JPS61120094A - Closing mechanism of storage cask for spent nuclear fuel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an extremely reliable spent fuel storage cask closure mechanism that can be leak tested during subsequent long-term spent fuel storage.

Dry storage casks are one route to the problem of long-term storage of spent fuel. After being stored for perhaps 10 years or so, in which the heat generated by the fuel assemblies reaches a certain amount, say 0.5 to 1.0 kilowatts per assemblage, the spent fuel is transferred to casks, which are then sealed. Transfer to above ground storage area for long term storage.

The requirements placed on this type of cask are quite demanding. The cask must be resistant to chemical attack from the surrounding environment during long-term storage. Furthermore, it must be sufficiently mechanically robust that it will not suffer even the slightest tear or breakage during long periods of storage and during transportation where the cask may be subject to rough handling or fall ring accidents. No. Additionally, the cask must conduct heat generated from the spent fuel to the surrounding environment and shield the surrounding environment from radioactivity emanating from the spent fuel. To prevent deterioration of the zirconium alloy housing, the temperature of the fuel rods should be kept at the maximum allowable temperature, e.g.
Must be kept below 5°C. Means shall be provided to ensure that a chain reaction is not sustained within the cask. That is, the effective critical coefficient aff must be less than 1 to prevent self-sustaining reactions from occurring. The above conditions place severe demands on the cask,
The cask must perform its storage function in a completely reliable manner.

During storage, the casks may be filled with helium, which readily transfers heat in the small gaps between the edges of each plate and each wall of the multiple channel sections. The cask, which can weigh more than 100,000 kg (100 tons) when filled with spent fuel, has a height of approximately 4.8 kg, excluding cooling fins.
A typical cask is one with a diameter of approximately 2.5 m.

The main object of the present invention is to have a closing mechanism that hermetically seals spent fuel inside the cask in an extremely reliable manner.
To provide a cask capable of performing a sealing (window sealing) test against gas leakage during cask closing work and after closing the cask.

In view of the above problems, the present invention comprises a cask base member for receiving spent nuclear fuel having a stepped mouth area including a first horizontal portion, and a lid assembly for closing said mouth area. a cask closure mechanism, the lid assembly comprising: a first cover having a peripheral area that rests on a first horizontal portion of the stepped mouth; and means for attaching the first cover to the mouth; a first gas seal disposed between the first cover and the first horizontal portion to prevent gas from leaking from inside the cask; and the first cover includes a first gas seal. a first gas passageway for injecting gas into the interior of the seal to test the first gas seal; and an exterior of the first gas seal of the first cover from the first cover seal during testing. A second gas seal for receiving leaked gas is provided.

The shielding lid can withstand mechanical shocks caused by accidents such as falls. The shielding lid preferably has an elastomeric O-ring cooperating with the mouth area of the base member of the cask and a closable channel for housing the water supply pipe or standpipe, and for temporarily inserting into the standpipe. It consists of a closable gas passageway that injects gas into the cask while removing water via an installed hose. After removing the water, the first cover is installed and the seal cover is then welded in place, the weld forming a second gas seal. The seal cover is provided with a closable gas passage to allow for redundant checking of gas leaks past the primary seal and for leak testing during long-term storage when the cover is installed.

Preferably, a pressure sensing device in communication with the interior of the cask is mounted within the cavity of the cask base member so that an alarm signal is issued when the pressure within the cask reaches a predetermined level. This alarm signal indicates an abnormal situation that requires immediate attention, or simply indicates that the transducer scale should be calibrated as a precaution. A closable gas passage leading to the cavity is provided within the cask body member to permit injection of gas to calibrate the transducer scale when an alarm signal is issued.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

A typical fuel assembly 20 for supplying fuel to a nuclear reactor is shown in FIG.
As shown in the figure. Fuel assembly 20 has a lower nozzle 22 and an upper nozzle 24 with a plurality of elongated fuel rods 26 disposed between the nozzles. Each fuel rod 26 has a cylindrical housing made of a zirconium alloy, such as commercially available Zircalloy-4, and is filled with fissile fuel pellets enriched with U-235. fuel rod 2
In the middle part of the nozzles 22 and 24 inside the assembly of No. 6,
A tubular guide (not shown) is provided, and a movably attached control rod (not shown) and measuring instruments (not shown) are housed therein. Nozzles 22 and 24 are attached to the ends of these tubular guides to form a skeletal support for the fuel rods 26, but the fuel rods are not permanently attached to the nozzles 22 and 24. .

Commercially available fuel assemblies include support grids 28 having a plurality of holes through which fuel rods 26 extend and tubular guides extending to bind these members together. , consisting of 179 to 264 fuel rods, depending on the design. A typical fuel assembly is approximately 4.1 meters long, 17.9 cm wide, and has a mass of approximately 585 kg.

After about three years of use in a pressurized wooden nuclear reactor, the enrichment of U-235 in the fuel assembly 20 decreases. Furthermore, various fission products with different half-lives are present in the fuel rods 26. These fission products are transferred to the fuel assembly 2
When 0 was removed from the reactor, it was emitting strong radioactivity and heat, so the assembly 20 was moved to a boule containing a boric acid solution dissolved in water (hereinafter referred to as "boric acid water") for a short period of time. Must be stored for a while. Such a Boolean is shown at 30 in FIG.

Boule 30 is typically 12.2 meters deep. A number of spent fuel racks 32 located at the bottom of the boule 30 are provided with storage slots 34 for storing a plurality of fuel assemblies 20 vertically. Boolean 3
A cask pad 36 is arranged on the bottom surface of 0.

During the period that fuel assembly 20 is stored within boule 30, the composition of the spent fuel within fuel rods 26 changes. Isotopes with short half-lives decay and, accordingly,
The proportion of isotopes with relatively long half-lives increases. Therefore, the radioactivity level and heat generated from the fuel assembly 20 within a certain period of time decrease relatively quickly, reaching a state where the rate of decrease in heat and radioactivity is extremely small. However, even at this reduced level, fuel rods 26 must be reliably isolated from the surrounding environment indefinitely into the future.

In FIG. 3, cask base member 38 of cask 40
has a floor 42 and a hollow interior 44 defined by a cylindrical wall 46.

The base member 38 is approximately 25 cm thick and is made of carbon steel, which serves to protect the surrounding environment from gamma radiation. For example, the section 48 covering the floor 42 and walls 46 may be made of stainless steel by placing the section 48 on a turntable and using a stainless steel welding rod to perform a continuous spiral weld to the inner section. A cover is provided and the stainless steel surface covers the interior 44 of the section 48 to protect the interior from chemical attack. The base portion 38 is surrounded by a neutron absorbing material 1 (52) having a thickness of about 7.0 cm, and resin can be used as the neutron absorbing material.

An example of a resin suitable for use as a neutron absorbing material is
Bisco Products, Inc., 1420 Renea Nance Drive, Park Ridge, Illinois.
oducts Inc,, 1420Renaissa
nce Drive, Park Ridge, l1
Product number NS-3 (5tack
No., N5-3). Surrounding object'i52 is an outer layer 54 of stainless steel that protects cask 40 from the surrounding environment. Cask 40 is
Additionally, it includes a plurality of cooling fins 56, preferably made of carbon steel, treated to protect the carbon steel from chemical attack by the surrounding environment. Fins 56 are welded to section 48 and extend through neutron absorber 52 and layer 54. Although not shown, a pair of trunnions are preferably attached to the top and bottom surfaces of base member 38 to facilitate handling.

With continued reference to FIG. 3, the lattice cage assembly 58 is
A first plate set 60 consisting of main plates 62.64.66.68 and 70, and main plates 74.76.78.8
and a second plate set 72 consisting of the blades o and 82. These aluminum sheets are approximately 3.7 meters high and approximately 2.5 meters thick. Can be made from ocm aluminum sheet. During manufacture, each of the metal plates of the first set 60 is provided with a plurality of upwardly oriented slots (not shown) extending from the bottom of each plate to the central portion of each plate, each having an approximately 26.3 cm diameter. arranged at intervals.

Each plate of the second set 72 is provided with a plurality of elongated slots (not shown) corresponding to the slots described above;
Extending from the top of each plate to the central portion, approximately 28.
They are placed at intervals of 3 cm. Assemble the first and second sets of plates by inserting their slots together (just put the combs facing each other and interlock them, then assemble the combs by bridging them together), then complete the entire length. Weld the mutual intersections by fillet welding. It can be easily seen that such a configuration creates a matrix (or lattice) of a plurality of elongated storage slots having a center-to-center spacing of approximately 26.3 cm and an axis extending generally perpendicular to the floor 42. The lattice cage assembly 58 includes a first set 60 of main plates.
and a number of small plates 86 welded to the periphery of the matrix by the second set 72. As shown in FIG. 3, the edge of each plate is movably inserted into an elongated channel section 88 extending generally perpendicular to floor 42 that is welded to wall 46. As shown in FIG. When the cask 40 is used, it forms a cross section in the positive direction and absorbs ``neutron poison J'' such as boron carbide.
An elongated cell (not shown) with stainless steel walls supporting a plurality of sheets of (neutron poison) material may be inserted into the storage slot 84.

Next, the fuel assembly 20 is inserted into the cell and stored in a helium atmosphere for a long period of time, during which time the heat generated from the fuel assembly 20 is transferred through the cage assembly 58 into the narrow helium channel section 88. The heat is transmitted to the wall portion 46 through the gap filled with heat and radiated. Alternatively, storage slots 84 may be utilized to provide an integrated metal container (c) without cells.
The fuel and fuel assemblies can also be stored in different storage slots of the same cask.

With the above summary in mind as background, a standpipe (water distribution pipe) 90 is shown in FIG. As shown in FIG. 4, a drain cutout 92 is provided at the bottom of the pipe 9o, so that the boric acid solution can reach the inside of the pipe 90 when draining the cask 40. This work will be described later. The lattice cage assembly 58 is spaced slightly upward from the floor 42 and does not obstruct the flow of water through the cutout 92. During water draining work, tilt the cask 40 slightly and cut out the notch 92.
Either make it easier for water to flow toward the floor 42, or provide the floor 42 with a slight slope toward the notch 92.

Continuing the explanation with reference to FIG.
4 and 74. Member 94 has an opening (not shown) through which pipe 90 passes, and member 94 is joined to pipe 94 by weld 96 . Similarly, a support member 98 with a triangular top surface having a hole through which the pipe 90 passes is welded to the plates 64 and 74. However, the pipes 100 are not welded to the member 98 and the pipes 90 will not stress the lattice cage assembly 58 even if the temperature within the cask 40 fluctuates and the thermal expansion of the assembly 58 and the pipe 90 differs. There is no such thing.

Next, referring to FIG. 5, the base member 38 has horizontal portion 1
04.106.108 and 110 terminate above the stepped mouth area 102. Portion 104 is vertical portion 1
12.114'BL and 116. A stainless steel cladding layer 50 extends above the stepped mouth area 102. Continuing with reference to FIG. 5, the lid assembly 1 has a threaded opening 118 in the horizontal section 104 and a threaded hole 120 in the horizontal section 106.
22 is a shielding lid 124 and a first cover 126 made of carbon steel.
and a seal cover 128 made of stainless steel. The shielding lid 124 consists of a carbon steel section 130 with a stainless steel cladding 132, which can be overlaid by welding as described above. Portion 130 is provided with an annular recess 134 into which it will extend during the coating operation. An annular recess 136 having a dovetail cross-sectional shape is cut into the stainless steel portion inside the recess 134. An elastomeric material O-ring 138 resides in the recess 136 and is permanently incorporated into the lid 124 during manufacture of the shielding lid 124, so that it is necessary to place the O-ring in place immediately before installing the lid 124 onto the cask base member 38. Rather, the portion of the stainless copper plated N50 on the horizontal portion 104 that receives the O-ring 138 is machined to form a flat surface to position the O-ring 138 in the proper position.

With further reference to FIG. 5, the channel 140 has a conical lower portion 142 and a head portion 146 of a stainless steel plug 148.
and an upper portion 144 shaped to receive the. An annular recess 156 within the head 146 receives a metal O-ring 152. The sides of the Q-ring 152 are connected to the wall of the recess 150 by applying a small amount of adhesive dotted on the wall of the recess 150 at several points during the manufacture of the plug 148, but the sealing of the O-ring 152 is Take care to avoid contaminating surfaces with adhesive. Alternatively, a plurality of recesses (
The O-ring 152 is captured and held in the plug 148 by providing two clamps (not shown) that are incorporated into the O-ring 152 and extend toward the O-ring 152 to hold the O-ring 152 in place. You can also do it. With O-ring 152 captured and held in the manner described above, plug 148 can be handled as an integral unit. A ring of unthreaded holes 154 on the periphery of the head 146 receives a cap screw 156 having a lower portion received in the threaded hole 124 of the shield lid 124.

Continuing the explanation with reference to FIG.
(Stand Vibe 90)
), the valve 159 includes a valve housing 161 welded to M124, a valve body 163 movably mounted inside the housing 161, and a valve body 163 that is movably attached to the inside of the housing 161.
A metal O-ring 165 captured at one end of 3 and 1
67, and a valve body 16 fixed to the lower end of the housing 161.
3 upwardly, and a spring 173 that presses downwardly a ball 171 movably housed inside the valve body 163. The valve 159 cooperates with a valve driver 175 inside the stand vibe 90, as explained below. The valve driver 175 has a plate 177 fixed inside the stand vibe 90 and having a passage ring 179 inside. Due to the thickness of the plate 177 and the shielding ring 183 around the standpipe 90, the spring 1
69 and 173 are protected from being completely affected by the neutron flux.

Still referring to FIG. 5, the gas passageway 160 consists of an enlarged middle portion 162 and an upper portion that houses the head 166 of a threaded stainless steel plug 168. The head 166 is provided with an annular recess, and the metal circle 17 is inserted in the same manner as described above.
0 is captured and held in the annular recess. Part 162
The upper section 172 of is threaded to receive the plug 168. Inside the upper section 172 of the portion 162,
A quick disconnect coupling 174 is attached. Coupling 174 has an internal ball valve (not shown) and is configured to snap connect with a corresponding quick disconnect coupling on the gas line. Quick disconnect couplings suitable for this purpose are well known in the art and commercially available.

Further, in FIG. 5, the shielding lid 124 has a peripheral edge 176 of reduced thickness with an unthreaded aperture 178 extending therethrough.

Next, attachment of the shielding lid 124 will be explained with reference to FIGS. 5 and 8. Prior to lowering the cask base 38 to the cask pad 36 (see FIG. 2), and preferably during assembly of the base 38, a plurality of studs 180 are coated with sealant and the threaded holes are sealed. Screw into 118. To keep the water outside of the hole 120, the cap threads 182 are also coated with sealant and temporarily installed into the hole 120. The fuel assembly 20 is the base member 38
and member 38 is still in cask bad 3.
6, it is assembled to the shielding lid 124. At the initial stage of assembly, plug 148 (including captured O-ring) and plug 168 (including captured O-ring 170) are not present in shield M124. By remote control, the M124 is lowered and aligned with the base member 38, and the standpipe 90 is inserted into the portion 142 of the channel 140 and the stud 1
80 is positioned to be inserted into the hole 178. Nut 184 is then screwed onto stud 18o, water removal hose 186 is inserted into channel 140, and metal O-ring 167 seals hermetically with the end of hose 186. When further downward force is applied by the hose 186, the position of the valve body 68 changes and finally the metal O-ring 16
5 hits the end of the standpipe 90 and hermetically seals it. Immediately before the hermetic seal described below is performed, bottle 181 displaces ball 171, creating a flow path from standpipe 90 through passage 179 and valve 159 to hose 186. A hem ram supply tube 186 terminating in a quick disconnect coupling is inserted into the gas passageway 160 and the coupling 188 is snapped into engagement with the coupling 174 to open the valve within the coupling 174 . Helium is injected into the cask 40, and at the same time boric acid water is removed via the hose 186 and pumped to the contaminant removal equipment. Since the boric acid water is raised to boiling temperature by the spent fuel inside the cask 40 during the boric acid water removal, there is no need for a long drying period to remove the last traces of water. After draining the water,
Once the hose 186 and tube 188 are removed, the quick disconnect coupling 174 and valve 159 act as a suitable shield to keep the helium inside the cask 40. next,
The cask 40 is removed from the boule 30 and allowed to dry, the cap screws 182 are removed, and the horizontal portions 106 are sanded in preparation for installing the first cover. Although the lid 124 is in place, the radiation flux passing through the lid 124 is strong enough to be hazardous to health, so installation is done by remote control. The shield M124 serves as the first protection against impact caused by, for example, an accidental fall or an accident when the cask 40 is placed in a horizontal position during transportation.

Returning to FIG. 5, the first cover 126 consists of a ring with 36 unthreaded holes 192 around its periphery, the top of which is enlarged to accommodate the cap of the cap screw 182. are doing. Gas passageway 194 has an enlarged middle portion 196 and an upper portion that accommodates head 200 of threaded plug 202 . The head 202 has an annular recess;
This annular recess accommodates a metal O-ring 204,
The O-ring 204 is captured and held within the recess in the same manner as described above. Intermediate portion 196 has a threaded area 206 that receives plug 200. A quick disconnect coupling 208 is mounted within portion 196 at a location remote from threaded section 206.

Continuing the explanation with reference to FIG. 5, the gas passage 210 is
Enlarged intermediate portion 212 and head 21 of threaded plug 218
It has an upper part 214 for storing 6. Head 282 has an annular recess within which metal O-ring 220 is captured and held in the same manner as described above. middle part 2
12 has a threaded section 222 that receives plug 218. A quick disconnect coupling 224 is attached to portion 212 at a location remote from compartment 222.

Continuing the explanation with reference to FIG. 5, the first cover 12
On the lower surface of 6, there is an annular recess 2 having a dovetail-shaped cross section and capable of capturing and holding an O-ring 228 made of an elastomer material inside.
26 are provided. An annular recess 330 concentric with the recess 226 is also provided in the first cover 126 . The recesses 226 and 230 are adjacent to each other, and a region intermediate between the recesses 226 and 230 communicates with the gas passage 210. A spring biased C-ring 232 is captured and retained within the recess 230, as will be described below. During assembly of the cask base member 38, the sections of the horizontal portion 106 that receive the rings 228 and 232 are machined to form a flat surface.

6 and 7, C-ring 232 has a helical spring 234 partially surrounded by a stainless steel cover 236. Referring now to FIGS. Spring-loaded C-rings of this type are commercially available. The second cover 126 is further provided with three other recesses 336 at equal intervals around the recess 230. Mounting ears 238 are fixed to these recesses 236 by cap screws 240. Installation is on ear part 2
A plurality of 38 fingers 242 extend into the middle portion of the coil of spring 234 and press ring 232 inside recess 230.
to capture.

Next, the procedure for attaching the first cover 126 will be explained with reference to FIGS. 5 and 9. The rings 228 and 232 are captured and held, but the plugs 202 and 218 (captured and held
(with rings 204 and 220) positions the removed cover 126 on the base member 38 so that the holes 192 are aligned with the holes 120.

Next, screws 182 are installed. Spring biased C ring 232
provides the first gas seal for differential assembly 122, and the installation of gas passageways 194 and 210 provides a first gas seal for first cover 12.
6, the first seal can be tested. Helium supply tube 244 terminating in quick disconnect coupling 246 is inserted into passageway 194 and coupling 246 is snapped into engagement with coupling 208 . Similarly, a test tube 248 terminating in a quick disconnect coupling 250 is inserted into passageway 210.

Approximately 2 atmospheres of helium is injected through tube 244 into the space between shield M 124 and first cover 126 . Leakage is preferably measured by a gas chromatograph helium leak detector connected to tube 248.

Temperature source detectors suitable for this purpose are commercially available. A typical value for the maximum allowable leakage rate is 10"" cm3 of helium per second at standard temperature and pressure.

The spring biased C-ring 232 provides excellent sealing properties as long as it is seated in the initial installed position. The purpose of the test procedure described above is to know if ring 232 is seated in the proper position. Small grid particles, slight defects or recesses 330 or portions 106 in the C-ring
Slight imperfections in the sealing surfaces of can result in unacceptable leakage. The other rings of the cask closure mechanism of the present invention do not need to be tested as ring 232 is not as severely constrained. The plugs 148, 168, 202 and 218 are by nature extremely reliable and, in any case, of relatively small dimensions, even the slightest defects do not cause excessive leakage. O-rings 138 made of elastomeric materials do not provide a reliable gas seal over long periods of time and are degraded by the heat generated from the spent fuel. The primary purpose of O-ring 138 is to seal shield M 124 and base member 138 when helium is injected through passageway 160 during the water removal process. Similarly, O-ring 228, which is made of an elastomeric material, is not reliable for long-term sealing and was split around ring 232 for sampling with a helium detector during testing of ring 232. It merely plays the role of forming an area.

Referring to the ls diagram again, remove the stainless steel cover 12.
8 has a gas passage 252 within which a quick disconnect coupling 254 is mounted.

Passageway 252 has a threaded section 256 and a portion 258 that accommodates the head of threaded plug 260 . The head of the threaded plug 260 has an annular recess in which a metal O-ring 262 is captured and retained as described above.

Next, referring to FIGS. 5 and 10, the seal cover 12
8 will be explained below. After the cover 128 is positioned on the horizontal portion 108, the total radioactivity escaping from the lid assembly 122 is reduced to a reasonable level, so that subsequent sealing operations can be safely performed without remote control. . The peripheral edge of seal cover 128 is welded to horizontal portion 110 by weld 264, which provides the second gas seal of the closure mechanism of the present invention. Helium supply tube 266 terminating in a quick disconnect coupling 268 snap-engaged with coupling 254 within gas passageway 252
Approximately 2 atmospheres of helium is injected into the intermediate area between the first cover 126 and the sealing cover 128 via the helium pump. Test for leaks around weld 264 using a gas chromatograph helium leak detector or other leak detection device.

Thereafter, a top cover 270 having a thickness of approximately 7,0 c+n, a layer of neutron absorber [272] and an outer layer 274 of stainless steel is glued over the seal cover 128.

The cover 270 has a cylindrical opening 2 that accommodates a plug 278.
76, and if it is desired to test the weld 264 at a later date, the cylindrical opening described above can be used to access the plug 260. Plug 278 is made of neutron absorbing material 280 with a stainless steel outer layer 282.
Consists of. After covering with the upper cover 270, the main body made of the neutron absorbing material 286 and the outer body made of stainless steel X
The closure operation is completed by welding the ring cover 284 with 288 in place.

After that, it is stored inside the cask 40.

The spent fuel can be transported to distant storage locations. The storage location is illustrated in FIG. 11, in which the cask 40 is shown resting on a reinforced concrete support 290 in a secluded area;
It is desirable to provide an access road for use during transportation and periodic monitoring of the cask 40.

While Cask 40 is in storage, in addition to regular visual monitoring, continuous electronic monitoring will be performed to indicate any gas leaks that could result in fission products such as radon reaching the surrounding environment. It is desirable that an alarm be issued when the pressure inside the cask 40 drops to a predetermined position. Returning to FIG. 5, the closure mechanism of the present invention may be provided with a monitoring device if desired. In FIG. 5, the base member 38 includes a gas passage 296.
A cavity 294 is provided that communicates with the. Cavity part 29
4 communicates with the inside of the cask 40 via a gas passage 298. Cavity 294 has a first cylindrical passage 300 communicating with passage 298 , a second cylindrical passage 302 communicating with passage 300 , and a third cylindrical passage 304 communicating with passage 302 . Captured and held metal O-ring 3
A gas pressure sensing device 306 having 08 is installed inside the passageway 300. Transducers suitable for use with device 306 are commercially available. -For example, MKS Instrument Co., Burlington, Massachusetts (
MKS Instruments Inc., Burl
A manual differential capacitance manometer (trade name 221AD-10000) available from Johnston, Massachusetts.
IIlanometer) can be used. Passage 302
has a threaded section 310 incorporating a first plug member 312 having a captured and retained metal O-ring 314;
The hollow cylindrical portion 316 of the member 312 extends to the rear surface of the assembly 306 to force the assembly 306 into intimate contact with the end of the passageway 300 and to place the rear surface of the assembly 306 in communication with the gas passageway 296. There is. Plug member 312 is further provided with a vacuum-sealed feed-through plug 318 for communicating electrical signals through plug member 312. This type of feed-through plug is known in the art and has a wire extending through the ceramic material and into the metal forming the remainder of the plug member 312. A plurality of leads 319 from gas pressure device 306 connect portion 316 and plug 31.
Referring further to FIG. 5, the third passageway 304 has a threaded section 322 that extends the second plug member 324 into the threaded section. Can be screwed in. The member 324 is provided with a captured and held metal O-ring 326 and a vacuum-sealed intrusion plug 328 made of a ceramic material with a conductor passing through it as described above. The shape of the adjacent surfaces of plug members 312 and 324 defines a cavity 3 that accommodates a lead wire 332 extending between plugs 318 and 328.
30. As can be seen from the above description, the gas pressure sensing device 306 is exposed inside the cask 40 and three metal O-rings 308, 314 and 326 hermetically seal the cavity 294 and allow the gas pressure sensing device 306 to escape from the device 316. The signals are from lead wire 319, plug 318, and lead wire 332.
' and available externally via insert 328. An external monitoring device (not shown) of conventionally known characteristics is electrically connected to the device 306 to provide an alarm signal when the gas pressure detector 306 indicates that the internal pressure of the cask 40 has fallen to a predetermined limit. is served. This alarm signal may be sent to a monitoring station, which may be alerted by ringing a bell or otherwise, and is located adjacent to the cask 40 on the support 290 and is a visual station used for observation during periodic monitoring rounds. A device (not shown) may also be operatively set.

An alarm signal does not necessarily imply destruction of the integrity of the cask. Alarms are far more often the result of false signals from pressure transducers within device 306. The electrical characteristics of a transducer tend to change gradually over time, and such electrical "drift" occurs particularly when the transducer is exposed to the harsh atmosphere inside the cask 40. easy.

Therefore, when an alarm signal is received, the transducer scale may be recalibrated to ensure that the alarm signal is valid before returning the cask 40 to the pool 30.

Continuing with reference to FIG. 5, the gas passage 296 terminates in a recess 334 into which a plug member 336 is inserted. The plug member 336 includes a metal O-ring 338 that is captured and held, and a threaded hole 3 of the base member 38.
A smooth hole 340 is provided which is aligned with 42 . Cap screws 344 are used to attach member 336 and seal passageway 296. After assembly, the peripheral edge of plug member 336 is welded to form a second shield.

Metal O-rings 308 and 338 and plug member 33
It will be appreciated that the weld at 4 prevents gas from escaping through the passageway 296.

Continuing with FIG. 5, after welding the plug member 336, a cylindrical plug member 348 having a body of neutron absorbing material 350 approximately 7.0 cm thick and an outer layer 352 of stainless steel will be incorporated. Similarly,
After installing the plug member 348, the neutron absorbing material 35
A cylindrical plug member 354 having a body of 6 and an outer layer 358 of stainless steel is placed over the plug member 224.

Upon receiving an alarm signal, the technician first measures the radioactivity around the cask 40, particularly in the area of plug members 348 and 354. Measurements indicate that the radioactivity levels surrounding plug members 348 and 354 are within acceptable limits, indicating that member 348 may be removed and the machined welded hermetic sealing plug member 336 removed. It turns out. Remove the above weld and measure the radioactivity again, and if it is within the acceptable level, cap screw 34
Loosen and remove the screw No. 4 and remove the plug member 336. A tube terminating in a quick disconnect coupling is then inserted into passageway 296 and helium is injected to bring the pressure inside passageway 300 to a predetermined level to calibrate the transducer inside device 306. If this recalibration determines that the alarm signal is spurious and not a true alarm, plug members 336 and 348 can be reassembled.

As can be seen above, the spent fuel cask closure mechanism according to the present invention provides a mechanically robust closure with a reliable overlapping seal. The lid assembly is constructed to facilitate draining of the cask and to facilitate testing of the first and second gas seals. The closure mechanism of the present invention is capable of monitoring the internal pressure of the cask and recalibrating the monitoring transducer over an extended period of time.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a typical fuel assembly. FIG. 2 is a top view of a pool for short-term storage of spent fuel assemblies. FIG. 3 shows a base member of a storage cask and a storage channel matrix located inside the base member that accommodates spent fuel assemblies and conducts heat generated from the spent fuel assemblies to the cask wall. It is also a sectional view showing a lattice cage assembly that supports a standpipe used when draining water from a cask. FIG. 4 is a perspective view of the stand pipe shown in FIG. 3, and is a diagram showing how the stand pipe is attached to the lattice cage assembly. FIG. 5 is a cross-sectional view of a portion of the mouth area of the cask base member and the lid assembly after installation. FIG. 6 is a cross-sectional view showing a metal spring-loaded C-ring captured in the first cover to form a first gas seal. FIG. 7 is a perspective view showing a portion of the C-ring of FIG. 6. FIG. 8 is a perspective view showing the upper part of the cask base member after the installation of the shielding lid member, and shows the water drain hose and gas pipe connected to the internal closable opening when draining water from the cask. FIG. FIG. 9 is a perspective view of the top of the cask body member after the first cover has been installed and shows a plurality of gas pipes attached to the closable gas passage during testing of the first gas seal; FIG. It is. Figure N10 is a diagram showing the insertion of the gas pipe into the gas passage which can be closed during the second seal test after the seal cover is attached. FIG. 11 is a perspective view showing the cask during long-term storage. 38...Base member 102...Stepped mouth area 122...Fruit set 126...First cover 182...Screw 194...First gas passage 210...Second gas passage 232...First gas seal

Claims (1)

[Claims] 1. A cask closing mechanism comprising a cask base member for receiving spent nuclear fuel having a stepped mouth area including a first horizontal portion, and a lid assembly for closing the mouth area. the lid assembly comprising: a first cover having a peripheral area that rests on a first horizontal portion of the stepped spout; means for attaching the first cover to the spout; a first gas seal disposed between one cover and the first horizontal portion to prevent gas from leaking from inside the cask; a first gas passageway for testing the first gas seal by injecting gas into the first gas seal;
and a second gas seal external to the seal for receiving gas leaking from the first cover seal during testing. 2. Another seal is further disposed between the first cover and the first horizontal portion outside the first seal, and the second gas passage is additionally disposed between the first seal and the first horizontal portion. A closure mechanism according to claim 1, characterized in that it communicates with the intermediate portion of the seal. 3. The first gas seal comprises a spring-loaded metal C-ring with a groove that captures the C-ring within the first cover, and the additional seal is located within the first cover. 3. A closure mechanism as claimed in claim 2, comprising a flexible O-ring disposed around said C-ring and located within another groove formed therein. 4. A first means for closing the first gas passage is provided;
4. Closing device according to claim 1, 2 or 3, characterized in that second means are provided for closing the second gas passage. 5. The stepped mouth area has a first horizontal portion and a second horizontal portion disposed above the first horizontal portion, and the lid assembly is located on the second horizontal portion. having a seal cover with a peripheral portion and a seal weld extending between said seal cover and said mouth area to form a second gas seal to prevent gas from escaping from the interior of the cask to the exterior; the second gas seal is configured to move inside the second gas seal to receive gas injected between the first cover and the seal cover to test the second gas seal; 5. A closing mechanism as claimed in any one of claims 1 to 4, characterized in that the seal cover is provided with a removable seal that can be attached or removed. 6. a shielding lid, the mouth area having a third horizontal portion disposed below the first horizontal portion, and the lid assembly having a peripheral portion located above the third horizontal portion; A shielding lid is attached to the mouth area. means for discharging a further gas in contact with both said shielding lid and said third horizontal portion;
a means for serving as a sealing portion; the shielding lid is provided with a fourth gas passage, and the fourth gas passage has a removable seal disposed inside the yet another gas sealing portion. A closing mechanism according to any one of claims 1 to 5, characterized in that: 7. The shielding lid has a channel with an enlarged lower portion, a standpipe extends into the interior of the cask body member, and means are provided for sealing the channel. A closing mechanism according to any one of claims 1 to 6. 8. Inside the cask base member, there is a cavity extending from the outside of the base member, and a fifth gas passage extends between the cavity and the inside of the cask, and a fifth gas passage is provided between the cavity and the cask. A sixth gas passage extends between the outside of the base member and a sixth gas passage that detects gas pressure and receives gas through the fifth gas passage inside the cavity. A pressure monitoring device for resetting the scale reading of the pressure monitoring device through the pressure monitoring device is attached, and means for sealing the cavity and the sixth gas passage are provided. A closing mechanism according to any one of claims 1 to 7.