JPH02184791A - High conversion and high combustion pressure tube type nuclear reactor - Google Patents

Abstract

PURPOSE:To utilize fuel effectively by stacking an upper and a lower fuel assembly axially across a connection part detachably, and arranging an inert gas area and a heavy water moderator area in a calandria tank. CONSTITUTION:The fuel assembly 7 in a pressure tube 4 is formed by stacking the upper fuel assembly 7a and lower fuel assembly 7b in two stages across the connection part 10. The coupling part 10 is arranged at the border between the heavy water moderator area 5 and inert gas area 6 in the calandria tank 1, but the water level of the heavy water moderator can be varied properly. Consequently, the ratio of the part of the assembly 7a which is converted at a high level in the area 6 and the part of the assembly 7b which is put in high fuel operation in the area 5. The coupling end parts are formed at the upper and lower end parts of the assemblies 7a and 7b and when a fuel exchange is made, the assembly 7a is taken out and a new fuel assembly is loaded and reloaded in a core, so that the assembly 7a is brought into high combustion.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a heavy water-moderated pressure tube nuclear reactor that can achieve high conversion and high burnup in order to effectively utilize nuclear fuel.

[Conventional technology]

In heavy water-moderated nuclear reactors that use heavy water as a moderator, which has the best nuclear properties among many moderators, if the coolant is other than heavy water, it is necessary to separate the coolant from the moderator. It is conventionally known that a pressure tube type heavy water reactor is employed. Usually, a pressure tube heavy water reactor is
A calandria tank that stores heavy water moderator and inert gas inside, a large number of calandria pipes arranged in the vertical direction of the calandria tank, and a heavy water moderator that passes through the calandria pipes and distributes the heavy water moderator inside. It has a furnace body consisting of a pressure pipe and a fuel assembly inserted into the pressure pipe. In order to achieve high conversion of the new fissile material produced by the neutron absorption reaction in the pressure tube type heavy water reactor described above, it is necessary to harden the neutron energy spectrum, specifically by reducing the lattice pitch or voiding the heavy water moderator. Although it is necessary, reducing the lattice pitch makes it difficult to install pressure pipes, voiding the heavy water moderator is difficult due to criticality issues if all the heavy water moderator is voided, and partial voiding poses a problem in terms of core stability. . On the other hand, in order to increase the burnup of nuclear fuel, it is necessary to increase the amount of heavy water moderator, but in this case, the coolant void coefficient deteriorates, which poses a safety problem. In this way, attempting to achieve both high conversion and high burnup at the same time requires different core configurations. So far, a method has been considered in which the reactor core is divided into two regions: an inner gas region and an outer heavy water region, and the fuel assemblies, which are highly converted in the inner gas region, are combusted at a high rate in the outer heavy water region. .

[Problems to be Solved by the Invention] As mentioned above, in order to divide the reactor core into two, the inner gas region and the outer water region, a double tank configuration is required.1 However, the area occupied by the reactor core is expanded. This poses a structural problem as the pressure vessel is made larger. There was also a problem in terms of neutron economics, where it is important to minimize unnecessary absorption and leakage of neutrons. An object of the present invention is to provide a high conversion/high combustion pressure tube nuclear reactor that can utilize fuel effectively without changing the structure of the above-mentioned ordinary pressure tube heavy water reactor.

[Means to solve the problem]

In order to achieve the above object, in the present invention, the fuel assembly inserted into the pressure pipe has a structure in which an upper fuel assembly and a lower fuel assembly are stacked in the axial direction via a connecting part so as to be detachable. The calandria tank is arranged in an inert gas region and a heavy water moderator region, and a high conversion region and a high combustion region are arranged above and below. It is preferable to form connection parts at the upper and lower ends of each fuel assembly to which other fuel assemblies can be connected.

[For use]

The upper fuel assembly disposed in the inert gas region where there is no heavy water is highly converted by high-energy neutrons, and the lower fuel assembly disposed in the heavy water region is highly combusted by low-energy neutrons. By changing the water level of the heavy water moderator in the calandria tank, the ratio of the high conversion region and the high combustion region arranged above and below can be varied. When the upper fuel assembly, which has been highly converted in the inert gas area at the top of the core, is then reassembled to the lower fuel assembly position and loaded again, it is burned at a high rate in the heavy water area at the bottom of the core, resulting in a high conversion rate.・High burnup can be achieved.

【Example】

In the embodiment shown in the drawings, a calandria tank 1 surrounded by a heavy water reflector 2 has a heavy water moderator region 5 and an inert gas region 6 such as helium or nitrogen. A fuel assembly 7 is arranged in a pressure pipe 4 inserted into a large number of calandria pipes 3 provided vertically through the calandria tank 1, and fuel flows into the pressure pipe 4 from an inlet pipe 8 at the bottom of the reactor core. The coolant boils in the reactor core and exits the outlet pipe 9.
It's starting to leak more. A fuel exchange device installed below the reactor and a seal plug attached to the lower end of the pressure pipe 4 to enable fuel exchange even during operation are not shown. The fuel assemblies 7 in the pressure pipe 4 are stacked in two stages, including an upper fuel assembly 7a and a lower fuel assembly 7b via a connecting portion 10. The connecting portions 10 are preferably formed at the upper and lower ends of each fuel assembly 7 a s 7 b for reasons described later. In the figure, the connecting portion 10 is arranged at the boundary between the heavy water moderator region 5 and the inert gas region 6 in the calandria tank 1, that is, at almost the same level as the water level of the heavy water moderator in the calandria tank 1. However, the water level of the heavy water moderator can be changed as appropriate. As a result, the ratio of the portion of the upper fuel assembly 7a located in the gas region 6 and subjected to a high conversion action to the portion of the lower fuel assembly 7b located in the heavy water moderator region 5 and subjected to a high combustion action can be changed as appropriate. If the connecting portions 10 are formed at the upper and lower ends of each of the fuel assemblies 7a and 7b, the lower fuel assemblies 7
b is taken out and placed in a spent fuel pool (not shown), then the highly converted upper fuel assembly 7b is taken out, a new fuel assembly is installed on top of it, and then reloaded into the reactor core. The upper fuel assembly 7a, which has been rearranged into the lower part, can now be highly combusted. Incidentally, when mainly aiming at high conversion, there is no need to rearrange the upper fuel assembly 7a in the upper part of the core so that it is facing downward. By extracting the residual material and reprocessing it, highly converted Pu is obtained, and this Pu is supplied as fuel for other reactor types.

【Effect of the invention】

According to the present invention, since the high conversion region and the high combustion region are arranged above and below, the reactor structure can be the same as the conventional one. Problems and neutron economic problems will be eliminated. Moreover, by adjusting the water level of the heavy water moderator in the calandria tank, the upper fuel assembly part is placed in the inert gas area and receives a high conversion effect, and the lower part is placed in the heavy water area and receives a high combustion effect. The ratio of the fuel assembly portions can be changed as appropriate. Furthermore, the upper fuel assembly, which has achieved high conversion in the inert gas region at the top of the core, can be rearranged and placed in the heavy water region at the bottom of the core, making it possible to achieve high burnup. can.

[Brief explanation of the drawing]

The figure is an explanatory diagram of a high conversion, high combustion pressure tube reactor according to the present invention. DESCRIPTION OF SYMBOLS 1... Calandria tank, 3... Calandria pipe, 4... Pressure pipe, 5... Heavy water moderator area, 6...
Inert gas region, 7... Fuel assembly, 7a... Upper fuel assembly, 7b... Lower fuel assembly, 10... Connection portion.

Claims (1)

[Claims] 1. A calandria tank that stores a heavy water moderator and an inert gas inside, a large number of calandria tubes arranged in the longitudinal direction of the calandria tank, and a calandria tube arranged to penetrate inside the calandria tubes. In a pressure tube nuclear reactor, the reactor body is composed of a pressure tube through which coolant flows, and a fuel assembly inserted into the pressure tube.The fuel assembly inserted into the pressure tube is The upper fuel assembly and the lower fuel assembly are stacked in the axial direction via a connecting part so that they can be attached and detached freely,
The upper fuel assembly and the lower fuel assembly are arranged in the inert gas region and heavy water moderator region in the calandria tank, and the high conversion region and the high combustion region are arranged above and below. A high-conversion, high-combustion-pressure tube nuclear reactor. 2. The high conversion, high combustion, heavy water moderated pressure tube nuclear reactor according to claim 1, wherein the upper and lower ends of each fuel assembly are formed with connecting portions to which other fuel assemblies can be connected.