JPH11142583A - Method and device for replacing fluid in used-fuel transporting container - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a fluid in a used-fuel transporting container relatively quickly replaceable with another fluid. SOLUTION: A replacing fluid in a storage tank 21 is injected into a used-fuel transporting container 10 containing a fluid by means of a pump 23 through a drain valve 26 provided to the bottom of the container 10 in the vertical direction. Since the temperature of the replacing fluid is relatively lower than that of the fluid in the container 10, a stratified interface is formed between the fluid contained in the container 1 and the injected replacing fluid. Because of the rising effect of the interface caused by the density difference between the fluids above and under the interface, the fluid contained in the container 10 can be replaced with the replacing fluid in a relatively short time even when the container has a complicated internal structure. The replaced internal fluid is discharged to a recovery tank 22 from a vent valve 29 and a pressurizing valve 30 provided to the top of the container 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for replacing fluid in a spent fuel transport container, that is, boric acid water.

[0002]

2. Description of the Related Art A spent fuel assembly in a nuclear power plant is housed in a spent fuel transport container (hereinafter, also referred to as a cask) usually called a cask and transported to a reprocessing plant. . The cask is filled with several thousand ppm of boric acid water for protection because the spent fuel assembly generates heat and is high in temperature and emits radiation.

Before the reprocessing in the reprocessing plant, it is necessary to discharge boric acid water in the cask. It is necessary to replace the water with pure water in a nuclear power plant, and it is also important that this replacement be performed efficiently and in a short time because the object is radioactive.

FIG. 3A shows an example of a conventional replacement method by mixing / dilution.
, And is diluted by convection and mixing with the boric acid water inside, and then drained and collected from the bottom 3.

[0005]

However, in such a method, as can be seen from FIG. 3B, the temperature and the concentration decrease slowly, and a long replacement time is required until the desired degree is reached. There is a problem. Accordingly, it is an object of the present invention to provide a method and apparatus capable of relatively quickly replacing the fluid inside a shipping container.

[0006]

For this purpose, a first aspect of the present invention is provided.
According to the present invention, a fluid having a relatively lower temperature than the internal fluid is injected from a vertically lower portion of the spent fuel transport container including the internal fluid, and the fluid is injected between the internal fluid and the injected low-temperature fluid. Spent fuel transport that forms a stratified interface and replaces the internal fluid with the injection fluid due to a rising effect of the stratified interface caused by a density difference between the internal fluid and the low-temperature fluid above and below the stratified interface. A method for replacing a fluid in a container is provided.

Further, in order to achieve this object, a second aspect is provided.
In the present invention according to the present invention, for replacing the internal fluid in the spent fuel transport container having a drain valve at the bottom and a vent valve and a pressure valve at the top with a replacement fluid having a relatively lower temperature than the internal fluid. The apparatus includes a storage tank for storing the replacement fluid, a replacement fluid supply line connecting the storage tank to the drain valve of the spent fuel transport container, and the storage fluid interposed in the replacement fluid supply line. A pump for supplying the replacement fluid in a tank to the spent fuel transport container, and a discharge line connected to the vent valve and the pressure valve of the spent fuel transport container for discharging the internal fluid to the outside And a recovery tank connected to the discharge line.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals indicate the same or corresponding parts. In FIG. 1, the used fuel transport container 10 itself may have a known structure.
A radiation fin assembly 12 surrounding the top and bottom of the shielding layer 11, and upper and lower lid assemblies 13, 1
4 sealed. A plurality of channels 16 for vertically storing the fuel assemblies 15 are provided vertically inside the shielding layer 11, and the channels 16 are supported by a large number of support rings 17 which are spaced apart in the axial direction. I have.

In order to discharge the fluid inside the transport container 10, the drain valve 2 is attached to the lower lid assembly 14.
6 is provided, the top lid assembly 13 is provided with a vent valve 29 for adjusting the fluid level and a pressurizing valve 30 for injecting a pressurized gas so that the fluid does not boil. ing. Before the replacement, the transport container 10 is filled with a boric acid solution (not shown) having a concentration of several thousand ppm at 100 ° C. or less. The replacement method and apparatus according to the present invention are intended to efficiently replace high-temperature and high-concentration boric acid water in the transport container 10 with pure water.

In FIG. 1, a fluid displacement device or system 20 supplies pure water into a transport container 10,
Equipped with a pure water tank 21 of a capacity corresponding to the transport container,
To accommodate the boric acid solution recovered from the transport container 10,
A collection tank 22 having a similar capacity is provided. The pure water tank 21 is in fluid communication with the inside of the transport container 10 from the lower lid assembly 14 via a drain valve 26 via a pure water supply line 25 including a pure water supply pump 23 and an appropriate valve 24, and a recovery tank. Numeral 22 is in fluid communication with the interior of the transport container 10 from the upper lid assembly 13 via a vent valve 29 by a drain line 28 including a valve 27. Also,
A drain line 28 communicating with the collection tank 22 is provided with a valve 3
1 also communicates with the pressure valve 30 described above.

In this fluid replacement system 20, when each valve is opened and the pure water supply pump 23 is operated, the pure water in the pure water tank 21 is injected into the transport container 10 via the pure water supply line 25, and Boric acid water is vent valve 2
The water is collected in the recovery tank 22 by a buoyancy effect via a drain line 28 composed of a water level adjustment line including the pressure line 9 and a pressure line including the valves 30 and 31.

FIG. 2A is a diagram for explaining the buoyancy effect. At the start of the replacement process, the transport container 10 is filled with hot boric acid water,
The specific weight of water is smaller. Under such circumstances,
When the vent valve 29 and the pressurizing valve 30 are opened, and relatively low-temperature pure water (having a large specific weight) is injected from the drain valve 26 at the lower portion of the transport container 10, the injected pure water PW is injected due to a difference in specific gravity. A stratified interface A is formed at the boundary between the high temperature boric acid water BW and the high temperature boric acid solution BW. In the vicinity of the stratified interface A, heat transfer occurs, but the stratified interface itself gradually rises as indicated by symbols A ′ and A ″ in FIG. And, overall, it forms a piston flow that flows as if the piston moved.

Accordingly, the high-temperature boric acid solution BW is pushed out of the transport container 10 by the pure water PW from below, and is discharged to the collection tank 22 of FIG. 1 through the vent valve 29 and the pressure valve 30.

According to the replacement method of the present invention, when the stratified interface passes through a certain position in the height direction of the transport container,
Since the temperature and concentration in the vicinity of the position decrease rapidly, as understood from FIG. 2B, the time required for replacement is greatly reduced as compared with the conventional example of FIG. 3B. it can.

As described above, the present invention has been described with reference to the preferred embodiments. However, the present invention is not limited to these, and various modifications such as the following are possible. 1) The internal fluid of the transport container was boric acid water having a concentration of several thousand ppm, but any fluid including the concentration may be used as long as it has a cooling / control function, and the replacement fluid is not limited to pure water. 2) Although it is advantageous in terms of cost to use the drain valve, the vent valve, and the pressurization valve provided in the transport container for injecting the replacement fluid and discharging the internal fluid, the embodiment is configured as such. Depending on the form of the transport container to which the present invention is applied, for example, one corresponding to a drain valve and a vent valve and a pressurizing valve,
If there is no corresponding one, a separate valve device may be installed. 3) In order to discharge the internal fluid, it is preferable to use both the vent valve and the pressurizing valve, but it is not essential to use them, and only one of them may be used. 4) In the embodiment, the spent fuel is a fuel assembly,
The spent fuel may be other fuels, radioactive materials or radioactive waste, as long as the fluid in the transport container needs to be replaced.

[0016]

As described above, in the method according to the first aspect, a fluid having a relatively lower temperature than the internal fluid is injected from a vertically lower portion of the spent fuel transport container including the internal fluid, A stratified interface is formed between the internal fluid and the injected cryogenic fluid, and the internal stratified interface is formed by a density difference between the internal fluid and the cryogenic fluid above and below the stratified interface, thereby increasing the internal stratified interface. In the apparatus according to claim 2, the fluid is replaced with the injection fluid, and the internal fluid in the spent fuel transport container having a drain valve at a lower portion and a vent valve and a pressurizing valve at an upper portion is replaced with the internal fluid. An apparatus for replacing the replacement fluid with a replacement fluid having a relatively lower temperature than the fluid includes a storage tank for storing the replacement fluid and a replacement fluid supply that connects the storage tank to the drain valve of the spent fuel transport container. A line, a pump interposed in the replacement fluid supply line to supply the replacement fluid in the storage tank to the spent fuel transport container, and the vent valve and the pressurization valve of the spent fuel transport container And a recovery line connected to the discharge line for discharging the internal fluid to the outside, and a recovery tank connected to the discharge line. Therefore, according to the first and second aspects of the present invention, the replacement fluid having a relatively low temperature is injected from the lower portion of the transport container, and the internal fluid of the transport container is discharged from the upper portion. A stratified interface is formed between the replacement fluid and the upper internal fluid due to a buoyancy effect or a density difference effect. The stratification interface gradually rises as the replacement fluid is continuously injected. Since a large temperature difference and a large concentration difference occur between the two fluids above and below the stratification interface, even in a transport container with a complicated internal structure, the internal fluid is replaced with the replacement fluid in a relatively short time. be able to. In addition, the inside of the transport container can be effectively decontaminated by the effect of the piston flow in which the stratified interface rises as described above.

[Brief description of the drawings]

FIG. 1 is a system diagram illustrating a replacement method and a replacement device for implementing the replacement method according to an embodiment of the present invention.

2A is a schematic diagram illustrating a piston flow generated by a buoyancy effect according to the present invention, and FIG.
It is a graph which shows the temperature / concentration change of the specific part in the transport container in which the piston flow occurs.

FIG. 3 (a) is a schematic diagram for explaining a conventional mixed dilution method, and FIG. 3 (b) is a graph showing changes in temperature and concentration at a specific portion in a transport container when the mixed dilution method is performed. is there.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Spent fuel transport container, 15 ... Fuel assembly (spent fuel), 20 ... Replacement apparatus or system, 21 ...
Pure water tank (storage tank), 22 ... Recovery tank, 23 ...
Pure water supply pump, 25: pure water supply line (replacement fluid supply line), 26: drain valve, 28: drain line (discharge line), 29: vent valve, 30: pressurizing valve,
A: stratified interface, PW: pure water (substitution fluid), BW: boric acid water (internal fluid).

(72) Inventor Shozo Taguchi 1-1-1 Wadazakicho, Hyogo-ku, Hyogo-ku, Kobe-shi, Hyogo Prefecture Inside Mitsubishi Heavy Industries, Ltd. 1-1-1 Cho-cho Mitsubishi Heavy Industries, Ltd., Kobe Shipyard

Claims (2)

[Claims] 1. A fluid having a relatively lower temperature than the internal fluid is injected from a vertically lower portion of a spent fuel transport container including an internal fluid, and a layer is formed between the internal fluid and the injected low-temperature fluid. A spent fuel transport container that forms an interface and replaces the internal fluid with the injection fluid by a rising effect of the same stratified interface caused by a density difference between the internal fluid and the low-temperature fluid above and below the stratified interface. Fluid replacement method. 2. An apparatus for replacing an internal fluid in a spent fuel transport container having a drain valve at a lower portion and a vent valve and a pressurizing valve at an upper portion with a replacement fluid having a relatively lower temperature than the internal fluid. A storage tank for storing the replacement fluid, a replacement fluid supply line connecting the storage tank to the drain valve of the spent fuel transport container, and the replacement fluid supply line A pump for supplying the replacement fluid in a storage tank to the spent fuel transport container, and a discharge connected to the vent valve and the pressurization valve of the spent fuel transport container for discharging the internal fluid to the outside A device for replacing fluid in a spent fuel transport container comprising a line and a recovery tank connected to the discharge line.