JPH0523398B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for removing sodium remaining in a fuel bucket used in a fuel transfer facility for replacing spent fuel in a nuclear reactor with new fuel.

[Prior art and its problems] A fuel transfer device in a nuclear reactor that uses sodium, a liquid metal, as a coolant is installed so as to straddle a reactor vessel 1 and an external fuel storage tank 8, as shown in FIG. a fuel bucket 3 for storing and moving fuel within the slopes 5 and 7; a hoisting device 4 for raising and lowering the fuel bucket 3; and a hoisting device 4 for changing the slope. It is composed of a movable slope 6. The spent fuel in reactor 2 is
It is gripped by a fuel gripping device 12 and held at the position shown in the figure. On the other hand, the fuel bucket 3 is lowered by the hoisting device 4, slides down the ramp 5 to just below the fuel gripping device 12, stores the waiting spent fuel, and is lowered by the hoisting device 4. It is pulled up to the inside of the movable ramp 6, and then the movable ramp 6 is swung and connected to the ramp 7, and in this state, the fuel bucket 3
The spent fuel is transferred to and stored in the extra-core fuel storage tank 8. On the other hand, new fuel stored in the extra-core fuel storage tank 8 is stored in the fuel bucket 3, and is loaded into the reactor 2 by following the same route in the opposite direction. In this way, the fuel is replaced with new fuel. Spent fuel is heated to a high temperature by the decay heat generated during the process of converting fissile material produced by nuclear fission into more stable material, so it is possible to prevent damage to fuel elements. To prevent this, the fuel bucket 3 is filled with sodium for cooling. After the fuel exchange is completed as described above, the fuel bucket 3 is pulled up into the movable slope 6,
The movable slope 6 is set vertically and waits until it is time to replace the fuel, but leaving the sodium in the fuel bucket 3 as it is for a long period of time will reduce the radioactivity level of the sodium due to the handling of spent fuel. Because of the extremely high When it becomes necessary to enter
It becomes impossible to approach. Therefore, from the perspective of reducing the radiation exposure of workers during inspections of equipment for fuel exchange and during repairs, as much sodium as possible is removed from the fuel bucket 3 on standby to reduce the radioactivity level in the fuel transfer cell 14. It is necessary. Furthermore, if the fuel bucket 3 is left filled with sodium and left on standby for a long period of time, the sodium will solidify unless the holding temperature is maintained above the sodium melting temperature, so this removal work may involve high levels of radioactivity. It will be extremely difficult.

Conventionally, sodium in the fuel bucket 3 has generally been removed by providing a drain plug at the bottom of the fuel bucket 3. In this method, if the plug loosens for some reason and the cooling sodium leaks, the fuel element itself will melt due to the decay heat of the fuel stored in the fuel bucket 3 due to insufficient cooling, resulting in a high level of radioactivity. There was a danger that the fuel would be released into the fuel transfer cell 14, and countermeasures were urgently needed.

[Object of the Invention] The present invention was made in view of the above-mentioned technical problems, and it provides a fuel bucket interior that makes it possible to easily discharge sodium from the fuel bucket in a nuclear reactor vessel or an extra-core fuel storage tank. The object of the present invention is to provide a method for removing residual sodium using a simulated fuel assembly for removing residual sodium.

[Structure of the Invention] The present invention provides a simulated fuel assembly, which is a sodium discharge fitting made of a non-radioactive metal, to remove residual sodium remaining in the fuel bucket in the ex-core fuel storage tank at the time when fuel exchange is completed. In this method, residual radioactive sodium is reduced by inserting the fuel into the fuel bucket instead of the replacement fuel and removing it by the volume of the simulated fuel assembly, and then moving the fuel bucket into the fuel transfer cell and leaving it on standby. This makes it possible to reduce the level of radioactivity within the fuel transfer cell.

Note that the shape of the simulated fuel assembly does not necessarily have to be the same as that of the fuel assembly, and may be cylindrical as long as it can be inserted into the fuel bucket.

[Example] An example of the method for removing residual sodium according to the present invention will be described in detail. The fuel bucket 3 is guided by wheels 9 on a guide track 10 provided in a cylindrical slope 5 as shown in FIG. 2, and is raised and lowered by a hoisting device 4 shown in FIG. There is. Two cylindrical bodies 15 are installed in the fuel bucket 3 as shown in FIGS. 1 and 2, and a fuel assembly is inserted into one of the cylindrical bodies 15 as shown.

Now, when the spent fuel has been discharged from the reactor vessel 1 and new fuel has been loaded to complete the fuel exchange, the fuel baguette 3 is filled with sodium from directly below the reactor vessel fuel gripping device 12.
is lifted up by the hoisting device 4 and stored in the movable slope 6, and then the slope 6 is swung and connected to the slope 7, and the slope 7 is slid down to release the fuel gripping device 12 in the fuel storage tank 8. Install it in the designated position directly below it. Since the position where the fuel bucket 3 is installed is below the sodium liquid level, the fuel bucket is filled with sodium. Here, the simulated fuel assembly 17, which is a metal fitting for discharging sodium stored in the fuel storage tank 8, is inserted into the fuel bucket 3 by the fuel gripping device 12 as shown in FIG. After expelling and removing the sodium contained therein, the fuel bucket 3 is lifted out of the ramp 7 by the hoisting device 4 and raised, stored in the movable ramp 6, and the movable ramp 6 is lifted from the ramp 7. Leave it uncut and vertical until the next fuel change.

[Effects of the Invention] As detailed above, in the present invention, the simulated fuel assembly is inserted into the fuel bucket, so the amount of radioactive sodium contained in the fuel bucket can be reduced. Therefore, when the fuel bucket is placed on standby in the fuel transfer cell, it is possible to lower the radioactivity level inside the fuel transfer cell, significantly reducing the radiation exposure of workers during inspection and repair of fuel exchange equipment. You can.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a fuel bucket into which a simulated fuel assembly of the present invention is inserted, FIG. 2 is a sectional view showing the simulated fuel assembly inserted into a fuel bucket in a ramp, and FIG. FIG. 2 is a longitudinal sectional view showing the entire fuel exchange device. 1...Reactor vessel, 3...Fuel bucket, 5,
6... Slope, 6... Movable slope, 8... Ex-core fuel storage tank, 17... Simulated fuel assembly.

Claims (1)

[Claims] 1. A simulated fuel assembly that simulates the shape of a fuel assembly is inserted into a fuel bucket in an ex-core fuel storage tank, and residual sodium is removed from inside the fuel bucket. A method for removing residual sodium from fuel buckets. 2. The method for removing residual sodium from a fuel bucket according to claim 1, wherein the simulated fuel assembly has a cylindrical shape.