JP2005201909A - Gas leakage test device for cask for storing radioactive waste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a gas leakage test when providing a metal seal in a lid of a cask. <P>SOLUTION: This leakage test device 200 performs the test by applying a load onto a central block 201 by a cylinder 220. The block 201 moves downward by the load application, so that an airtight state by a metal gasket 204 changes. Helium gas is supplied into a minute gap 208 from a gas cylinder 211, and its leakage amount is changed by the change of the airtight state of the metal gasket 204. The leaked helium gas is leaked into a gap 209 between the metal gasket 204 and an O-ring 206, and is detected by a gas detection device 213 through a passage 212 communicating with the gap 209. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gas leak test apparatus for a radioactive waste storage cask which is a pressure vessel for storing a spent fuel assembly which has been burned and which can be easily transported.

  A nuclear fuel assembly at the end of the nuclear fuel cycle that cannot be used after combustion is called a spent fuel assembly. Since the spent fuel assembly contains a highly radioactive substance such as FP and needs to be thermally cooled, it is cooled in the cooling pit of a nuclear power plant for about 60 years. Then, it is stored in a cask which is a radioactive waste shielding container, and is transported and stored in a reprocessing facility by a truck or the like. Since a highly radioactive substance is contained in the cask, strict caution is required for sealing the cask.

  FIG. 7 is a cross-sectional view showing the structure of a conventional cask. FIG. 8 is a partially enlarged view of the cask shown in FIG. This cask 500 is made of a stainless steel or carbon steel body 1, an outer cylinder 2 constituting the outer surface of the cask, and a polymer resin containing hydrogen filled between the body 1 and the outer cylinder 2. 3, a bottom plate 5 which is welded to the lower portion of the trunk body 1 and encloses the resin 4, and a primary lid 7 and a secondary lid 8 provided on the flange portion 6 of the trunk body 1. As shown in FIG. 8, the primary lid 7 and the secondary lid 8 are fixed to the flange portion 6 with bolts 9 and 10. The secondary lid 8 is filled with a resin (not shown).

  A basket 11 for accommodating a spent fuel assembly is provided in the trunk body 1. The basket 11 is made of a composite material of boron (B) and aluminum. The cask 500 is filled with helium gas in a negative pressure state. On the other hand, the inside of the space 12 of the primary lid 7 and the secondary lid 8 is positive pressure, thereby forming a pressure barrier between the inside and outside of the cask 500. The secondary lid 8 is provided with a hole 13 for measuring the pressure in the space between the lids, and a pressure sensor 14 is provided at the outlet of the hole 13.

  The primary lid 7 is provided with a valve 15 for replacing the gas in the cask 500. The valve 15 is covered with a valve cover 16. Metal gaskets 20 and 21 are provided between the primary lid 7 and the trunk body 1 and between the secondary lid 8 and the trunk body 1. A metal gasket 22 is also interposed between the valve cover 16 and the primary lid 7.

  FIG. 9 is an enlarged view showing a seal portion of the cask. Note that the seal portions between the primary lid 7 and the trunk body 1 and between the secondary lid 8 and the trunk body 1 have the same seal structure. The gasket groove 23 is formed by machining. As shown in FIG. 10, the metal gaskets 20 and 21 to be used have a double structure of inner and outer rings. These metal gaskets 20 and 21 are coated with an inner envelope 25 that also uses Inconel on a coil spring 24 made of Inconel (trade name: nickel-based alloy containing 16% chromium and 7% iron) that is resistant to corrosion and high-temperature oxidation. Furthermore, the inner and outer rings are collectively covered with an outer envelope 26 made of aluminum.

  The gasket groove 23 is fixed by a bolt 27 at the outer envelope 26 portion. As the metal gaskets 20 and 21, for example, “Triback” manufactured by Nippon Valqua Industries Co., Ltd., “Helicoflex Seal” manufactured by CEFILAC of France, etc., which has been used for nuclear casks, can be used.

  It can be expressed by the Larson-Miller Parameter (LMP), which is the temperature and time dependency of the plastic deformation rate and sealing performance of the metal gaskets 20 and 21. Development "(Kato, Ito, Saegusa, Journal of the Atomic Energy Society of Japan, Vol. 38, No. 6, 95-101, 1996). In general, the long-term sealing performance of a metal gasket is performed by obtaining the LMP of the sealing retention limit and predicting the limit time at a predetermined temperature.

  Further, an O-ring groove 29 into which a synthetic rubber O-ring 28 is fitted is machined on the inner peripheral side of the metal gaskets 20 and 21. The O-ring groove 29 is a dovetail for fixing the O-ring 28. Normally, when the cask 500 is stored, the synthetic rubber O-ring 28 is not used. The reason why the O-ring 28 made of synthetic rubber is not used during storage is that it cannot withstand use under high temperature (around 150 ° C.) and high dose conditions.

On the other hand, the metal gasket 20 is unsuitable for use during cask transportation for the following reasons. When the metal gasket 20 is used, the amount of deviation between the secondary lid 8 and the trunk body 1 with respect to the leakage rate (1 × 10 ⁻⁹ Pa · m ³ / s) which is the manufacturer's recommended value is about 0.2 to 0.3 mm. However, the amount of deviation when an actual cask is dropped is several mm. Therefore, when the cask 500 fitted with the metal gasket 20 falls, the recommended value may be exceeded. For this reason, in the conventional cask 500, the metal gasket 20 of the secondary lid 8 is replaced with a synthetic rubber O-ring 28 during transportation before transportation.

  On the other hand, it has been confirmed that the O-ring 28 made of synthetic rubber can maintain a sufficient sealing function against a lateral shift due to dropping if it is several months even under high temperature and high dose.

  In the conventional cask 500, in order to replace the metal gasket 20 and the synthetic rubber O-ring 28, it is necessary to remove the secondary lid 8 once and attach the O-ring 28 to the O-ring groove 29 of the secondary lid 8. There is. Then, the secondary lid 8 is attached to the cask 500 and a confidential inspection is performed. Since all of these operations are performed under a high dose, the operations must be performed with the cask 500 submerged in the pool. Alternatively, it is necessary to install the cask 500 in a hot cell surrounded by a thick shielding wall and perform the work remotely. For this reason, there has been a problem that it takes time and cost to replace the seal.

  There was also a question about the necessity of replacement work. That is, in the past, the leakage rate of the metal gasket 20 was evaluated based on the value recommended by the manufacturer of the metal gasket 20, but this value is for a case in which the product to be used is not limited. The evaluation is not limited to 500. Conventionally, there is no testing apparatus for testing the leakage rate when the metal gasket 20 is used in the cask 500, and there is no specific standard for the leakage rate during cask transportation. For this reason, in the conventional handling, based on a general manufacturer recommended value, it was evaluated that the replacement work of the sealing device was necessary when the cask was transported.

  Further, in the above-described conventional cask 500, it is difficult in terms of manufacturing technology to suppress the displacement of the secondary lid 8 to about 0.3 mm, and the secondary device has been replaced. The present condition is not made in particular so that the space | interval of 8 and the flange part 6 of the trunk | drum main body 1 may be shortened, and the deviation | shift amount at the time of cask transportation may be suppressed.

  In order to achieve the above-mentioned object, a gas leakage test apparatus for a radioactive waste storage cask according to the present invention forms a metal seal holding structure such as a seal groove or protrusion on the first flat surface, and A first block holding a metal seal, and a second plane part in surface contact with the first plane part of the first block, and the second plane part comes into contact with the metal seal to partially form a sealed space. A second block to be formed; a gas supply passage which is formed in the first block or the second block and supplies a test gas to the sealed space; a gas detection means for detecting gas leaked from the sealed space; and the first block Or a load applying means for applying a load to the second block and relatively moving the first block and the second block on the contact surface, and the metal seal is used for sealing the cask. It is obtained so as to test the leakage of internal gas.

  By applying an impact by the impact applying means, the first block and the second block move relative to each other, and a deviation occurs between the second flat portion and the metal seal. On the other hand, since the gas is supplied to the sealed space formed by the metal seal through the gas supply passage, the sealing is broken by the shift and the gas leaks. The gas detection means detects the leaked gas. This test apparatus enables a leak test when a metal seal is provided on the cask lid.

  In the gas leakage test apparatus for a radioactive waste storage cask according to the present invention, a first block in which a metal seal holding structure such as a seal groove and a protrusion is formed on the first plane portion, and the metal seal is held in the holding structure; A second block having a second planar portion in surface contact with the first planar portion of the first block and forming a sealed space in part by contacting the second planar portion with a metal seal; Alternatively, a gas supply passage formed in the second block for supplying a test gas to the sealed space, a gas detection means for detecting gas leaked from the sealed space, and a load applied to the first block or the second block And a load applying means for moving the first block and the second block relative to each other on the contact surface, and testing for leakage of internal gas when the metal seal is used for sealing a cask. It was. For this reason, since this test apparatus enables a leak test when a metal seal is provided on the lid of the cask, the leak rate of the cask using the metal seal can be evaluated.

  Hereinafter, the present invention will be described in detail with reference to the drawings. The present invention is not limited to the best mode for carrying out the invention.

  1 is a partial cross-sectional view showing a cask according to a first embodiment of the present invention. The cask 100 is characterized in that an O-ring groove into which a synthetic rubber O-ring is inserted is omitted, and only a gasket groove 101 for a metal gasket is provided. Since the configuration other than this is the same as the conventional example, the description thereof is omitted. In the present invention, providing only the gasket groove 101 of the metal gasket 20 means that the O-ring groove is omitted and only the gasket groove 23 of the conventional metal gasket 20 is provided, as shown in FIG. The case where the groove and the O-ring groove are shared (not shown) includes the case where the metal gasket 20 is dedicated. In any case, since it is not necessary to process a dovetail shape such as an O-ring groove, the processing is simplified. For example, a simple U-shaped groove as shown in FIG.

  In addition, the cask 100 does not require replacement work with an O-ring during transportation on the sea or on public roads. As described above, the manufacturer's recommended value is conventionally used for the leak rate of helium gas in the cask, but this recommended value is a general value and not a value specific to the cask. In addition, such a leak test has not been performed so far, and the leak rate required for the cask has not been verified. For this reason, the replacement work to the O-ring is performed based on the recommended value, and a great burden is imposed by this replacement work.

The inventors of the present application focused on this point and verified the leakage rate required for the cask 100. First, the cask 100 is required to be sealed for a long time, but since this storage is performed for about 60 years, the required leakage rate is relatively high. Specifically, the leakage rate that can maintain the negative pressure in the cask 100 during storage under the condition that the temperature of the metal gasket portion is 150 ° C. is within 1 × 10 ⁻⁷ Pa · m ³ / s. Next, when the cask falls during transport of the cask and the secondary lid 8 is displaced by this, the negative pressure in the cask 100 needs to be maintained for one month, and the leakage rate at that time is 1 × 10 ⁻⁴ Pa · m ³ / s.

  Next, the configuration of the leakage test apparatus will be described. FIG. 2 is a configuration diagram showing a leakage test apparatus. This leak test apparatus 200 has a configuration in which a central moving block 201 is sandwiched between disc-shaped flanges 202 and 203. A gasket groove 205 of a metal gasket 204 and a synthetic rubber made of synthetic rubber are formed on the contact surface of the flange 202 and the block 201. An O-ring groove 207 for receiving the O-ring 206 is provided. A minute gap 208 is formed inside the gasket groove 205. The minute gap 208 is supplied with helium gas from a gas supply passage 210 formed in the flange 202.

  Helium gas is supplied from a gas cylinder 211 installed outside. Further, a passage 212 for detecting a leaked gas is formed in a gap 209 formed between the metal gasket 204 and the O-ring 206. The passage 212 is connected to a gas detection device 213 installed outside the flange 202. A heater 214 is installed on the outer surface of the flange 202. A thermocouple 215 is disposed in the vicinity of the minute gap 208, and this temperature output is sent to the processing device 216 of the heater 214.

  A displacement meter 217 is installed below the flange 202 and the block 201. The displacement data of the displacement meter 217 is sent to the processing device 216. Both flanges 202 and 203 are installed on the base 218. A cylinder 220 for applying an impact load is provided on the upper portion of the block 201. The movement of the cylinder 220 is controlled by the processing device 216 based on the output of the displacement meter 217.

  The leak test apparatus 200 is performed by applying a load to the central block 201 by the cylinder 220. By applying this load, the block 201 moves downward, and the airtight state of the metal gasket 204 changes. Helium gas is supplied to the minute gap 208 from the gas cylinder 211, and the amount of leakage changes depending on the change in the airtight state of the metal gasket 204. The leaked helium gas leaks into the gap 209 between the metal gasket 204 and the O-ring 206, and is detected by the gas detection device 213 through the passage 212 communicating with the gap 209.

  The displacement of the block 201 is measured by the displacement meter 217, and the measurement data is sent to the processing device 216. The processing device 216 controls the position of the cylinder 220 based on the measurement data from the displacement meter 217. The vicinity of the minute gap 208 is heated by the heater 214, and this temperature is constantly monitored by the thermocouple 215. The processing device 216 acquires temperature data, displacement data, and leakage amount and stores them in the memory. The leak test apparatus 200 may be a horizontal type as well as a vertical type.

The conditions of the metal gasket 204 used for the leak test, the flange 202 and the block 201 in contact with the metal gasket 204 are shown below.
(1) Metal gasket Type: Tripack (elastic cored metal O-ring), manufactured by Nippon Valqua Industries, Ltd. Structure: Outer coating ... Aluminum
Inner coat ... Inco. 600
Coil spring: Inco. X-750
Dimensions: Hoop inner diameter 176 mm, wire diameter (cross-sectional diameter) 5.6 mm
(2) Flange and block gasket groove dimensions: OD208mm x ID175.6mm x t4.5mm
Material: SUS304, Rmax 3-9 μm

  FIG. 3 shows the results of a shift test using the leakage test apparatus 200 shown in FIG. The test conditions are shown in Table 1 below.

As a result of this test, it was confirmed that the leak rate increased as the deviation amount increased. Further, under all the test conditions, a result of an allowable leakage rate during transportation (1 × 10 ⁻⁴ Pa · m ³ / s) or less could be obtained. In particular, even when the amount of deviation was 3 mm, the leakage rate was 1 × 10 ⁻⁷ Pa · m ³ / s, which was confirmed to be significantly below the allowable leakage rate. In addition, it has been found that the leakage rate decreases at high temperatures. This is considered to be due to the improvement of the conformability due to the softening of the material. In addition, a clear tendency could not be found in the influence of the flange roughness on the leakage rate.

  This test result shows that the smaller the deviation amount, the smaller the leakage rate. Therefore, the amount of bending is suppressed by fixing the secondary lid 8 of the cask 100. FIG. 4 is a partial cross-sectional view showing such a cask. In the cask 150, the secondary lid 8 is inserted into the flange portion 151, and a positioning member 153 having a wedge-shaped cross section is fitted into the gap 152 between the secondary lid 8 and the flange portion 151. The positioning member 153 is a ring-shaped member divided as shown in FIG. 5 and is fitted evenly into the three portions of the gap 152.

  Thereby, since the secondary lid 8 can be fixed to the flange portion 151, the amount of displacement of the secondary lid 8 can be suppressed to some extent. For this reason, the leakage rate can be further reduced. The positioning member 153 may be, for example, a nail-like member or a plate material made of a soft material other than the divided annular members. Further, the positioning member 153 may be attached when the cask is stored, or may be attached when the cask 100 is transported. When attaching only at the time of cask transportation, it is economical because the positioning member 153 can be reused.

  According to the casks 100 and 150 of the above invention, since it is not necessary to replace the synthetic rubber O-ring, the transportation work of the casks 100 and 150 can be greatly simplified. Furthermore, since the secondary lid 8 is fixed by the positioning member 153, the displacement of the secondary lid 8 can be suppressed and the amount of leakage can be reduced.

  FIG. 6 is a partial cross-sectional view showing a cask according to a second embodiment of the present invention. The cask 300 is characterized in that the secondary lid is omitted. A lid 303 is provided on the flange portion 302 of the trunk main body 301, and the lid 303 and the flange portion 302 are airtightly held by the metal gasket 20 similar to the above. The lid 303 is provided with a valve 304 for replacing gas in the cask. The valve 304 is covered with a valve cover 305.

  A metal gasket 306 similar to the above is also interposed between the valve cover 305 and the lid 303. The lid 303 is fixed to the trunk main body 301 with a bolt 307. In this cask 300, the O-ring replacement at the time of cask transportation is omitted and the metal gasket 20 is conveyed as in the above.

  According to the cask 300, in addition to being transportable using the metal gasket 20, the weight of the cask 300 can be reduced by the amount that the secondary lid is omitted. In addition, operations related to the secondary lid, such as vacuum drying, helium filling, piping installation and removal, and leaked confidentiality inspection are not required, and handling is facilitated. Further, since the differential pressure between the inside of the cask and the outside air becomes small, it becomes difficult to leak.

  From the above description, the following invention is disclosed. A radioactive waste storage cask according to the present invention is provided with a barrel body containing a spent fuel assembly in a bottomed cavity and a neutron shield around the basket body, and attached to a flange portion of the trunk body For sealing the cask, only the seal groove for the metal seal is provided at the contact portion between the flange portion and the lid, and the metal seal is inserted into the seal groove.

  Since this cask has a configuration in which only a seal groove for a metal seal is provided, a seal groove for an O-ring made of synthetic rubber is unnecessary. For this reason, the trouble of processing the seal groove for the O-ring as in the prior art can be saved. The cask refers to a pressure vessel for storing radioactive waste such as spent fuel assemblies and storing them. This cask is served for storage and transport using only a metal seal. That is, since only the seal groove for the metal seal is provided, there is no room for replacing the rubber O-ring, and internal gas leakage during transportation can be suppressed to a range where there is no problem with exposure by using only this metal seal.

  It was common knowledge that conventional casks should be replaced with O-rings based on general recommendations from metal seal manufacturers. Therefore, the inventors of the present application have conducted a test of the cask leakage rate, which has not been conventionally performed, by developing a dedicated test apparatus described below. From this test, it was found that there was no problem with leakage when transported using only a metal seal. For this reason, since the cask can be transported with the metal seal while omitting the replacement work to the O-ring, the labor of transportation is greatly reduced. Moreover, the trouble of processing the seal groove for the O-ring can be saved. The metal seal includes a metal gasket and a metal O-ring.

  In the cask for radioactive waste storage according to the present invention, a barrel main body provided with a basket for containing spent fuel assemblies in a bottomed cavity and provided with a neutron shield around the basket is attached to a flange portion of the trunk main body. For sealing the cask, only the metal seal seal groove is provided in the contact portion between the flange and the lid, and the metal seal is inserted into the seal groove. Since it can be omitted, the labor of transporting the cask is greatly reduced. Moreover, the trouble of processing the seal groove for the O-ring can be saved.

  The radioactive waste storage cask according to the present invention is a cross section of the radioactive waste storage cask according to the present invention, in which a lid is attached to the flange portion and then fitted into a minute gap between the flange portion and the lid. It has a wedge-shaped positioning member.

  The reason why the metal seal becomes a problem in the above-described conventional art is that the amount of displacement of the lid with respect to the flange portion is large. However, it is difficult to reduce the gap between the flange portion and the lid so that no deviation occurs. For this reason, a lid is once attached to the flange portion, and a positioning member having a wedge-shaped cross section, for example, a ring member having a wedge-shaped cross section is fitted into the gap between the flange portion and the lid to fix the lid. In this way, since the amount of displacement of the lid when the cask falls can be reduced, gas leakage due to displacement from the metal seal can be reduced.

  In the radioactive waste storage cask according to the present invention, the cask has a wedge-shaped positioning member that fits into the minute gap between the flange portion and the lid after the lid is attached to the flange portion of the cask. The amount of displacement of the lid when dropped can be reduced. For this reason, the gas leakage by the shift | offset | difference with a metal seal can be decreased.

  Further, according to the method for transporting the radioactive waste storage cask according to the present invention, the spent fuel assembly is accommodated in the cavity of the trunk body, a lid is attached to the flange section of the trunk body, and the flange section and the lid are attached. Sealing is performed with a metal seal, and the sea and public roads are transported in this state.

  Conventionally, due to the risk of damage to the seal when the cask is dropped, the metal seal is replaced with a synthetic rubber O-ring when the cask is transported. On the other hand, the inventors of the present application have confirmed that the cask can be transported also by a metal seal as a result of the above-described test research. Therefore, the replacement procedure is omitted, and the transportation procedure can be greatly simplified.

  In the method for transporting a radioactive waste storage cask according to the present invention, a spent fuel assembly is accommodated in a cavity of a trunk body, a lid is attached to the flange portion of the trunk body, and a gap between the flange portion and the lid is provided. Is sealed with a metal seal, and the sea and public roads are transported in this state, so that the replacement procedure is omitted and the transport procedure can be greatly simplified.

  Further, the method for transporting a radioactive waste storage cask according to the present invention is the above-described method for transporting a radioactive waste storage cask, further comprising: attaching the lid to the flange portion; A positioning member having a wedge-shaped cross section is fitted into a minute gap with the lid.

  In this way, the lid can be fixed by fitting the positioning member having a wedge-shaped cross section between the flange portion and the lid during cask transportation. For this reason, since it becomes difficult for a lid to slip | deviate when a cask falls, the sealing failure of a metal seal becomes difficult to occur.

  In the method for transporting a radioactive waste storage cask according to the present invention, after a lid is attached to the flange portion of the cask, a positioning member having a wedge-shaped cross section is provided in the minute gap between the flange portion and the lid when the cask is transported. I tried to fit it. For this reason, since the amount of displacement of the lid when the cask falls can be reduced, gas leakage due to displacement from the metal seal can be reduced.

  Moreover, the radioactive waste storage cask according to the present invention is the above-described radioactive waste storage cask, further comprising only one lid of the cask. In this way, the structure can be simplified while maintaining the airtightness of the cask.

  In the radioactive waste storage cask according to the present invention, since the cask has only one lid, the structure can be simplified while maintaining the airtightness of the cask.

  As described above, the gas leak test apparatus for a cask for radioactive waste storage according to the present invention is useful for a gas leak test of a cask for storing radioactive waste, particularly when a metal seal is provided on the lid of the cask. Suitable for gas leak tests.

It is a partial cross section figure which shows the cask concerning Example 1 of this invention. It is a block diagram which shows a leak test apparatus. It is a figure which shows the result of having performed the shift test using the leak test apparatus shown in FIG. It is a partial cross section figure which shows the modification of the cask shown in FIG. It is explanatory drawing which shows the positioning member shown in FIG. It is a partial cross section figure which shows the cask concerning Example 2 of this invention. It is sectional drawing which shows the structure of the conventional cask. FIG. 8 is a partially enlarged view of the cask shown in FIG. 7. It is an enlarged view which shows the seal part of a cask. It is explanatory drawing which shows a metal gasket.

Explanation of symbols

1 Body 6 Flange 7 Primary lid 8 Secondary lid 12 Space 20, 21 Metal gasket 101 Gasket groove 100 Cask

Claims (1)

Forming a metal seal holding structure such as a seal groove or a protrusion on the first flat portion, and a first block holding the metal seal in the holding structure;
A second block having a second plane portion in surface contact with the first plane portion of the first block, and forming a sealed space in part by contacting the second plane portion with a metal seal;
A gas supply passage formed in the first block or the second block for supplying a test gas to the sealed space;
Gas detection means for detecting gas leaked from the sealed space;
A load applying means for applying a load to the first block or the second block and relatively moving the first block and the second block on the contact surface;
A gas leakage test apparatus for a radioactive waste storage cask characterized by testing leakage of internal gas when the metal seal is used for sealing a cask.

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