FR2573238A1 - USED NUCLEAR FUEL STORAGE CASTLE CLOSURE SYSTEM - Google Patents

Abstract

THE CASTLE CLOSING SYSTEM, ACCORDING TO THE INVENTION, INCLUDES A HOLLOW BASE ELEMENT 38 OF A CASTLE HAVING A STAGE OPENING 102 DESIGNED FOR THE RECEPTION OF USED NUCLEAR FUEL, AND A COVER ARRANGEMENT 122 FOR CLOSING THIS OPENING AND PROVIDED WITH AN ELEMENT PRIMARY CLOSURE 126 OF WHICH A PERIOD REGION IS PLACED ON A FIRST HORIZONTAL PART, A MEANS 182 TO FIX THIS ELEMENT 126 ON THE OPENING 102, AND A PRIMARY GASKET 232 PROVIDED BETWEEN THE PRIMARY CLOSING ELEMENT AND THE FIRST HORIZONTAL SECTION TO PREVENT LEAKAGE OF GAS FROM THE CASTLE. THE CLOSING ELEMENT 126 INCLUDES A FIRST PASS 194 FOR GAS LOCATED INTERNALLY RELATIVE TO THE PRIMARY GASKET 232 GAS-TIGHT FOR THE PURPOSE OF A GAS INJECTION IN ORDER TO CONTROL THE PRIMARY GASKET 232 AND A SECOND GAS-PASS 210 OUTSIDE IN RELATION TO THE PRIMARY GASKET 232 GAS-TIGHT AND DESIGNED TO RECEIVE ANY GAS THAT LEAKED THROUGH THE PRIMARY GASKET 232 GAS-TIGHT DURING THE TEST.

Description

SYSTEM FOR CLOSING A CASTLE OF STORAGE

USED NUCLEAR FUEL.

  The present invention relates to the long-term storage of spent fuel which has been removed from a nuclear reactor and, more particularly, to a castle for the storage of spent fuel comprising

  an extremely reliable closure system whose

  can be controlled at the time of filling the castle and later during long-term storage

spent fuel term.

  Dry storage castles are a form of long-term storage of spent fuel. After the heat produced by a fuel assembly has dropped to a predetermined amount, such as 0.5 to 1.0 kilowatt per assembly, after perhaps 10 years of storage, the spent fuel is transferred to the castle

  which is then closed off and transported to a sto-

  ckage above ground rFour un. long-term storage.

  The requirements to be imposed on such a castle are particular

  particularly strict. The castle must be sheltered from

  chemical attack during long-term storage.

  In addition, it must be mechanically robust enough to

  prevent even tiny ruptures or frac-

  tures during long-term storage and during trans-

  port o the castle risks being manhandled or subjected to accidents such as falls. In addition, the castle must be able to emit into the environment the heat produced by the spent fuel while protecting the environment against the radiation produced by the spent fuel. The temperature of the bars must be maintained below a maximum temperature such as 375 C to avoid deterioration of the Zircaloy sheath. Arrangements should also be made for

  ensure that a chain reaction cannot be

  held inside the castle, that is to say that the fac-

  effective multiplier Keff remains less than one, so that a self-sustaining reaction does not occur. These conditions impose requirements

  strict with the castle which must fulfill its function of sto-

  ckage in an absolutely reliable way.

  During storage, the castle can be drowned in helium, which easily transmits heat through the narrow intervals between the edges of the plates and the walls of the U-profiles. The castle, which can have a mass of more than one hundred thousand kilograms once loaded with spent fuel, is normally 4.8 meters high and about 2.5 meters in diameter without the

cooling fins.

  The main object of the present invention is to produce a castle having a closure system which confines the spent fuel inside the castle to a degree

  high reliability and which makes it possible to control the tightness

  gas leakage during the closing of the castle

and then.

  Considering this objective, the present invention

  resides in a castle closure system,

  nant a hollow basic element of a castle comprising a

  stepped opening region designed to receive

  spent nuclear fuel, and a cover arrangement for closing this opening region, characterized in that said cover arrangement comprises a primary closing element, a region of the periphery of which is placed on a first horizontal part, means for fixing

  the primary closure on the opening region

  ture, and a gas-tight primary seal disposed between the primary closure member and the first horizontal portion to prevent leakage of gas from the castle and in that the primary closure member has a first gas passage located internally by relative to the gas-tight primary seal for injecting gas to control the gas-tight primary seal and a second passage located externally to the gas-tight primary seal designed to receive any gas that has leaked through the gas-tight primary seal

during the test.

  Protective cover can withstand shocks

  mechanical damage caused by accidents such as falls.

  The protective cover preferably includes a

  elastomer O-ring which cooperates with the region of

  greening of the basic element of the castle, a conduit ob-

  turable to receive a vertical pipe, and a closable passage for gas in order to inject gas into

  the castle while discharging water through an ins-

  temporarily installed in the vertical pipe. Once the water has been removed, the primary closure element is installed, then a sealed closure element is put in place by welding, the weld constituting the joint.

  secondary gas tight. The sealing element

  che has a closable gas passage to carry out a redundancy check for gas leaks through

  the primary seal when installing the element

  closing and in order to check for leaks during

long-term storage.

  Preferably, a gas pressure sensor arrangement communicating with the interior of the castle is installed in a cavity of the base element of the castle to produce an alarm signal if the pressure inside the castle reaches a level predetermined. Such an alarm signal can either indicate an abnormal situation requiring immediate attention, or be harmless by simply indicating that the sensor calibration has changed. A closable gas passage to the cavity is present in the basic element of the castle to allow the injection of gas to recalibrate the sensor

if an alarm signal is issued.

  The invention will be described in more detail with reference to the appended drawings in which:

  FIG. 1 is a perspective view of an assembly

typical fuel line;

  Figure 2 is a top plan view of a pis-

  cine for the short-term storage of spent fuel assemblies;

  Figure 3 is a sectional view showing the element

  base of a storage castle, and a grid basket arrangement in the base element to make a matrix of storage housings to receive spent fuel assemblies, to transmit to the wall of the castle the heat produced by the worn assemblies and to support a vertical pipe used during the emptying of the castle; Figure 4 is a perspective view of the vertical pipe shown in Figure 3 and shows the mounting of the vertical pipe on the grid basket arrangement; Figure 5 is a sectional view showing part of the opening region of the base element of the castle and the cover arrangement after its installation;

Z573238

  Figure 6 is a sectional view showing a C-shaped metal ring actuated by a spring which is

  retained in the primary closure for reacting

  read the gas-tight primary seal; Figure 7 is a perspective view showing part of the C-ring of Figure 6; Figure 8 is a perspective view showing the upper part of the basic element of the castle

  after installing the protective cover, and

  feels a water evacuation pipe and a gas pipe connected therein to closable openings during the evacuation of water out of the castle; Figure 9 is a perspective view of the part

  upper part of the basic element of the castle after installation

  lation of the primary closure element, and represents gas piping installed in closable gas passages during checks of the gas-tight primary seal; Figure 10 is a perspective view of the part

  upper part of the basic element of the castle after the

  tallation of the sealing element and represents a gas piping introduced into a closable gas passage during the checks of the secondary seal; and figure 11 is a perspective view of the castle

during long-term storage.

  FIG. 1 represents a fuel assembly 20

  typical to supply a nuclear fuel reactor

  area. The assembly 20 comprises a lower end piece 22 and an upper end piece 24 between which are arranged elongated fuel rods 26. Each fuel rod 26 comprises a cylindrical sheath made of zirconium alloy such as the "Zircalloy-4" existing on the market and is filled with pellets of fissile fuel enriched in U-235. In the assembly of bars

  26 of fuel, tubular guides (not shown

  felt) are arranged between the ends 22 and 24 to

  receive command bars (not shown) my-

  movable tees and measuring instruments (not shown. The ends of these tubular guides are fixed to the ends 22 and 24 to form a skeleton for supporting the fuel rods 26, which are not permanently fixed to the ends 22 and 24 Grids 28 have openings through which fuel rods 26 and tubular guides extend to group these elements into bundles.Fuel assemblies in the industry include 179 to 264 fuel rods, depending on the model. A typical fuel assembly is approximately 4.1 meters

  long, about 19.7 cm wide and weighs about 585 kg.

  After a useful life of approximately 3 years in a pressurized water reactor, the enrichment in U-235 of a fuel assembly 20 is exhausted. In addition, various fission products having different periods are present in bars 26. These fission products give off intense radioactivity and heat when

  the assemblies 20 are taken out of the reactor and the assemblies

  20 blages are therefore placed in a swimming pool containing boron salts dissolved in water (hereinafter called boron for short-term storage.

  pool is designated by the reference 30 in FIG. 2.

  Pool 30 has a typical depth of 12.2 meters.

  A number of racks 32 of spent fuel placed at the bottom of the pool 30 are provided with housings

  storage 34 for vertically receiving assemblies

  combustible cans 20. A castle support block 36

is at the bottom of the pool 30.

  During the storage of the fuel assemblies 20 in the pool 30, the composition of the spent fuel changes in the bars 26. The isotopes with a short period decay and, consequently, the proportion of fission products with a relatively long period increases. As a result, the level of radioactivity and heat produced by a fuel assembly 20 decreases relatively quickly for a time and ultimately reaches a state in which heat and radioactivity decrease very slowly. Even at this reduced level, however, the bars 26 must be reliably isolated from the environment for an indefinite future. Considering FIG. 3, the basic element 38 of the castle 40 has a bottom 42 and a hollow interior 44 produced

  by a cylindrical wall 46. The basic element 38 comprises

  carries a part 48 made of carbon steel about 25 cm thick which serves to protect the environment against gamma rays. A coating layer 50 of steel

  stainless covering the bottom 42 and the wall 46 is applied

  on part 48, for example, by placing part 48 on a turntable and rotating it while welding a continuous spiral around the inside using stainless steel welding rods so that a stainless steel surface completely covers the interior 44 of the part 48 in order to protect it from chemical attack. The element of

  base 38 is surrounded by a layer of approximately 7.0 cm thick-

  neutron absorbing material 52 which may be a resin. A suitable resin to serve as material 52 is sold by Bisco Products Inc., 1420 Renaissance Drive, Park Ridge, Illinois 60068, under the reference NS-3o The enveloping material 52 is an outer layer 54 of stainless steel to protect the castle 40 against the environment. Chateau 40 also has ailerons =

  carbon steel cooling 56

  rence treated to protect carbon steel against chemical attack from the environment. The fins 56 are welded to the part 48 and extend through the material 52 and the layer 54. Although they are not shown, it is advantageous to fix two pins

  at the top and bottom of the base member 38 in order to easily

read the manipulations.

  Still with reference to FIG. 3, the grid basket arrangement 58 comprises a first set 60 of large plates 62, 64, 66, 68 and 70 and a second

  set 72 of large plates 74, 76, 78, 80 and 82.

  These plates can be made of aluminum sheets about 3.7 meters high and 2.0 cm thick. During their manufacture, the metal plates of the first set 60 are provided with elongated notches oriented upwards (not shown) which extend from the bottom to the middle of the plates and which are spaced apart by about 26.3 cm. The plates of the second set 72 are provided with corresponding elongated notches (not

  shown) which extend from the top of the plates to the middle

  instead of these and which are also spaced 26.3 cm apart. The plates of the first set 60 and

  of the second set 72 are then assembled intro

  combining the notches into each other (so

  very comparable to combs that are hung in orient-

  both their teeth towards each other and then oar-

  the combs against each other) then by welding the intersections obtained by long welds in corcon. It appears that such a construction gives a matrix of elongated storage housings 84 spaced apart by 26.3 cm from one center to the other and having an axis which extends substantially perpendicular to the floor 42. The arrangement grid basket 58 also includes a number of small plates 86 welded around the periphery of the matrix produced by the first and second sets 60 and 72 of large plates. As shown in FIG. 3, the edges of the various plates are removably introduced into elongated U-shaped profiles 88 which are welded to the wall 46 and which extend substantially perpendicular to the floor 42. When using the castle 40 , elongated hollow cells (not shown) with stainless steel walls which form substantially square transverse sections and which support sheets of "neutron poison" material such as boron carbide can be introduced into the storage compartments 84. Fuel assemblies 20 are then introduced into the cells for long-term storage under a helium atmosphere, the heat produced by the assemblies 20 being transmitted via the grid basket arrangement 58 through narrow spaces filled with helium at U-profiles 88, up to wall 46 to be dissipated Storage compartments can also be used to store solidified fuel in s metal solidification containers 0 without

  cells, or solidified fuels and assemblies

  Combustible fuel can be stored in storage boxes.

  Storage elements different from the same châteauo In the context of the preceding overview, FIG. 3 also represents a vertical pipe 90 which is fixed to the grid basket arrangement 58 and which extends substantially perpendicular to the floor 42 As shown in Figure 4, the lower end of the pipe 90 is provided with drain holes 92 so that borated water can reach the inside of the pipe 90 during the emptying of the castle 40, an operation which will be described later in detail. It is necessary

  note that the grid basket arrangement 58 is light

  spaced apart above the floor 42 and does not interfere

  the flow of water to the holes 92. During the operation

  drain ration, the castle 40 can be tilted slightly to facilitate the flow of water to the holes 92, or the floor 42 can have a slight

slope towards the holes 92.

? 3238

  Still with reference to FIG. 4, a tri-

  lower angular support 94 made of aluminum is welded to the plates 64 and 74 of the arrangement 58. The element 94 has

  an opening (not shown) through which the

  tightens the piping 90, the element 94 being connected to the piping 90 by a weld 96. In the same way, the upper triangular support element 98, which has

  an opening 100 through which the pipe extends

  terie 90, is welded to plates 64 and 74. However, the pipe 100 is not welded to the element 98, thereby preventing the pipe 90 from deforming the grid basket arrangement 58 if the arrangement 58 and piping experiencing different expansion rates

  when the temperature inside the castle varies.

  Considering now FIG. 5, the basic element 38 ends upwards in a stepped opening region 102 which comprises horizontal parts 104, 106, 108 and 110. The region 104 also comprises vertical parts 112, 114 and 1-16. The coating layer 50 of stainless steel extends over the stepped opening region 102. Threaded bores 118 are present in the horizontal part 104 and threaded bores 120

  are present in the horizontal part 106.

  Still with reference to FIG. 5, the cover arrangement 122 comprises a protective cover 124, a primary closing element 126 made of carbon steel

  and a sealing element 128 made of stainless steel

  devil. The waterproof cover 124 has a portion 130 of carbon steel with a coating layer 132 of stainless steel which can be applied by welding as indicated above. The part 130 is provided with an annular recess 134, which, during the coating operation, is filled with stainless steel so that the layer 132 extends into the recess 134. An annular recess 136 having a dovetail cross section is cut from the stainless steel of the recess 134. An elastomeric O-ring 138 is "trapped" in the recess 136 so that it can be permanently installed when the protective cover 124 is produced, and therefore does not have to be installed just before mounting the cover 124 on the base element 38 of the castle. It should be noted that the part

  of the stainless steel coating 50 on the horizontal part

  zontale 104 which receives the O-ring 138 is machined to make a flat surface in order to wedge the

O-ring 138.

  Still with reference to FIG. 5, the conduit 140

  has a conical lower part 142 and an upper conical part

  lower 144 designed to receive the head 146 of a plug 148 made of stainless steel. The annular recess 150 of the head 146 receives a metal O-ring 152o. The O-ring 152 is "trapped" in the plug 148 by the application of small dots of glue connecting the sides of the O-ring 152 to the walls of the recess 150, in several places, during the production of the plug 148, taking care to avoid contamination of the sealing surfaces of the O-ring 152 by the adhesive. The O-ring 152 can also be retained on the plug 148 by two collars (not shown) installed in recesses (not shown) contiguous to the recess and extending towards the O-ring 152 to hold it in place. With the O-ring 152 trapped in this way, the plug 148 can be handled in one piece. A ring of unthreaded holes 154 on: the periphery of the head 146 receives round head screws 156, the threaded lower part of which is housed in

  threaded bores 158 of the protective cover 124.

  Still with reference to FIG. 5, a valve 159 is placed in the pipe 140 slightly above the upper end of the vertical pipe 90 (for

257323-8

  allows thermal expansion thereof), and

  carries a valve chamber 161 which is welded to the cover 124, a valve member 163 so mounted

  sliding in chamber 161, metallic O-rings

  ques 165 and 167 retained at the two ends of the element 163, a spring 169 which is fixed to the lower end of the chamber 161 and which pushes up the valve 163, a ball 171 movable inside the item 163

  and a spring 173 which pushes the ball 171 downwards.

  As will be discussed below, the valve 159 cooperates

  with a valve control 175 in the vertical line

  shim 90. The valve control 175 has a plate

  177 which is fixed on the inside of the vertical pipe

  wedge 90 and which contains a crown of passages 179.

  An axis 181 is fixed to the plate 177 in a position under the ball t171. The thickness of the plate 177, and the protective ring 183 around the vertical pipe protect the springs 169 and 173 against the effects

direct from the neutron flux.

  Still with reference to FIG. 5, the passage 160 for gas has an enlarged intermediate part 162 and an upper part 164 which receives the head 166 of the threaded plug 168 made of stainless steel. Head 166 has a

  annular recess in which the metal O-ring

  lique 170 is retained in the manner described above.

  The upper region 172 of the part 162 is threaded to receive the plug 168. A quick coupling 174 is

  mounted in part 162 under the upper region 172.

  The fitting 174 has an internal ball valve (not shown) and is designed to snap into place with a fitting

  corresponding fast on a gas pipe. Rac-

  quick couplings suitable in known in the art

and exist on the market.

  Still with reference to FIG. 5, the protective cover 124 also has a region 176 of the periphery of reduced thickness through which extend 36

unthreaded holes 178.

  Considering then the two figures 5 and 8, the ins-

  Installation of the protective cover 124 will now be described. Before the base element 38 of the castle is lowered onto the base 36 of the castle (Figure 2), and preferably during the manufacture of the element 38, the studs 180 are coated with sealant and screwed into the threaded bores 118. To avoid the penetration of water into the bores 120, the round head screws 182 are also coated with sealant and installed at low cost in the bores 120. The protective cover 124 is installed after loading the fuel assemblies 20 in base element 38 and while element 38 is still placed on the

  base 36 of the castle. During the first stage of the in-

  tallation, the plug 148 (with the captive gasket 152) and the plug 168 (with the O-ring 152

  prisoner) are absent on the protective cover 124.

  By remote control, the protective cover 124 is deposited on the base element 38 and aligned with it, so that the vertical pipe 90 is in place to be introduced into the part 142 of the pipe 140 and so that the studs 180 are in place to be introduced through the holes 178. Nuts 184 are then screwed onto the studs 180 and a water drain pipe 186 is introduced into the conduit 140, where the joint

  metallic toric 167 seals with the

  down from pipe 186. A further downward pressure from pipe 186 moves the valve member 168 until the O-ring 165 presses down.

  watertight against the end of the vertical pipe 90.

  Just before the creation of this last seal, the axis 181 moves the ball 171 to create a flow path from the vertical pipe 90, via the passages 179 and the valve 159 to the pipe 186. The piping 186 helium supply terminating in quick connector 188 is introduced into the passage 160 for gas, where the connector 188 snaps into place with the connector 174 and opens the internal valve of the connector 174. Helium is injected into the château 40 and, at the same time, the borated water is evacuated via the pipe 186 and pumped to a decontamination installation. It should be noted that the spent fuel raises the internal temperature of the castle 40 to the boiling point of the borated water during the evacuation of the borated water, so that a long drying time is unnecessary to remove the last traces of water. At the end of the emptying, the

  hose 186 and piping 188 are removed, and the connector

  rapid cord 174 and the valve 159 serve as adequate protections to retain the helium in the castle 40. The castle 40 is then taken out of the pool 30 and allowed to dry, the round head screws 182 are removed and

  polishes the horizontal part 106 to prepare the installation

  lation of the primary closure element 126. This ins-

  tallation is carried out by remote control since, although the cover 124 is in place, the radiation flux

  through the cover 124 remains sufficient to present

  ter health risks. It should be noted that the cost

  protective ring 124 constitutes the primary protection against impacts, for example impacts caused by accidental falls or incidents while the castle 40

  is placed in a horizontal position during transport.

  Returning to FIG. 5, the closing element

  mayor 126 has a crown of thirty six non-threaded holes 192 on its periphery, the upper part of the holes 192 being enlarged to receive the heads of the

  round head screw 82. The passage 194 for gas has a par-

  enlarged intermediate tie 196 and an upper part 198 which receives the head 200 of the threaded plug 202. The head 202 has an annular recess which receives the metal O-ring 204, the O-ring 204 being trapped in the recess as described above. The intermediate part 196 has a threaded region 206 to receive the plug 200. A quick connector 208 is mounted in the part 196 in a position away from the

threaded region 206.

  Still with reference to FIG. 5, the passage 210 for gas has an enlarged intermediate part 212 and an upper part 214 which receives the head 216 of the threaded plug 218. The head 282 has an annular recess

  in which a metal O-ring 220 is imprisoned-

  deny in the manner described above. The inter-

  middle 212 has a threaded region 222 for receiving

  see the plug 2180 A quick coupling 224 is mounted in the part 212 in a position away from the region 222. Still with reference to FIG. 5, the underside of the primary closing element 126 has an annular recess 226 which has a dovetail cross section so that the elastomeric O-ring 228 can be retained there. The annular recess 330, which

  is concentric with respect to the recess 226, is also

  Lement made in the primary closure element 126.

  It should be noted that the recesses 226 and 230 are located one beside the other, and that the passage 210 for gas communicates with the region between the recesses 226 and 230. The ring in C 232 actuated by a spring is retained in the recess 230 as described below. During the assembly of the base element 38 of the castle, the region of the horizontal part 106 which receives the seal 228 and the ring 232 is machined to

create a flat surface.

  Then considering Figures 6 and 7, the C 232 ring includes a helical spring 232 partially surrounded by a cover 236 made of stainless steel. These rings

  in C operated by a spring exist on the market.

  In three places separated by the same space around the recess 230 are made other recesses 236 in the primary closure element 126. Mounting tabs 238 are fixed in the recesses 236 by round head screws 240. The branches 242 of mounting lugs 238 extend between the turns of the spring

  234 to retain the ring 232 in the recess 230.

  Then considering the two figures 5 and 9, we

  describe the installation methods for the shutter element

  primary structure 126. The closing element 26, with the seal 228 and the ring 232 trapped therein but devoid of plugs 202 and 218 (with their O-rings 204 and 220), is placed on the base. 38 so that the holes 192 are aligned with the bores 120. The screws 182 are then installed. The C ring 232 actuated by a spring makes the gas-tight primary seal for the cover arrangement 122, and passages 194 and 210 are provided so that this primary seal can be checked during the installation of the primary closure element 126. The helium supply pipe 244 terminating in the quick connector 246 is

  introduced in passage 194, where fitting 246 fits

  clicks with the connector 208. In the same way, the tubing 248 for sealing verification ending with the quick connector 250 is introduced into the passage

  210. Helium, under a pressure of about two atmospheres

  spheres, is injected into the space between the protective cover 124 and the primary closure element 126 via the piping 244. Leaks are then measured, preferably using a helium leak detector using a chromatograph. gas phase, connected to piping 248. Appropriate leak detectors exist on the market. A typical figure for a leak rate

  maximum allowable is 10-4 cubic centimeters of ha

  lium per second under normal temperature conditions

erasure and pressure.

  The spring-actuated C 232 ring can pre-

  feel an excellent seal for long periods if it is correctly wedged during its initial assembly. The determination of correct setting of the ring 232 is the object of the control procedure described.

  above. A tiny foreign body or a slight im-

  perfection of the ring 232 or the sealing surfaces of the recess 330 or of the part 106 may cause

  an unacceptable leak rate. The other pre-

  smells in the castle closure system of the present invention are nowhere near as critical as

  the ring 232 and therefore do not have to be checked. The -

  metal O-rings such as seals 152, 170, 204 and 220 prisoners respectively of caps 148, 168, 202 and 218 have a high intrinsic reliability and, in any case, have relatively small dimensions and, therefore, are not likely to have excessive leak rates even if small faults appear. The elastomeric O-ring 138, which is not reliable for long-term gas tightness, can deteriorate over time due to the heat produced by the spent fuel. The O-ring 138 is present mainly to make the protective cover 124

  watertight with respect to base element 138 during installation

  helium junction via passage 160 during the water draining operation. Likewise, the elastomeric seal 228 is not reliable for long-term sealing; the seal 228 is only used to define an isolated region around the ring 232 for the samples taken

  by the helium detector when checking the ring 232.

  Considering once again FIG. 5, the sealed closure element 128 made of stainless steel has a passage 252 for gas in which a quick coupling 254 is mounted. The passage 252 has a threaded region 256 and

  a part 258 which receives the head of the threaded plug 260.

  The head of the threaded plug 260 has a recess

  ring in which a metal O-ring is retained

  262, as previously described.

  Considering then the two figures 5 and 10, the ins-

  tallation of the waterproof closure element 128 will

  holding be described. Once the closure element 128 is in place on the horizontal part 108, the level of all of the radiation from the body escaping through the cover arrangement 122 is relatively modest,

  so that it is safe for the following operations

  shutter rations are performed without remote control. The periphery of the sealed closure element 128 is welded to the horizontal part 110 by a weld 264, which makes the gas-tight secondary seal of the closure system of the present invention. This seal is controlled by injecting helium under pressure

  of about two atmospheres, via a supply pipe

  Helium tation 266 which ends with a quick connector 268 which engages with the connector 254 in the passage 252 for gas, in the region between the primary closure element 126 and the sealed closure element 128. The leaks around weld 264 using a chromatographic helium leak detector

  gas phase or other leak detection device.

  Next, an upper closure member 270 having

  a layer of approximately 7.0 cm thick of new material

  trophage 272 and an outer layer 274 of stainless steel

  Stainless steel is adhered to the sealed closure member 128. The closure member 270 has a cylindrical opening 276 for receiving the plug 278, which allows access to the plug 260 if it is desired to subsequently control the weld 264. The plug 278 has a body of neutron absorbing material 280 with an outer layer 282 of stainless steel. After the upper closure element 270 has been put in place, the annular closure element 284, which has a body of neutron absorbing material 286 and an outer layer 288 in

  stainless steel, is put in place by welding to

  undermine the closing operation. Chateau 40 and the

  bustible worn in it can then be transported

  tees to a remote storage site 0 Such a storage site is represented in FIG. 11, which shows a castle 40 resting on a support 290 of reinforced concrete in rural areas, an access road 292 being desirable when bringing the castle on support 290 and at

  time of the periodic inspections of the château 40.

  In addition to the periodic visual monitoring, during the storage of the castle 40, it may be desirable to provide a continuous electronic monitoring which emits an alarm signal if the pressure inside the castle 40 drops to a predetermined level, indicating that a gas leak has occurred and there is a danger that fission products such as radon gas will reach the environment. Returning to the figure, the closure system of the present invention provides an optional monitoring system. In FIG. 5, a cavity 294 communicating with the passage 296 for gas is produced in the base element 38. The cavity 294 communicates with the interior of the castle 40 via the passage 298 for gas. The cavity 294 comprises a first cylindrical part 300 communicating with the passage 298, a second cylindrical passage 302 communicating with the passage 300, and a third passage 304 communicating with the passage 302. A gas pressure detector arrangement 306, which comprises a prisoner metal O-ring 306, is installed in passage 300. It

  there are sensors on the market suitable for agen-

  cement 306. For example, a differential pressure gauge

* capacity, marketed by> KS Instruments Inc., Bur-

  lington, Massachusetts 01803 under reference 221AD-

  10000, can be used. The passage 302 has a threaded region 310 for the installation of a first plug element 312, which comprises a captive metal O-ring 314. The hollow cylindrical part 316 of the element 312 extends to the rear of arrangement 306 for firmly pushing arrangement 306 in contact

  from the end of passage 300 while still allowing

  less at the rear of the arrangement 306 to communicate with the passage 296 for gas. The plug element 312 also includes a penetrating plug 318 hermetic under vacuum for

  allow the transmission of electrical signals through

  towards the plug element 312. Such plugs penetrate

  trantes are known in the art and have wires extending through the bonded ceramic material

  to the metal that forms the rest of the plug member 312.

  Conductors 319 from the gas pressure sensor arrangement 306 extend through the first plug member 312 through the hollow interior of the

part 316 and sheet 318.

  Still with reference to FIG. 5, the third passage 304 has a threaded region 322 into which a second plug element 324 is screwed. The element 324 is provided with a captive metal O-ring 326 and a hermetic penetrating plug 328 under vacuum made of bonded ceramic material through which conductors extend, as described above. The contiguous faces of the plug elements 312 and 324 are designed to produce a cavity 330 to receive conductors 332 extending between the plugs 318 and 328. It appears from the above indications that the arrangement 306 of detector of gas pressure gives on the inside of the castle 40, that three metal O-rings 308, 314 and 326 seal the cavity 294 and that the

  electrical signals from arrangement 316 are com-

  fitted externally via conductors 319, the plug

  318, the conductors 332 and the added contact 328.

  External surveillance equipment (not shown) of the conventional type is electrically connected to

  arrangement 306 for providing an alarm signal if the

  Gas pressure detection arrangement 306 indicates that the pressure inside the castle 40 has fallen to a predetermined limit. The alarm signal can be sent to a monitoring station where it sounds a buzzer or otherwise attracts attention, or can be used to trigger a visual indicator device (not shown) placed next to the castle 40 on the support 290 and which is observed during visits

surveillance periodicals.

  The presence of an alarm signal does not necessarily mean that the integrity of the castle has been compromised. It is much more likely that an alarm signal results from an erroneous signal produced by the pressure sensor in arrangement 306. The electrical properties of a sensor tend to change slowly over time, and a "drift" electric of this type is particularly

  likely if a sensor is subjected to harsh conditions

  reigns in the castle 40. Thus, upon receiving an alarm signal, it is first necessary to recalibrate the sensor, to ensure that the alarm signal is valid, before sending the castle 40 in the

swimming pool 30.

  Still with reference to FIG. 5, the passage 296 for gas ends in a recess 334 into which a plug element 336 is introduced. The plug element 336 is provided with a captive metal O-ring 338 and smooth holes 340 who are aligned

  with threaded bores 342 of the base member 38.

  Round head screws 344 are used to fix the element 336 and the sealed passage 296. After assembly, the periphery of the plug element 336 is welded to provide secondary protection. It appears that gas leaks through passage 296 are prevented by the seals

  metal toric rings 308 and 338 and the welding on the element

  plug 334. A quick connector 346, of the in-

  previously described, is mounted in passage 296 for gas.

  Still with reference to FIG. 5, the element cylin-

  drique 348 of cork having a body in new material

  trophage 350 of about 7.0 cm thick and an outer layer 353 of stainless steel is installed after

  the welding of the plug element 336. Similarly

  niere, the cylindrical plug element 354, having a

  body 356 absorbing neutrons and an outer layer

  358 stainless steel cover, covers element 224

once installed.

  If an alarm signal is received, a technician measures

  first the radioactivity around the castle, and in particular

  bottom in the region of the plug elements 348 and 354.

  Measurements of an acceptable level around the plug elements 348 and 354 indicate that it is not dangerous to remove the element 348 and to eliminate the weld which seals the plug element 336. Once this weld removed, the technician measures the radiation again and, if it is at an acceptable level, continues by unscrewing the round head screws 344 and removing the plug element 336. A pipe terminated by a quick connector is then introduced into passage 296, and helium is injected to bring the pressure

  in passage 300 to a predetermined level in order to

  read the sensor in arrangement 306 by applying

  a predetermined pressure on one of its sides. In addition

  stating that this recalibration establishes that the alarm signal was untimely, the plug elements are reinstalled

336 and 348.

  It appears from the above description that

  the spent fuel castle closure system of the present invention provides a mechanically robust closure with very reliable overabundant seals. A cover arrangement facilitates the emptying of the castle and the checks of a primary seal

  gas tight and a gas tight secondary seal.

  The closure system of the present invention allows

  also long-term monitoring of the internal pressure

  of the chateau and recalibration of the surveillance sensor.

Claims (8)

  1. Castle closure system, comprising a hollow base element (38 of a castle comprising a region   opening (102) designed to receive fuel   worn nuclear target, and a cover arrangement (122) for closing this opening region, characterized in that   that said cover arrangement (122) comprises an element   primary closure (126) with a region of its periphery placed on a first horizontal part, means (182) for fixing the primary closure (126) on the opening region (102), and a primary seal (232) gas tight arranged between the primary closure member and the first horizontal portion to prevent the escape of gas from the castle, and in that the primary closure member (126) has a first passage (194) for gas located internally with respect to the gas-tight primary seal (232) for injecting gas to control the gas-tight primary seal (232) and a second passage (210) located externally with respect to the gas-tight primary seal (232) and designed to receive any gas that has leaked through the primary seal   (232) gas tight during the test.   2. System according to claim 1, characterized in that another seal (228) is disposed externally relative to the primary seal (232) between the primary closure element (126) and the first horizontal part, the second passage ( 210) for gas communicating with the region between the primary seal (232) and said other seal (228).   3. System according to claim 1 or 2, charac-   terized in that the gas-tight primary seal (232) comprises a metal spring-actuated C-ring, in that a groove (330) is provided for retaining the C-ring in the primary closure element (126 ), and in that said other seal (228) comprises an elastomeric O-ring disposed around the C-ring in another groove (226) formed in the closure element primary (126).   4. System according to any one of the claims   1, 2 or 3, characterized in that first means (202, 204) are provided for sealing the first passage (194) for gas and second means (218, 220) are provided for sealing the second passage (210 ) for gas.   5. System according to any one of the claims   1 to 4, characterized in that the stepped opening region (102) has a first horizontal part (106) and a second horizontal part (108) arranged above the first horizontal part (106), and the arrangement cover (122) further comprises a sealing element (128) having a region of its periphery placed on the second horizontal part (108), in that a sealing weld (264) extends between the sealing element (128) and said opening region (102) for making a gas-tight secondary seal in order to prevent the gas inside the castle from escaping outside, and in that the the sealing member (128) has a third gas passage (252) having a removable gasket (260, 262) located internally with respect to the gas-tight secondary seal (264) for receiving gas injected between the primary closure member (126) and the sealed closure member (128) for controlling the   secondary seal (264) gas tight.   6. System according to any one of the claims   1 to 5, characterized in that the opening region (102) comprises a third horizontal part (104) arranged under the first horizontal part, and in that the cover arrangement (122) further comprises a protective cover (124) of which a region of the periphery is placed on the third horizontal part (104), means (180, 184) for fixing the protective cover (124) on the opening region, and a means (138) in contact both with the protective cover (124) and the third horizontal part (104) to make another gas-tight seal (138), the protective cover having a fourth passage (160) for gases with   a removable seal (168, 170) located inside   relatively to said other seal (138) gas.   7. System according to claim 6, characterized in that the protective cover (124) has a duct (140) having an enlarged lower part (142), and in that a vertical pipe (90) penetrates inside. of the basic element (38) of the castle and in that means (146, 150) are provided for sealing the duct (140).   8. System according to any of the claims   1 to 7, characterized in that the base element (38) of the castle has an internal cavity (294) extending from the outside of the base element (38), in that a fifth passage ( 298) for gas extends between the cavity (294) and the interior of the castle (38), and a sixth passage (296) for gas extends between the cavity (294) and the exterior of the element base (38) of the castle, in that pressure control means (306) are mounted inside the cavity (294) to detect the pressure of the gases via the fifth passage   (298) for gas and for receiving gas to recalibrate   ner the means (306) of pressure control via the sixth passage (296) for gas and in that there are means (312, 314, 324, 326) for sealing the cavity (294) and means (336, 338) to seal the sixth passage (296) for gas.