light water to fuel is around 2, and in ATR, which is a single heavy water reactor, the ratio of heavy water to fuel is around 8.

As can be seen from Figure 2, light water reactors have the advantage of having a large infinite multiplication factor and a small ratio of light water to fuel, so less fuel is required to generate a certain amount of output, and the reactor core can be smaller. It is. However, due to the low fuel conversion ratio, fuel economy is poor and a large amount of uranium resources are required.

In addition, in order to prolong operation, highly enriched fuel is indispensable, which increases fuel costs. At the same time, excess reactivity increases at the beginning of operation, making it necessary to add burnable poisons such as gadolinia, which reduces core operation. Problems arise such as reduced reliability and fuel economy. An example of a PWR fuel loading pattern is shown in Figure 3 (``Methods and Analysis of Nuclear Fuel Management'' by HoW Graves), but unless complex fuel shuffling is performed, the power distribution will not be flattened, making it difficult to improve core management. Heavy water reactors have the advantage of having a high fuel conversion ratio and better fuel economy than light water reactors, but they have a small infinite multiplication factor and the ratio of heavy water to fuel is low. The large ratio increases the amount of fuel required to produce a given power output, increases the core size, and increases plant costs.

As mentioned above, both light water reactors and heavy water reactors have their own characteristics, but at present it is difficult to say that core characteristics that fully utilize the strengths of light water and heavy water have been achieved. be.

[Purpose of the invention]

An object of the present invention is to provide a nuclear reactor that has a high fuel conversion ratio, is small in size, can be operated in a long cycle, has low plant cost, and is excellent in fuel economy.

[Summary of the invention]

The basic idea of the present invention is to achieve the above object by effectively utilizing the respective advantages of light water and heavy water, which are coolants for nuclear reactors. That is, it utilizes the high conversion ratio characteristics of a single heavy water moderator and the high reactivity characteristics of a light water moderator.

The core of a nuclear reactor is divided into two in the radial direction, and the moderators in the inner and outer regions are heavy water and light water, respectively. In the present invention, first, during the first half of the stay in the reactor core, fuel assemblies are loaded into the inner region where the conversion ratio is high to generate heat through combustion and to accumulate plutonium. Then, in the latter half of the stay in the core, the fuel assemblies whose reactivity has decreased due to combustion are moved to the outer high reactivity area, and the accumulated plutonium is effectively heated. In this manner, the present invention makes full use of the convertibility characteristic of nuclear fuel to increase fuel utilization efficiency.

Further, according to the present invention, since a high reactivity region is provided in the periphery of the reactor core where the contribution of neutron leakage is large and the power distribution decreases, it is possible to simultaneously achieve flattening of the power distribution.

[Embodiments of the invention]

The present invention will be explained in detail based on examples.

FIG. 1 shows an embodiment of the present invention. This figure shows the fuel loading pattern for a nuclear reactor.

It is divided into an inner region with heavy water as moderator 1 and an outer region with light water as moderator 2. The inner region is loaded with fuel for 1 to 3 years of stay, and the outer region is loaded with fuel for 4 years of stay. Fuel is loaded. That is, this embodiment is for a core loaded in four batches (one quarter of the fuel in the entire core is replaced).

Figure 4 shows the change in the infinite multiplication factor of the fuel assembly due to combustion. The characteristics of the fuel assembly in the conventional core are shown by the broken line.The new fuel when loaded into the core has a high infinite multiplication factor, but it monotonically decreases as it burns and is removed from the core at the end of the fourth year. The fuel assembly of the present invention shown in this figure has an average enrichment lower than that of the conventional example in order to match the operating period with that of the conventional reactor core.

The infinite multiplication factor of the present invention is for fuel that stays for 1 to 3 years,
This value is lower than in conventional cores due to the lower average enrichment and the loading of heavy water into the low-reactivity region (inner region). However, the rate of decrease in the infinite multiplication factor due to combustion is small, and because the reactor is in a high conversion ratio region, it can be seen that more plutonium is accumulated than in conventional cores. Then, in the fourth year, it is loaded into the high reactivity region (outer region), so the infinite multiplication factor increases greatly, as shown in the figure. The above-mentioned plutonium accumulation effect also contributes to this increase in the infinite multiplication factor. In this way, since the fourth-year fuel in the outer region shares most of the core reactivity, operation with the same cycle length as the conventional core can be achieved even if the fuel enrichment is lowered. In addition, since the infinite multiplication factor of the outer core is increased, the radial power distribution is flattened, eliminating the need for fuel shuffling or control rod insertion to flatten the power distribution as in conventional light water reactors.
There is no need to separately produce highly enriched fuel for the outer regions, as in conventional heavy water reactors.

As described above, according to the present invention, when operating the same cycle length as a conventional core, it is possible to reduce fuel costs and save uranium resources by lowering the fuel enrichment, and to operate without fuel shuffling. It is possible to improve the efficiency and simplify core operation by reducing the number of insertion controls.

Figure 5 shows the change associated with combustion of the infinite multiplication factor when the effect of the present invention is applied to lengthen the operating cycle by using the same enrichment as the conventional core without lowering the fuel enrichment. be. By the same principle as above.

A longer cycle can be achieved.

In this case as well, the effects of the present invention are the same as described above.

In the above explanation, heavy water is used in the inner region, but if it becomes necessary to increase the reactivity in this region, a mixture of heavy water and light water can also be used as a moderator. . In this case, the conversion ratio in the inner region deteriorates a little, so the effects of reducing fuel costs and saving uranium resources are reduced, but effects such as flattening the power distribution and simplifying core operation can be achieved.

FIG. 6 shows a fuel loading pattern when the present invention is applied to a 3-batch loaded core, and FIG. 7 shows a fuel loading pattern when the present invention is applied to a 5-batch loaded core. As you can see from these examples,
The present invention is applicable regardless of the number of batches.

〔Effect of the invention〕

According to the present invention, it is possible to reduce fuel costs and save uranium resources by reducing fuel enrichment or prolonging the operating period. Furthermore, since the power distribution can be flattened, fuel shuffling is no longer necessary, improving operating efficiency, reducing the number of control rods inserted, and simplifying core operation.

[Brief Description of the Drawings] Fig. 1 is a fuel loading pattern diagram showing one embodiment of the present invention, Fig. 2 is a nuclear characteristic diagram of a light water reactor and a heavy water reactor when the moderator to fuel ratio is changed, FIG. 3 is a fuel loading pattern diagram of a conventional PWH, FIGS. 4 and 5 are combustion characteristic diagrams of the fuel of the present invention, and FIG. 6 is a diagram. FIG. 7 is a pattern diagram showing another embodiment of the present invention. 1... Heavy water moderator, 2... Light water moderator.

Claims (1)

[Claims] 1. In a nuclear reactor configured such that fuel containing fissile material and a coolant that removes heat generated by the fuel flow between the fuel, the core region of the reactor is divided into two regions in the radial direction, and an inner region is divided into two regions. A nuclear reactor is characterized in that the core is filled with heavy water, the outer region is filled with light water, fuel for the first half of the stay period is loaded into the inner region, and fuel is loaded into the outer region during the second half of the stay period.