JPH0129279B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a cask for transporting spent fuel assemblies immersed in liquid coolant to a reprocessing facility, but is not particularly limited to the following:
The present invention relates to a cask with a large heat removal capacity suitable for transporting spent fuel of a liquid metal cooled fast breeder reactor. In the following, a sodium cask for transporting spent fuel assemblies of a liquid metal cooled fast breeder reactor will be explained as an example, but the present invention can also be used for other purposes, such as a water cask for transporting spent fuel assemblies of a light water reactor. are also available. Conventionally, the method of handling spent fuel assemblies from liquid metal cooled fast breeder reactors is to remove the fuel assemblies stored in the external fuel storage tank one year or more after the reactor has been shut down, and to remove the fuel assemblies with liquid metal still attached. The "gas cask transport method" transports the fuel assemblies to the reprocessing plant in casks filled with inert gas, or the fuel assemblies stored in the fuel storage tank outside the reactor are removed approximately six months to one year after the reactor has been shut down. A common method was the ``water cask transport method,'' in which the adhering liquid metal was cleaned and then transported to a reprocessing plant in a cask filled with water. As described above, in the prior art, the reason why the fuel must be stored in the external fuel storage tank for a long period of time after the reactor is shut down is because the heat removal capacity of the transport cask is small. Incidentally, in the case of gas casks, argon gas is filled in, and the current practical use is for each fuel assembly.
It is about 1KW, and although the heat removal capacity improves slightly by filling it with helium gas, it is still 2 to 2.5KW.
is the limit. On the other hand, the heat removal capacity of water casks can be up to about 4KW per aggregate, but this is not a sufficient value. These conventional methods are applied to relatively small-scale cases such as experimental reactors and prototype reactors, but when applied to practical reactors (for example, output 1000 to 1500 MWe),
The following disadvantages arise. Firstly, from the perspective of plutonium productivity, the gas cask transport method has the longest fuel cycle period, which leads to a decline in plutonium productivity, which is disadvantageous in terms of energy strategy.On the other hand, the water cask transport method is more effective than the gas cask transport method. those that shorten the fuel cycle period;
From the viewpoint of plutonium productivity, further shortening is desired. Second, in the case of a 1000 MWe liquid metal cooled fast breeder reactor, the aggregate storage number of ex-core fuel storage tanks is approximately 540 for the gas cask delivery method;
Approximately 400 water cask delivery systems are required each.
Even in the latter case, the size of the ex-core fuel storage tank is, for example, about 9 m in diameter, which cannot be ignored compared to the size of the reactor vessel. Therefore, from the viewpoint of simplifying the equipment for the extra-core fuel storage tank, the gas cask removal method is the most disadvantageous, and the water cask removal method is also not very preferable. In particular, when an ex-core fuel storage tank is installed inside the reactor containment vessel, the size of the ex-core fuel storage tank is one of the factors that determines the reactor containment vessel diameter. It is important. Thirdly, the construction cost of the total system including the power plant and reprocessing plant (per unit output)
The processing capacity of a utility-scale reprocessing plant is 1,500 tons/year (equivalent to a power plant capacity of 50,000 MWe),
Assuming that the unit capacity of the power plant is 1000 to 1500 MWe, the cost of the water cask removal method will be higher than that of the gas cask removal method. This is because the water cask delivery method requires a fuel cleaning function at each power plant. As is clear from the results of the above considerations, in order to shorten the fuel cycle period and achieve economic efficiency in a commercial reactor, it is necessary to develop a cask with a larger heat removal capacity, and to develop a cask that has a larger heat removal capacity, and to It is required that the bodies be transported to a reprocessing plant as soon as possible after the reactor is shut down. Therefore, a ``sodium cask delivery method'' in which fuel assemblies are packaged in sodium-filled casks and transported was considered. For example, in order to take out and transport fuel assemblies stored in the external fuel storage tank 100 days after reactor shutdown, the heat removal capacity per fuel assembly is approximately
A cask of 7KW or more is required. However, although research and development is being carried out in various countries, sodium casks with such high heat removal capacity have not yet been put to practical use, and are currently undeveloped. The present invention was made in view of the actual state of the prior art as described above, and its purpose is to use a liquid coolant and to create a system in which the liquid coolant flows smoothly in a fixed direction. An object of the present invention is to provide a spent fuel assembly transport cask with high heat removal performance. By using such a transport cask, it is possible to shorten the fuel cycle period and improve plutonium productivity, thereby realizing the conditions for establishing a practical liquid metal cooled fast breeder reactor from the perspective of the nuclear fuel cycle. In addition, by minimizing the number of spent fuel assemblies required to be stored in the external fuel storage tank, it is possible to simplify the reactor equipment and reduce the quantity, contributing to the economic efficiency of the reactor. , to minimize the construction cost (per unit output) of the total system including the power plant and reprocessing plant,
It is possible to achieve comprehensive economic efficiency in a practical liquid metal cooled fast breeder reactor. Hereinafter, the present invention will be explained in detail based on the drawings.
An embodiment of a spent fuel assembly transport cask according to the present invention is shown in FIGS. 1 to 4. This cask is
It mainly consists of a cylindrical container body 1 with a bottom, a container lid 2 for sealing it, and a biological shield 3 that covers these, and a liquid coolant 4 such as liquid sodium is freely stored inside the cask. The spent fuel assembly 7 (indicated by a phantom line) is housed in the liquid coolant completely immersed in the liquid coolant. A partition wall 8 is installed at the center of the container body 1, and a lattice member 9 is installed at a position closer to the bottom than the center.
is installed. As shown in FIG. 2, the lattice member 9 has a conical opening 11 in the center into which the base of the entrance nozzle 10 of the spent fuel assembly 7 is inserted, and around the conical opening 11 there are a number of lattice-shaped openings. A hole 12 is formed therein. The partition wall 8, as shown in FIG.
It has an opening 13 in the center into which the spent fuel assembly 7 is inserted, a gas communication hole 14 and a plurality of coolant flow holes 15 are formed in the periphery, and each coolant flow hole 15 has a partition wall. A valve 16 is provided. This bulkhead valve 16 is installed when the cask is placed horizontally (see Fig. 6).
The check valve is installed so that the coolant outside the fuel assembly 7 can flow only from the handling head 17 to the entrance nozzle 10. Further, a handling head holder 6 is provided on the container lid 2 and protrudes toward the inside of the container body 1.
As can be seen from the sectional view shown in FIG. 4, the handling head holder 6 has an opening 18 in the center into which the handling head 17 of the fuel assembly 7 just fits.
A large number of holes 19 are formed in a lattice pattern around the periphery, and a large number of holes 20 are also formed in the peripheral wall. and the handling head 1
A handling head valve 21 is attached to the opening 18 into which the handle 7 fits. This handling head valve 21 is a check valve installed so that the coolant inside the fuel assembly 7 can flow outward through the handling head 17 when the cask is placed horizontally (see Fig. 6). . The fuel assembly 7 includes these partition walls 8, lattice members 9,
and is firmly held by the handling head holder 6. Bulkhead valve 16 and handling head valve 2
The first structure does not require any special function and may be simply attached by a hinge. These are closed due to their own weight when the cask is placed horizontally (see FIG. 6), but they open easily when the fluid pressure of the coolant acts in the above-mentioned permitted direction. Also, when the cask is placed vertically, the first
As shown in FIG. 5, the bulkhead valve 16 becomes open due to its own weight, and the handling head valve 21
is brought into an open state by the gravity of the weight 22 connected thereto. Note that legs 25 for vertical placement are provided on the bottom of the cask, and legs 26 for horizontal placement are provided on the peripheral wall. In general, methods for transporting spent fuel assemblies from power plants to reprocessing plants using casks include ship transport, road transport, rail transport, and combinations thereof. However, no matter which method is used, the cask is not necessarily transported immediately after the spent fuel assembly is packed up after being taken out from the ex-core fuel storage tank. Expect to wait up to 4 to 5 months for shipping. Therefore, the cask is required to be capable of removing decay heat from the fuel assembly not only during transportation but also during storage on land. During storage on land, as shown in FIG. 5, it is held vertically by vertical legs 25. As mentioned above, the bulkhead valve 16 and the handling head valve 21 are in the open state in this case, so the coolant is heated by the decay heat of the pellets in the fuel pin 30 inside the fuel assembly and near the container wall surface. The density difference due to cooling induces natural circulation in the direction of the arrow, thereby achieving steady heat removal. Next, when transporting casks in general, compatibility with the transport method must be considered. In other words, the total length of a fuel assembly for a practical liquid metal-cooled fast breeder reactor is approximately 4.8 to 5.4 m, and transporting the long cask containing this in a vertical state as in storage is difficult for road transport. In some cases, the height limit for transported objects stipulated by the Road Traffic Act (3.8m above ground)
exceeds and is impossible. Furthermore, railway transport is obviously impossible, and ship transport would also be considered extremely inconvenient. Therefore, the cask must be transported horizontally, but in this case it cannot be expected to remove heat through natural circulation as during vertical storage. The present invention overcomes these difficulties and can achieve good heat removal not only during storage but also during transportation. During transportation, the cask according to the present invention is held in a horizontal position using horizontal placement legs 26, as shown in FIG. As mentioned above, in this installation state, the bulkhead valve 16 only flows from the handling head side to the entrance nozzle side, and the handling head valve 21 only flows from the inside of the fuel assembly 7 to the outflow direction through the handling head 17. As shown in the arrow direction, the coolant passing through the bulkhead valve 16 passes through the grid member 9 to the entrance nozzle 10.
It can flow only along a loop such that it enters the interior of the fuel assembly 7 through the orifice hole 31 of the fuel assembly 7, exits from the handling head 17 through the handling head valve 21, and reaches the bulkhead valve 16. FIG. 7 shows the flow of coolant inside the cask during ship transportation. When the cask tilts due to the movement of the transport ship and the handling head side becomes lower, the coolant flows only inside the fuel assembly 7 and moves to the handling head side (see A and B in the same figure). At this time, the coolant is heated by the fuel pin. Next, when the inclination of the cask is reversed, the coolant flows only outside the fuel assembly 7 and moves toward the entrance nozzle, and at this time, the coolant is cooled by the container wall (see C and D in the same figure). ).
Thus, the movement of the transport ship causes the coolant to continue to flow intermittently in a fixed direction within the container, thereby achieving heat removal from the fuel assembly 7. In addition, in FIG. 7, the horizontal level HL is indicated by a chain double-dashed line. In the case of road and rail transport, the cask itself does not tilt, but the acceleration and
The acceleration during deceleration causes the coolant to flow in the same way as during ship transportation, making it possible to efficiently remove heat from the fuel assembly. The procedure for packing spent fuel assemblies into this cask is as follows: In the case of a sodium cask, the container body is placed vertically in an inert gas atmosphere.
After injecting liquid metal sodium to a predetermined level and inserting the fuel assembly 7, the container lid 2 may be fixed to the container body 1. The procedure for opening the cask is the reverse of the above, but if the sodium has solidified at this time, the fuel assembly 7 is removed after melting the sodium in the cask by high-frequency heating. The procedure for packing the water cask is also substantially the same as above, but in this case it is not necessary to carry out the above operation in an inert gas. The constituent material of this cask is sodium.
In the case of a cask, stainless steel is suitable for the container body as well as all parts that come into contact with sodium, as it is compatible with sodium and can withstand the decay heat of the fuel assembly. In addition, as biological shielding,
Suitable materials include lead or carbon steel. The same material is suitable for water casks for similar reasons. When designing a cask, it goes without saying that the required functionality and design standards should be met. It is also effective to take reinforcing measures as necessary to increase mechanical strength, and to provide heat dissipation fins on the outer surface of the cask to improve heat transfer performance. In addition, the present invention uses not only liquid metal such as sodium or Nak as a coolant but also water to transport cleaned liquid metal cooled fast breeder reactor fuel using the conventional water cask delivery method. It can also be used as a water cask for transporting spent fuel for light water reactors and as a water cask for transporting spent fuel for light water reactors, and its heat removal performance can be improved compared to conventional casks. In the above embodiment, one fuel assembly is housed in each cask, but the present invention is not necessarily limited to this. In the case of the conventional gas cask transport method and water cask transport method, casks that can store about 8 to 12 fuel assemblies are common, and the cask of the present invention can also store a large number of fuel assemblies in this way. It can also be designed like this. Table 1 shows the effectiveness of the cask according to the present invention in comparison with the conventional technology.

【table】

[Table] As described above, the present invention uses a liquid coolant,
It is designed to allow the coolant to flow smoothly in a fixed direction not only when placed vertically but also when placed horizontally, making it possible to obtain a cask for transporting spent fuel assemblies with extremely high heat removal performance. can. When the cask according to the present invention is used as a sodium cask for transporting spent fuel assemblies of liquid metal cooled fast breeder reactors, the following remarkable effects can be obtained. First, it becomes possible to remove spent fuel assemblies from the external fuel storage tank and transport them to a reprocessing plant approximately 100 days after reactor shutdown, thereby minimizing the fuel cycle period and increasing plutonium productivity. The conditions for establishing a practical liquid metal cooled fast breeder reactor from the viewpoint of the nuclear fuel cycle can be advantageously realized. Second, it is not necessary to install spent fuel assembly cleaning equipment at each power plant, but only at the reprocessing plant.
The construction cost (per unit output) of the total system consisting of multiple power plants and one reprocessing plant will be the lowest, and overall economic efficiency will be achieved in a commercial liquid metal cooled fast breeder reactor. Thirdly, spent fuel assemblies stored in the external fuel storage tank will be able to be removed 100 days after the reactor is shut down.
As a result, the number of spent fuel assemblies required to be stored in the ex-core fuel storage tank can be reduced compared to the conventional water cask transport method and gas cask transport method. Therefore, a reduction in the size of the ex-core fuel storage tank container is achieved (see Table 1). Furthermore, when the present invention is used as a water cask for transporting spent fuel assemblies for liquid metal cooled fast breeder reactors and light water reactors, its heat removal performance is improved compared to conventional water casks, and the decay waiting time from reactor shutdown to transport is improved. It becomes possible to shorten the time.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing one embodiment of a transport cask according to the present invention, FIG. 2 is a sectional view thereof, and FIG.
The figure is - sectional view, Figure 4 is - sectional view, 5th figure is - sectional view.
The figure is an explanatory diagram showing the state when stored vertically, Figure 6 is an explanatory diagram showing the state when transported horizontally, and Figure 7 A, B,
C and D are explanatory diagrams showing coolant flow conditions during transportation by ship. 1... Container body, 2... Container lid, 3... Biological shield, 4
...Liquid coolant, 7. Spent fuel assembly, 8. Partition wall, 9. Grid member, 10. Entrance nozzle,
16... Bulkhead valve, 17... Handling head, 1
8... Handling head holder, 21... Valve for handling head, 25... Legs for vertical placement, 26... Legs for horizontal placement.

Claims (1)

[Claims] 1 A container body, a lid body, and a biological shield surrounding them, in which a liquid coolant is sealed with a free liquid level, and the spent fuel assembly is fixed and transported immersed therein. A cask for use in the cask, wherein the cask interior space is divided approximately at the center by a partition wall having an opening into which the fuel assembly is inserted;
The bulkhead has a gas passage hole, a coolant flow hole, and a bulkhead valve installed near the coolant flow hole, and the lid has a handling head into which a handling head of the fuel assembly fits. A head holder protrudes inward from the container body, and a handling head valve is provided in the opening, and when the cask is placed vertically, the two types of valves are open, and the opening is provided with a handling head valve. When the cask is placed horizontally, the above two types of valves are closed, but the bulkhead valve allows the coolant to flow from the handling head side to the entrance nozzle side outside the fuel assembly, and the handling head valve is closed. A cask for transporting spent fuel assemblies, characterized in that the coolant inside the fuel assemblies is allowed to flow out from a handling head.