EP1580764B1

EP1580764B1 - Metal gasket, method of manufacturing metal gasket, and radioactive-material container

Info

Publication number: EP1580764B1
Application number: EP04007344A
Authority: EP
Inventors: Katsunari c/o Kobe Shipyard & Machinery W Ohsono; Shinji Ookame; Masaharu Minami
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2004-03-26
Filing date: 2004-03-26
Publication date: 2012-03-07
Anticipated expiration: 2024-03-26
Also published as: EP1580764A1; ATE548736T1

Abstract

A metal gasket includes a coil spring (30a, 30b) that forms a ring, an inner cover (40a, 40b) that covers the coil spring (30a, 30b), and an outer cover (50) that covers the inner cover (40a, 40b). The inner cover (40a, 40b) has a hole (101a, 101b) for draining water from the metal gasket.

Description

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a technology for sealing a radioactive-material container.

2) Description of the Related Art

A nuclear fuel assembly that is at the end of a nuclear fuel cycle and finishes its combustion is called a recycle fuel assembly. The recycle fuel assembly is cooled at a cooling pit of a nuclear power plant for a long period of time because the recycle fuel assembly contains highly radioactive materials such as fission product (FP) and requires thermal cooling. Then, the recycle fuel assembly is contained in a radioactive-material container, and conveyed to a reprocessing facility to be stored. Since the radioactive-material container contains highly radioactive materials, a strict care should be paid on its sealing for a storing period of 40 years to 60 years. Generally, the radioactive-material container is sealed by using a metal gasket between a lid and a body. An example of a structure for securing the sealing of the metal gasket is a single-ring-type metal gasket that suppresses a leak by forming a solid lubricating clad between an inner cover and an outer cover (see Japanese Utility Model Application Laid-Open, H5-75154 ).
When containing the recycle fuel assembly in the radioactive-material container, a containing process is conducted in a pool. Therefore, after containing the recycle fuel assembly, it is necessary to remove water inside the radioactive-material container by vacuum drying. However, the conventional metal gasket formed by a double structure, where the outer cover is configured to grasp a ring-shaped coil spring, the water collects inside the inner and the outer rings and it is hard to drain the water to the outside. Moreover, since the conventional metal gasket is configured to wind a plate-shaped inner cover around the coil spring, it is necessary to wind the inner cover in such a manner that an opening faces the outside when manufacturing. Consequently, it is difficult to have the openings of both inner rings faced the inside to make it easy to remove the water inside the metal gasket.
GB-A-2 001738 discloses a metal gasket for a lid of a container with the features of the preamble portion of claim 1.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve at least the problems in the conventional technology.
The metal gasket according to the present invention includes the features of claim 1.
The method of manufacturing a metal gasket, according to another aspect of the present invention includes the feaures of claim 3.
The radioactive-material container according to still another aspect of the present invention includes a body that includes a cavity to accommodate a recycle fuel assembly; a lid that is arranged on an opening of the cavity; and the metal gasket according to the above aspects between the body and the lid.
The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross section of a radioactive-material container;
Fig. 2 is a partial enlarged-view of the radioactive-material container;
Fig. 3 is a schematic for illustrating a sealing structure of a metal gasket according to the first embodiment;
Fig. 4 is a cross section of the medal gasket according to the first embodiment;
Fig. 5 is a cross section of a metal gasket
Fig. 6 is a perspective view of a coil spring of the metal gasket according to the first embodiment of the present invention;
Fig. 7 is a side view of the coil spring of the metal gasket according to the first embodiment;
Fig. 8 is a cross section of a metal gasket; and
Fig. 9 is a cross section according to a modification of this metal gasket.

DETAILED DESCRIPTION

Exemplary embodiments of a metal gasket, a method of manufacturing the metal gasket, and a radioactive-material container according to the present invention will be explained below in detail with reference to the accompanying diagrams. An applicable scope of a metal gasket according to the present invention is not especially limited, and, for example, the metal gasket may be applied to a sealing part of a radioactive-material container, and a sealing part of a reactor container. The metal gasket is suited to an application that requires maintaining the sealing performance for decades in the comparatively-high-temperature environment, more especially a radioactive-material container that accommodates the recycle fuel assembly, is conveyed, and stores the recycle fuel assembly for a long period of time.
Fig. 1 is a cross section of a radioactive-material container 500. Fig. 2 is a partial enlarged-view of the radioactive-material container 500. The radioactive-material container 500 includes a body 501, which is made of stainless or carbon steel, an external cylinder 502, which composes an external surface of the radioactive-material container 500, a resin 503, which is a polymeric material that contains hydrogen and fills a space between the body 501 and the external cylinder 502, a bottom plate 505, which is welded to the bottom of the body 501 and in which a resin 504 is enclosed, and a lid 520, which is arranged on an opening 509o of a cavity 509. The opening 509o corresponds to a flange member 506, which is welded to the body 501. The lid 520 includes a primary lid 507 and a secondary lid 508. The flange member 506 may be formed to unite with the body 501. A basket 513 that accommodates the recycle fuel assembly is arranged inside the cavity 509 of the body 501. The primary lid 507 and the secondary lid 508 are fixed using bolts 510, 511 on the flange member 506, and a resin 512 is enclosed in the secondary lid 508.
The basket 513 is composed of a plurality of cells that is formed of boron/aluminum composite. The cavity 509 is filled with negative-pressure helium gas while a space between the primary lid 507. On the other hand, the secondary lid 508 is positive pressured, so that a pressure barrier is formed between the inside and the outside of the radioactive-material container 500. A hole 514 is arranged in the secondary lid 508 to measure the pressure of a space between the primary lid 507 and the secondary lid 508, and a pressure sensor 515 is arranged on the outlet of the hole 514.
A valve is arranged in the primary lid 507 to replace the gas inside the radioactive-material container 500, and covered with a valve cover. To seal a gap between the primary lid 507 and the body 501 or a gap between the secondary lid 508 and the body 501, a metal gasket 20 that is heat resistant, corrosion resistant, and highly durable is used because the gaps must be sealed for a long period of time. Using the bolts 510, 511, the metal gasket 20 is fixed to a gasket gap 525 that is formed by machining. For example, when the secondary lid 508 and the body 501 are fixed, the metal gasket 20 is tightened and transformed, and a sealing function is exerted.
Fig. 3 is a schematic for illustrating a sealing structure 10 of a metal gasket 20 according to the first embodiment. A feature of the sealing structure 10 is that a hole for draining water is made in an inner cover. Various types of metal gaskets are applied to the present invention. For example, the metal gasket 20, shown in Fig. 4, is so-called double ring type and includes two coil springs 30a, 30b. The coil springs 30a, 30b have the different hoop diameters Df, are arranged concentrically, and covered with an outer cover 50. Another example that is not covered by the present invention is the metal gasket 20', shown in Fig. 5, that includes an outer cover 50' with a flat upper part, and a single-type metal gasket (not shown). The coil springs 30a, 30b may be called a coil spring 30 or coil springs 30 in the following explanation for convenience.
The sealing structure 10 and the metal gasket 20 shown in Fig. 3 is arranged between the secondary lid 508 and the flange member 506 of the body 501, and may be arranged between the primary lid 507 and the flange member 506of the body 501. In the metal gasket 20, inner covers 40a, 40b respectively cover the coil springs 30a, 30b that are substantially circular, so that an outer ring 70a and an inner ring 70b are formed. Then an outer cover 50 covers the outer ring 70a and the inner ring 70b. The outer cover 50 has bolt holes and bolts penetrate the bolt holes to fix the metal gasket 20 on the gasket gap 525. The inner covers 40a, 40b may be called an inner cover 40 or inner covers 40 in the following explanation for convenience.
A water drain hole 101 a, which is a hole for draining water, is made where the inner cover 40a is not covered with the outer cover 50, namely, where the inner cover 40a is not disturbed by the outer cover 50, and is made in the inner cover 40a to face the center of the body 501. Therefore, the water drain hole 101 a is made only in the inner cover 40a. After the water drain hole 101 a is made in the inner cover 40a, the inner covers 40a, 40b are wound round the coil springs 30a, 30b respectively to have a ring section.
In the metal gasket 20, the hoop diameter is approximately 2 meters, the sectional diameters D of the outer ring 70a and the inner ring 70b are approximately 5.5 millimeters, the thickness of the outer cover 50 is 0.4 millimeter, and the thickness of the inner cover 40s is 0.4 millimeter. As a material of the outer cover 50, a soft metal, such as aluminum, silver, copper, and nickel, is used to ensure the seal performance. As a material of the inner cover 40 and the coil spring 30, a nickel alloy, such as Inconel (a registered trademark), which is corrosion resistant and oxidation resistant at high temperature, or stainless, is used to maintain the elasticity in a high-temperature environment. Nimonic (a registered trademark), which has a high Co content, or the like may be used to improve the durability at higher temperature.
As a material of the secondary lid 508, the primary lid 507, and the body 501, stainless steel or carbon steel is used to block off the radiation and maintain the mechanical strength. The secondary lid 508, the primary lid 507, and the body 501 make a contact with the outer cover 50, which is made of a soft metal. Therefore, for example, when the outer cover 50 makes a contact with the body 501, a voltage potential difference is caused by a contact between the different metals, so that the galvanic corrosion occurs and the air tightness is broken. To prevent the galvanic corrosion, SUS317 or SUS625, which contains molybdenum, is used as a material of the secondary lid 508 and the body 501.
SUS317 and SUS625 have a good weldability and are generally suited to an application that has a lot of parts to be welded, such as a radioactive-material container. SUS625, SUS314, SUS316, SUS326, and SUS345 may be used as an alternative to SUS 317 and SUS625. Instead of making the secondary lid 508 and the body 501 of SUS317 or the like, a sealing surface 90 may be made on the secondary lid 508 and the body 501 to have a bulge portion using the same material.
Another way to prevent the galvanic corrosion, aluminum may be used as a material of the outer cover 50. Aluminum has the higher corrosion potential than SUS317 or SUS 625, which is a material of the secondary lid 508 and the body 501. Therefore, when the outer cover 50 is made of aluminum, the outer cover 50 is corroded more easily than the secondary lid 508 and the body 501. However the outer cover 50 can be replaced more easily than the secondary lid 508 and the body 501, so that the secondary lid 508 and the body 501 can be protected from the galvanic corrosion. The same explanation is applied to a combination of the primary lid 507 and the body 510.
Fig. 6 is a perspective view of a coil spring of the metal gasket according to the first embodiment of the present invention. Fig. 7 is a side view of the coil spring of the metal gasket according to the first embodiment. The coil spring 30 is formed by winding a wire material closely. If the wire material is not wounded closely, when the medal gasket 20 is sealed and squashed, the spring coil 30 can not push back the inner cover 40 and the outer cover 50 with a uniform force and the seal performance can not be exerted for a certain distance in long-time usage.
Generally, to gain the higher seal pressure in the metal gasket 20, the coil spring 30 is required to have the larger wire diameter d, that is, to have the higher flexural rigidity. The wire diameter d of the coil spring 30 gets larger, winding the wire gets harder and the winding diameter D1 of the coil spring 30 gets larger. Consequently, when the metal gasket requires a high seal-pressure, the cross-sectional diameter D of the metal gasket 20 gets larger comparing to that of what requires a low seal-pressure. The flexural rigidity of the coil spring 30 indicates how hard to transform the coil spring 30 to the radial direction when force P is acted on the coil spring 30 to the radial direction.
After the radioactive-material container 500 is sunk in the pool and the recycle fuel assemblies are accommodated in the radioactive-material container 500, the radioactive-material container 500 is lifted up from the pool and the water is removed from the radioactive-material container 500 by vacuum drying. In this process, the water enters inside the coil springs 30a, 30b. Especially, the water enters inside the coil spring 30b easily because the inner cover 40b has an opening 40s inside the metal gasket 20. Even if the inner cover 40a does not have the opening, the water enters from a gap between the outer cover 50 and the inner cover 40a and collects inside the coil spring 30a or the water enters from a surface between the outer cover 50 and the inner cover 40a and collects inside. When the water is removed from the radioactive-material container 500, the water that collects inside the metal gasket 20 is removed. In the outer ring 70a, the water is removed from the water drain hole 101 a to the outside. In the inner ring 70b, the water is removed from the opening 40s and the inner cover 40b is required to have a hole for draining the water. However, the inner cover 40b is required to have a water drain hole 101b, which is a hole for draining the water from a gap between the inner cover 40b and the outer cover 50, in the cases of:

1) draining the water when the opening 40s is arranged on the comparatively upper side and the inner cover 40b has the shape that collects the water easily; and
2) draining the water that collect at the surface between the outer cover 50 and the inner cover 40b. The water drain holes 101 a, 101 b may be called a water drain hole 101 or water drain holes 101 in the following explanation for convenience.

In the production of metal gasket 20, after the water drain holes 101 are made in the inner covers 40, the inner covers 40 are wound round the coil springs 30. Consequently, the water drain holes 101 can be made in the inner covers 40 without paying an attention not to damage the coil springs 30.
The water drain hole 101 a is made to face the center of the body 501, substantially corresponding to the direction of a line L that joins a center of the outer ring 70a to that of the inner ring 70b, because if the water drain hole 101 a faces obliquely downward, the opening of the water drain hole opening may be covered with the sealing surface 90 and may not work. Preferably, the angle between the direction that the water drain hole 101 faces and the line L is ±45 degrees.
Fig. 8 is a cross section of a metal gasket 200 not covered by the present invention. Fig. 9 is a cross section according to a modification of this metal gasket. The metal gasket 200 includes inner covers 210a, 210b and the inner covers 210a, 210b have a ring section, have the different hoop diameters, and are arranged concentrically. The hoop diameter of the inner cover 210a is larger than that of the inner cover 210b. An outer cover 50 covers the inner covers 210a, 210b and forms a double-ring. A metal gasket that is applied to the present invention is not limited to the type of the metal gasket 200, but may be a metal gasket 250 so-called a single type metal gasket. The metal gasket 250 includes an outer cover 220c and an inner cover 210c that has a water drain hole 230c, which is a hole for draining the water. Hereinafter, the inner cover 210a, the inner cover 210b, and the inner cover 210c may be called an inner cover 210 or inner covers 210 and the outer cover 220A and the outer cover 220C may be called an outer cover 220 for convenience.
As a material of the outer cover 220, a soft metal such as aluminum, silver, copper, and nickel is used to ensure the sealing performance. As a material of the inner cover 210, a nickel alloy such as Inconel (registered trademark), which is corrosion resistant and oxidation resistant at high temperature, is used to maintain the elasticity. Nimonic (registered trademark), which has a high Co content, may be used to improve the durability at higher temperature. The inner covers 210 are formed substantially circular and to have a ring section by winding a plate material while overlapping an edge of the plate material with another edge, so that the inner covers 210 works as a spring. When the metal gasket 200 or the metal gasket 250 is compressed by sealing, an overlapped part 210s slides and a diameter of the inner cover 210 gets smaller. Though the diameter of the inner cover 210 gets smaller, a contact surface with the inner cover 210 does not get larger, therefore, sealing can, be performed certainly.
Water drain holes 230a, 230a', and 230b are formed in the inner covers 210a and 210b. The water drain holes 230a and 230a' formed in the inner cover 210a is for draining the water from the metal gasket 200. The water drain hole 230b is for draining the water from a gap between the inner cover 210b and the outer cover 50. The inner covers 210 may have a plurality of holes for draining the water. The water drain holes 230a, 230b are made to face the center of the body 501 and arranged at substantially middle of the height of the metal gasket 200. If the water drain holes 230a, 230b are made below, the water drain holes 230a, 230b are disturbed by the outer cover 220A, therefore the outer cover 220A needs to have a hole for draining the water. If the water drain holes 230a, 230b are made close to a sealing surface, the water easily enters between the outer cover 220A and the sealing surface but it is hard to remove the water. Preferably, the water drain holes 230a, 230b are arranged to make an angle of at most 45 degrees to a line I, which joins a center of an outer ring 270a to a center of an inner ring 270b.
On the other hand, the inner ring 270b has the overlapped part 210s arranged inside the metal gasket 200. Therefore, the water drain hole 230a' needs to be arranged where the water drain hole 230a' is not covered with the overlapped part 210s when the overlapped part 210s slides by sealing.
The metal gasket 250 is formed in the following order:

1) making the water drain hole 230c in the inner cover 210c that is plate-shaped;
2) overlapping the inner cover 210c with the outer cover 220c that is plate-shaped; and
3) winding the inner cover 210c and the outer cover 220c that are overlapped and have two edges in such a manner the two edges have an overlap and a cross section is a ring shape. The water drain hole 230c is made at substantially middle height of the metal gasket 250. If the inner cover 210c and the outer cover 220c are separate plates, the water drain hole 230c is made in the opposite surface of the overlapped part 210s.

According to the present invention, the seal performance can be sufficiently exerted for a long time by removing the water inside the metal gasket.

Claims

A metal gasket (20) for a lid (520) of a radioactive-material container (500), said metal gasket (20) comprising:
a first coil spring (30a) that forms a first hoop-shaped ring having a first hoop diameter, and a second coil spring (30b) that forms a second hoop-shaped ring having a second hoop diameter which is smaller than that of the first coil spring (30a), wherein the first coil spring (30a) and the second coil spring (30b) are arranged concentrically;

a first inner cover (40a) that covers the first coil spring (30a) so as to form an outer ring (70a), and a second inner cover (40b) that covers the second coil spring (30b) so as to form an inner ring (70b) : and

an outer cover (50) that partially covers the first inner cover (40a) and the second inner cover (40b) and connects the outer ring (70a) with the inner ring (70b), wherein the part of the first inner cover (40a) which is not covered with the outer cover (50) is an arc segment of the first inner cover (40a) facing towards the inner ring (70b) and located within angles of +/-45 degrees with respect to a line (L) that connects a sectional center (E) of the first coil spring (30a) and a sectional center (E) of the second coil spring (30b);

characterized in that

a first water drain hole (101a) is provided in the first inner cover (40a) and a second water drain hole (101b) is provided in the second inner cover (40b), wherein said first and second water drain holes (101a, 101b) are configured to remove water from the metal gasket (20);

the first water drain hole (101a) and the second water drain hole (101b) are respectively provided such that the direction in which the holes face is in a range between +/-45 degrees with respect to the line (L) that connects the sectional centers (E) of the first and second coil springs (30a,30b); and the first water drain hole (101a) in the first inner cover (40a) is arranged in the part of the first inner cover (40a) which is not covered with the outer cover (50).
The metal gasket according to claim 1, wherein
the second water drain hole (101b) is configured to remove water collected at a gap between the second inner cover (40b) and the outer cover (50).
A method of manufacturing a metal gasket (20) for a lid (520) of a radioactive-material container (500), comprising:
forming a first hoop-shaped ring having a first hoop diameter from a first coil spring (30a);

forming a second hoop-shaped ring having a second hoop diameter which is smaller than that of the first coil spring (30a) from a second coil spring (30b);

making a first hole (101a) in a first plate material;

making a second hole (101b) in a second plate material;

forming an outer ring (70a) by winding the first plate material around an outer surface of the first coil spring (30a) such that the first plate material forms a first inner cover (40a) that covers the first coil spring (30a);

forming an inner ring (70b) by winding the second plate material around an outer surface of the second coil spring (30b) such that the second plate material forms a second inner cover (40b) that covers the second coil spring (30b);

arranging the outer ring (70a) and the inner ring (70b) concentrically; and

winding an outer cover (50) around the first inner cover (40a) and the second inner cover (40b) such that the outer cover (50) partially covers the first inner cover (40a) and the second inner cover (40b) and connects the outer ring (70a) with the inner ring (70b), wherein the part of the first inner cover (40a) which not covered with the outer cover (50) is an arc segment of the first inner cover (40a) facing towards the inner ring (70b) and located within angles of +/-45 degrees with respect to a line (L) that connects the sectional centers (E) of the first and second coil springs (30a, 30b);

wherein said first and second holes (101a;101b) are provided such that the direction in which the holes face is in a range between +/-45 degrees with respect to the line (L) that connects the sectional centers (E) of the first and second coil springs (30a, 30b);

wherein said first hole (101a) in the first inner cover (40a) is arranged in the part of the first inner cover (40a) which is not covered with the outer cover (50); and

wherein said first and second holes (101a;101b) are configured to remove water from the metal gasket (20).
A radioactive-material container (500) including
a body (501) having a cavity (509) for accommodating a recycle fuel assembly;
a lid (520) that is arranged on an opening of the cavity (509); and
a metal gasket (20) according to claim 1 or 2 arranged between the body (501) of the radioactive-material container (500) and the lid (520) to maintain air tightness of the cavity (509).