JPS62893A - Core for light water type 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 [Technical Field of the Invention] The present invention relates to a core for a light water nuclear reactor in which fuel assemblies are arranged in a lattice pattern.

[Technical background of the invention and its problems] The core of a conventional light water nuclear reactor will be explained using the core of a boiling water reactor. Here, FIG. 5 is a schematic plan view of the core of a boiling water reactor, and in the figure, the core 1 of the boiling water reactor is inside a shroud 4 installed in a reactor pressure vessel (not shown). is housed in. This core 1 is composed of a fuel assembly 2 and first and second control rods 3a and 3b. The fuel assemblies 2 are usually a set of four and are supported by fuel support fittings (not shown), and in the gaps between the four fuel assemblies 2 are the first control rods that are pulled out of the reactor core 1 during operation. 3b
A second control rod 3a, which is inserted into the reactor core during operation, is arranged.

When steady operation is performed with the above configuration, the second control rod 3a is inserted into the reactor core, and this control rod 3a
The operation is controlled by absorbing excess neutrons in the reactor core.

Currently, from the perspective of effective use of uranium, the use of moderators is reduced to suppress the effect of slowing down the neutron velocity, and accordingly, the amount of neutron absorption by U238 is increased to increase plutonium production and increase reactivity gain. A plan has been proposed to try to do so. However, in the current reactor core configuration, fast neutrons (nf) generated by nuclear fission collide with hydrogen atoms in light water, which is a coolant, to become thermal neutrons (n), and these thermal neutrons (nt) cause U23 to undergo nuclear fission. The structure is such that it generates heat. For this reason, U233 is Pu
There were no medium-speed neutrons (nm) 4;t to be converted to 23q, and there was a limit to the production of Pu.

[Object of the Invention] The present invention is to obtain a core for a light water reactor that can promote the production of plutonium and make efficient use of uranium.

[Summary of the invention]

The present invention provides a core for a light water reactor consisting of a large number of fuel assemblies arranged in a lattice pattern, in which a water exclusion rod that can be inserted into and removed from the core is arranged, and the water exclusion rod has a core material of magnesium,
A core for a light water nuclear reactor is characterized in that at least one member of magnesium oxide or depleted uranium is used, and the core material is coated with a zirconium alloy.

[Embodiments of the Invention] Hereinafter, a first embodiment of the present invention will be described with reference to FIG. Note that the same parts as in FIG. 5 are given the same reference numerals, and the explanation of the structure will be omitted. First, on the outermost periphery of the core 1, there is disposed a water removal rod 5 whose core material is depleted uranium and which is coated with zirconium. This water removal rod 5 is inserted into the reactor core 1 from the early stage to the middle stage, and is placed under the core 1 or pulled out of the reactor pressure vessel (not shown) at the final stage.

In the above configuration, the neutrons generated in the core 1
The water is absorbed by the water removal rod 5 arranged on the outermost periphery of the water. Then, the depleted uranium stored in the water exclusion rod 5 absorbs neutrons and becomes Pu23? become. Further, at the final stage of the reactor core, this water removal rod 5 is placed below the reactor core 1. Therefore, at the end of the reactor core, medium-speed neutrons and fast neutrons generated within the reactor core 1 are decelerated by the coolant and become thermal neutrons, which react with fuel within the reactor core to generate heat, as in the conventional case.

Therefore, the recovery rate of Pu is increased compared to the conventional core. Furthermore, since the neutrons that were directly absorbed in the reactor pressure vessel are absorbed by the water exclusion rod 5, the integrity of the reactor pressure vessel can be improved.

Next, a second embodiment of the present invention will be described with reference to FIGS. 2 and 3 showing a plan view and a side view of a water exclusion rod used in a pressurized water nuclear reactor. The water exclusion rod 10 of the pressurized water reactor shown in FIGS. 2 and 3 is provided with a connecting screw 12 above a zirconium alloy cladding tube 11, and the upper part of the cladding tube 11 and the connecting screw 12 are It is fixed by a spider 13.

The cladding tube 11 contains M(1), which has a small neutron absorption effect.
, MgO, MQ F2, or a Ma gold alloy.

In the above configuration, the water removal rod 10 is moved vertically by a drive device located at the upper part of the core, and is inserted into the core from the early to middle stages of core combustion. Further, at the final stage of core combustion, this water removal rod 10 is pulled out to the upper part of the core. As a result of this operation, the neutron spectrum within the core becomes as shown in FIG. Here, FIG. 4 shows a characteristic diagram of the reactor core with relative neutron flux plotted on the vertical axis and neutron energy plotted on the horizontal axis. In FIG. 4, JIIilA indicates the neutron spectrum when the water exclusion rod is withdrawn from the core, and line B indicates the neutron spectrum when the water exclusion rod is inserted into the core. As shown in Figure 4, by inserting water exclusion rods into the reactor core, thermal neutrons (n
t) decreases, and medium speed neutrons (nm) relatively increase. Therefore, if the reactor continues to operate with the water exclusion rod inserted, U will absorb the increased medium speed neutrons (ns+) and become Pu. Then, as thermal neutrons (nt) decrease, the consumption of U23' is suppressed, and furthermore, as fast neutrons (1'%f) increase, nuclear fission of U increases.

Therefore, by inserting water exclusion rods into the reactor core, the neutrons that were previously absorbed by the control rods act on U23t, thereby promoting the production of Pu and increasing the fission of U21t, thereby saving U231.

In addition, if the water exclusion rod remains inserted into the reactor core, there will be few thermal neutrons, so accumulated puz39 and saved U13
cannot be effectively combusted.

Therefore, by placing water removal rods in the upper part of the core at the end of combustion, thermal neutrons are increased, and the accumulated pulB' of many companies reacts with the saved U23', thereby making it possible to further extend the operation period.

Next, a third embodiment of the present invention will be described.

The third embodiment differs from the second embodiment in that the core material of the water removal rod is made of depleted uranium metal or an oxide of depleted uranium, which has a lower Ul'' content than natural uranium. With the above configuration, by using depleted uranium as the core material, U23!r absorbs medium-speed neutrons and p
It becomes u z3'l.

The Pu accumulated in the water exclusion rods can be taken out from the reactor after the reactor is shut down, reprocessed, and used again as fuel for the reactor.

In addition, in the second and third embodiments described above, the water removal rods shown in the second embodiment are arranged in the central region of the core,
If the water exclusion rods shown in the third embodiment are placed in other outer peripheral regions, medium-speed neutrons will increase in the central region of the reactor core, and the depleted uranium in the outer peripheral region can absorb the medium-speed neutrons. Plutonium can be produced more efficiently.

Although zirconium is shown as the coating material for the water exclusion rod, any material that has a small neutron absorption effect, good water contact resistance, and a high melting point that does not melt in a light water reactor can be used as the coating material in this example. It is possible.

[Effects of the Invention] The light water reactor core shown in the present invention has a water exclusion rod in which the core material is made of at least one of magnesium, magnesium oxide, or depleted uranium, and is therefore superior to conventional reactors. And Pu23? The recovery rate can be increased more effectively.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view of a core for a boiling water reactor according to a first embodiment of the present invention, and FIG. 2 is a plan view of a water exclusion rod for a pressurized water reactor according to a second embodiment of the present invention. 3 is a side view of the water exclusion rod shown in FIG. 2, FIG. 4 is a characteristic diagram of the reactor core showing the effect of the present invention, and FIG. 5 is a schematic plan view of the core for a conventional boiling water reactor. . 10. ...Core 2...Fuel assembly 3a, 3
b...Control rod 4...Shroud 5...Water exclusion rod 10...Water exclusion rod 11...
・Claying tube 12... Connection screw 13... Spider 14... Core material agent Patent attorney Ken Chika
Yu (1 other person) Figure 2 Figure 3 Figure 4

Claims (2)

[Claims] (1) In a light water reactor core consisting of a large number of fuel assemblies arranged in a lattice, a removable water removal rod is placed in the core, and the core material of this water removal rod is magnesium and magnesium oxide. Or, a core for a light water reactor, characterized in that at least one member of depleted uranium is used, and the core material is coated with a zirconium alloy. (2) The core for a light water reactor according to claim 1, wherein the water exclusion rod is arranged outside the outermost fuel assembly.