JPS63754B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control mechanism for a gas-cooled nuclear reactor in which the structure of the control rod channel of the reactor core is improved. Generally, in a gas-cooled nuclear reactor, a core 2 is constructed by stacking a large number of substantially hexagonal graphite fuel bodies 1, as shown in FIGS. 1 and 2. Control rod insertion holes 3 are formed in these graphite fuel bodies 1, and these control rod insertion holes 3 are arranged in a straight line to form a control rod channel 4 . A control rod guide tube 5 is provided opposite the upper ends of these control rod channels 4 , and a control rod 6 is slidably housed within this control rod guide tube 5. The control rod 6 is moved up and down by a control rod drive mechanism 7, and is configured to be inserted into the control rod channel 4 from above. Further, the control rod 6 includes a plurality of control rod elements 6.
a... are connected in a bendable manner, and the whole is configured to be freely bendable. These control rod elements 6a... have a double tube structure consisting of an inner cladding tube and an outer cladding tube, and a neutron absorbing material is filled between the inner cladding tube and the outer cladding tube. . By the way, during normal operation of such a nuclear reactor, the control rods 6 are in the upper third of the reactor core 2 , as shown in FIG.
It has been inserted to some extent. Further, the temperature in the upper part of the reactor core 2 is relatively low at about 400°C, and the temperature in the lower part of the reactor core 2 is high at about 1000°C or more. In this state, the temperature of the control rods 6 is approximately equal to the temperature of the upper part of the reactor core 2 . When the reactor is scrammed, this control rod 6 is used as shown in Figure 3.
is inserted to the bottom of the core 2 . In this case, the control rods 6 are heated by radiant heat from the inner surface of the control rod channel 4 , but the temperature in the lower part of the reactor core 2 is 1000°C.
This is a high temperature, and the radiant energy is generally proportional to the fourth power of the absolute temperature, so the heat flux flowing from the inner surface of the control rod channel 4 to the control rod 6 in the lower part of the reactor core 2 becomes extremely large. For this reason, the thermal load that the control rods 6 receive is extremely large, resulting in a large temperature difference between the outer cladding tube and the inner cladding tube of the control rod elements 6a, resulting in excessive thermal stress. For this reason, there was a problem that the control rod 6 suffered from large thermal fatigue and had a short lifespan. The present invention was made based on the above circumstances, and its purpose is to provide a gas-cooled type that can alleviate the thermal shock that the control rod receives when the control rod is inserted, and extend the life of the control rod. The objective is to obtain a nuclear reactor control mechanism. The present invention will be explained below with reference to an embodiment shown in FIGS. 4 to 6. That is, 102 is a reactor core, and this reactor core 102 is constructed by stacking a large number of hexagonal graphite fuel bodies 101 as shown in FIG. Control rod insertion holes 103 are formed in these graphite fuel bodies 101. These control rod insertion holes 103...
are arranged in a straight line, forming a control rod channel 104 extending from the top surface to the bottom of the core 102 . A control rod guide tube 105 is provided opposite the upper end of this control rod channel 104 . Although a large number of control rod channels 104 and control rod guide tubes 105 are actually provided, only one of them is shown in FIGS. 4 and 6. A control rod 106 is slidably accommodated in the control rod guide tube 105. This control rod 106 is suspended by a wire rope 107. The wire rope 107 is wound in and unwound out by a control rod drive mechanism 108, and the control rod 106 is raised and lowered to be inserted into the control rod channel 104 . The control rod 106 is formed by connecting a plurality of control rod elements 106a in a flexible manner, and the entire control rod 106 can be bent freely, even if the control rod channel 104 is slightly bent due to an earthquake or the like. It is configured so that it can be smoothly inserted into the control rod channel 104 . Further, the control rod elements 106a have a double tube structure consisting of an inner cladding tube and an outer cladding tube, and a neutron absorbing material is filled between the inner cladding tube and the outer cladding tube.
This control rod 106 is the fourth control rod during normal operation.
As shown in the figure, it has been inserted to about 1/3 of the upper part of the core 102 . A heat insulating layer 109 is formed on the inner surface of the control rod channel 104 below the insertion position of the control rod 106 during normal operation. As shown in FIG. 5, this heat insulating layer 109 serves as the control rod insertion hole 103 of the graphite fuel assembly 101.
A heat insulating tube 109a... is inserted into the fuel cell, and a graphite fuel body 101... in which such a heat insulating tube 109a... is fitted is placed below the position where the control rod 106 is inserted during normal operation. be.
The inner diameter of the control rod insertion hole 103 of the graphite fuel assembly 101 into which the heat insulating cylinder 109a is inserted is larger than the inner diameter of the other graphite fuel assembly 101, and the inner diameter of the heat insulating cylinder 109a is larger than that of the other graphite fuel assembly. The control rod channel 104 is formed to have the same inner diameter as the control rod insertion hole 103 of the control rod channel 101, and is configured so that no step is formed on the inner surface of the control rod channel 104 . And these heat insulating cylinders 109a
... are made of a material such as alumina, which is heat resistant and has a lower thermal conductivity than the graphite material used for the graphite fuel bodies 101. Further, an appropriate gap is formed between the outer peripheral surface of the heat insulating cylinder 109a and the control rod insertion hole 103, and this gap allows the heat insulating cylinder 109a to connect to the graphite fuel body 1.
It is configured to absorb the difference in thermal expansion with 01... and to increase the thermal resistance due to this gap. In one embodiment of the present invention constructed as described above, during normal operation, the control rods 106 are inserted to a position of about 1/3 of the upper part of the reactor core 102, as shown in FIG.
In this normal operation, the upper part of the reactor core 102 is relatively low at about 400°C, but the lower part of the reactor core 102 is at a high temperature of 1000°C or more, and the control rods 106 are approximately equal to the upper part of the reactor core 102 . It's the temperature. When the reactor is to be scrammed, the control rods 106 are inserted into the control rod channel 104 up to the bottom of the reactor core 102 , as shown in FIG. In this case, the control rod 106 is at a relatively low temperature while the lower part of the core 102 is at a high temperature, so the control rod 106 is heated by heat from the surroundings. In this case, the heat transfer path is from the graphite fuel bodies 101 to the inner surface of the control rod channel 104 by heat conduction via the heat insulating layer 109 , and then to the control rod channel 104 .
It is transmitted to the control rod 106 by radiation from the inner surface. In this case, since the lower part of the core 102 is at a high temperature, the amount of heat that can be given to the control rods 106 by radiation becomes extremely large. However, in this embodiment, the heat transferred to the control rod 106 must be transferred through the heat insulating layer 109 by thermal conduction .
Since 09 has a low thermal conductivity, it produces a large thermal resistance. Therefore, the heat flux transmitted to the control rod 106 is small, the thermal shock that the control rod 106 receives is small, and the life of the control rod 106 is therefore extended.
Furthermore, since the heat insulating layer 109 is provided below the position where the control rod 106 is inserted during normal operation, the thermal shock to the control rod 106 is further reduced. That is, if such a heat insulating layer is formed over the entire surface of the control rod channel 104 , the amount of heat transmitted to the control rod 106 during normal operation will also be reduced, so the temperature of the control rod 106 during normal operation will decrease. For this reason, this control rod 106 is connected to the reactor core 102.
When the control rod 106 is inserted into the lower part of the control rod 106, the temperature difference between the control rod 106 and the surroundings increases, the thermal shock increases accordingly, and the effect of reducing thermal shock decreases. However, in this embodiment, during normal operation, the control rod 1
Since the heat insulating layer 109 is not formed in the portion where the control rod 106 is inserted, the temperature of the control rod 106 during normal operation does not drop, and the effect of reducing thermal shock is not diminished. Note that the present invention is not limited to the above embodiment. For example, the material, structure, range of formation, etc. of the heat insulating layer are not necessarily limited to the above embodiment. Furthermore, the configurations of the graphite fuel body and control rods are not necessarily limited to those described above. As described above, in the present invention, a heat insulating wall made of a material having a lower thermal conductivity than the graphite material used in the graphite fuel assembly is formed on the inner surface of the control rod channel. Therefore, when a control rod is inserted into the lower part of a high-temperature reactor core, the heat insulating layer creates a large thermal resistance in the path of heat flowing to the control rod, reducing the amount of heat transferred to the control rod. Therefore, the thermal shock to which the control rod is subjected is reduced and the life of the control rod is extended, which has great effects.

[Brief explanation of the drawing]

Figures 1 to 3 show conventional examples, with Figure 1 being a vertical cross-sectional view of the reactor core during normal operation, Figure 2 being a perspective view with a part of the graphite fuel body cut away, and Figure 3 being a control FIG. 3 is a vertical cross-sectional view of the core when rods are inserted. 4 to 6 show an embodiment of the present invention, in which FIG. 4 is a longitudinal sectional view of the core during normal operation, FIG. 5 is a perspective view showing a part of the graphite fuel body cut away, and FIG. Figure 6 is a vertical cross-sectional view of the core with control rods inserted. 101...graphite fuel body, 102 ...core, 103...
Control rod insertion hole, 104 ...Control rod channel, 10
6... Control rod, 109 ... Heat insulation layer, 109a... Heat insulation cylinder.

Claims (1)

[Claims] 1. A reactor core configured by stacking a large number of graphite fuel bodies, a control rod channel formed in this core,
A gas-cooled nuclear reactor control mechanism characterized by comprising a heat insulating layer formed on the inner surface of the control rod channel and made of a material having a lower thermal conductivity than graphite. 2. The gas cooling device according to claim 1, wherein the heat insulating layer is formed on the inner surface of the control rod channel at a portion below the portion where the control rod is inserted during normal operation. Type nuclear reactor control mechanism.