WO1999063544A1

WO1999063544A1 - Dry storage vault

Info

Publication number: WO1999063544A1
Application number: PCT/GB1999/001726
Authority: WO
Inventors: Michael Blackbourn; Christopher Charles Carter
Original assignee: Alstom Uk Ltd.
Priority date: 1998-05-29
Filing date: 1999-06-01
Publication date: 1999-12-09
Also published as: ZA993676B; GB9811538D0; GB2337722B; AU4275099A; GB2337722A

Abstract

Canisters (400) containing hot spent nuclear fuel or waste are stored in respective chambers of a vault and are air-cooled by natural thermo-siphon convection. The vault is constructed from pre-cast concrete sections (120, 140, 150), assembled on-site and secured together by poured concrete (186). Each chamber has a stainless steel liner (450) defining inner and outer annular spaces (510, 520) between the hot wall (402) of the canister and the concrete wall (465) of the chamber through which cooling air flows by convection. Air from the outer space discharges via exit vents (172) cast into the concrete, air from the inner space via gap (436) between metal lid (430) and flanges (432, 434). Liner (450) shields the concrete from direct thermal radiation from the hot canister wall (402) and provides additional surfaces from which heat can be lost by convection. The inner metal-lined air path prevents very hot air from coming into direct contact with concrete. Slots (532) allow hot air to discharge via one of the exit vents in the event of blockage of the other. The concrete walls themselves are cooled by further ducts (not shown) formed as an integral part of the pre-cast structure.

Description

Dry Storage Vault

This invention relates to dry storage vaults. It particularly relates to dry storage vaults for the storage of spent nuclear fuel.

In a known type of dry storage vault, canisters of spent fuel are arranged vertically in a chamber and air is caused to pass horizontally over the canisters by convective thermo- siphon action. The chamber communicates with a flue which draws hot air upwards and causes cool air to be drawn into the chamber. Thus heat is removed by natural convection without the need for auxiliary cooling fans. Such a vault is described in "The developed technology of modular vault dry storage", Nuclear Engineering International April, 1984. While this has proved to be an effective method of dealing with spent fuel, it does have a number of disadvantages. The chamber and flue have to be built on-site to minimise the risk of radiation leakage from the fuel, the construction must be built on site using poured concrete. Suitable foundations must be provided for the resultant massive structure, and all the machinery required for vault construction must be transported to the site where the vault is to be constructed.

The invention provides an improved dry storage vault for heat-emitting radioactive material disposed within a canister, the vault comprising a concrete body having a chamber to accommodate the canister and means for air cooling the canister by convection, the vault further comprising: a liner arranged within the chamber to define a first air flow space between and coextensive with an inner wall of the chamber and an outer wall of the liner, the liner being arranged to accommodate the canister therein with a clearance between an inner wall of the liner and an outer wall of the canister, the clearance defining a second air flow space between and coextensive with the inner wall of the liner and the outer wall of the canister, air inlet means at a lower end of the chamber communicating with the first and second air flow spaces at their lower ends, and air outlet means at an upper end of the chamber communicating with the first and second air flow spaces at their upper ends. It should be understood that the word "concrete" as used in connection with this invention is to be given a wider meaning than merely a cured mixture of cement and aggregate and should be construed to include any type of composite material suitable for constructing such vaults (i.e. resistant to the effects of radiation and heat) and comprising fragments of structural material in a cured binder material.

Unlike the prior art, each canister has its own cooling arrangement. Unlike the prior art, a vault in accordance with the invention does not require the provision of a large building with its attendant massive foundation. It can be assembled from pre-cast units manufactured off- site and only requires a suitable hard surface capable of supporting the weight of the vault. The provision of the liner reduces the temperature to which the concrete structural members are subjected, resulting in reduced thermal stresses and reducing the likelihood of degradation of the concrete.

Further aspects of the invention will be apparent from the following description and the appended claims.

Embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which:

Figure 1 shows an isometric view of a first embodiment of the invention;

Figure 2 shows an exploded view of Figure 1;

Figure 3 shows an exterior plan view of Figure 1;

Figure 4 shows a sectioned elevation view of Figure 1 on line IV-IV of Figure 3; Figure 5 A shows hidden detail in area VA of Figure 4 on an enlarged scale;

Figure 5B shows detail in area VB of Figure 4 on an enlarged scale;

Figure 6 shows a sectioned partial view of Figure 1 along line VI- VI of Figure 3,

Figure 7 shows a scrap view of Figure 1 along line VTI-Vπ of Figure 3;

Figure 8 shows a side elevation view of Figure 1; Figure 9 shows an end elevation view of Figure 1 ;

Figure 10 shows a second embodiment of the invention; and Figure 11 shows a third embodiment of the invention.

Referring now to Figures 1 to 9, the vault consists of a number of pre-cast concrete components, namely, end base members 110, centre base members 120, end top members 5 130, centre top members 140, side wall members 150, and end wall members 160. The vault also comprises a number of cylindrical steel liners 450 (Figure 5) whose function will be described later, flange members 432, 434, and lids 430.

As described in more detail later, various of the members have channels which provide ducts 0 for flow of air therethrough when the vault is in its assembled state. The members 110, 120, 130, 140, 150 and 160 are also provided with steps, as for example indicated by reference number 141 in Figure 4, which fit into each other and serve both to assist assembly and to provide a tortuous path for radiation to prevent leakage thereof. The members also have channels 180, 182 which, after assembly, are filled with concrete 186 (Figure 4) to secure 5 the units together and provide a monolithic structure.

Referring now to Figures 2, 4 and 5, the lower and upper members 110, 130 and 120, 140 have cylindrical bores which, in the assembled vault, constitute chambers in which fuel canisters 400 are accommodated. Each chamber has a peripheral concrete wall surface 460. 0 Each chamber is provided with a stainless steel liner 450 arranged so as to provide a first annular gap 510 between the fuel canister 400 and the inner surface 452 of the liner 450, and a second annular gap 520 between the outer surface 454 of liner 450 and the concrete wall surface 460. These annular gaps allow passage of cooling air to flow therethrough as will be described later. As seen in Figure 5, the bottom of the cylindrical liner 450 is provided 5 with a number of apertures 530 to allow passage of air therethrough.

As best seen in Figure 2 and Figures 5 A, 6 and 7, the base members 110, 120, have air ducts 170 to allow entry of air from the atmosphere into the interior of the chamber 400 at the base thereof via slots 470 through the lower inner concrete wall 465 of the chamber, and to 0 pass into annular gaps 510, 520. The top members 130, 140 have ducts 172 to allow air 419 which has flowed via annular gap 520 to pass to the atmosphere via slots 475 through the upper inner concrete wall 466 of the chamber. The openings of ducts 170, 172 have screens to prevent entrance of birds or other animals.

It will be seen from Figures 2, 4 and 5B that the top of each liner 450 terminates in a first steel flange 434 which is sealed to its respective concrete member 130, 140. A second steel flange 432 is fitted over the first flange 434. A steel lid 430 is fitted over the second flange. The lid 406 of the canister 400 locates within but does not touch the first flange 434 so as to provide a labyrinthine passage 435 for hot air 437 rising through annular gap 510. The lid 430 and flange 432 are such to leave a gap 436 through which air 437 flowing via annular gap 510 passes to the atmosphere.

The air 437 at the top of the inner annular gap 510 can reach a temperature which could cause long-term damage to concrete. Hence this very hot air is normally allowed to escape by an air path whose walls are constituted by steel members, so that the hot air does not come into direct contact with concrete.

The air 419 at the top of the outer annular gap 520 will not reach a temperature sufficiently high to cause damage to concrete and hence can be allowed to escape via ducts cast into the concrete side members and whose walls consist of the concrete of which the walls are cast. This is also the situation if gap 436 is accidentally blocked.

A number of communication slots 532 in the liner 450 provide communication between the inner and outer annular spaces at the top of the chamber. This provides an alternative non- preferential flow path via annular gap 520 and ducts 172 to atmosphere in the event that gap 436 is accidentally blocked. The hot air from annular gap 510 is diluted and cooled by mixing with the cooler air flow in annular gap 520.

The base 453 of the liner 450 is supported by spacing members 480 above the base of the chamber to provide a space 490 to allow free passage of air 491 from inlet duct 170 to the entire lower periphery of the liner 450 via slot 470.

As shown in Figures 3 and 6, bars 310, 320 are located in grooves so as to cover the joins between adjacent base members 110, 120 and adjacent top members 130, 140. These bars provide a labyrinthine path to inhibit egress of stray radiation.

The vault is designed to be used in the open air. Having prepared a suitable hard standing, the individual units, which may be fabricated off-site using conventional concrete casting techniques, are assembled. Fuel canisters 400 are then placed in the chambers as required and the flanges 432 and lids 430 fitted. Heat from the fuel in the canisters causes the surface temperature of the outer cylindrical wall 402 of the canister 400 to rise. Part of this heat is carried away by convection by air in annular gap 510, the remainder as radiant heat.

Radiant heat from canister 400 falls on liner 450 and causes its temperature to rise. The liner's inner surface 452 is blackened to maximise absoφtion of heat and minirnise reflection of heat back to canister surface 402. Heat is lost from the liner's inner surface 452 by convection into the air in inner annular gap 510.

The remainder of the heat in liner 450 is lost from its outer surface 454, this being left as a natural stainless steel finish to minimise radiation of heat to the concrete surface 460 and thereby enhance carrying of heat away from the liner surface 454 by convection into the air in the outer annular gap 520. However, some radiant heat from liner surface 454 warms the concrete surface 460 of the chamber. Some of this heat is absorbed by the concrete by conduction, the remainder is again carried away from surface 460 by convection by the air.

Thus the air in both annular gaps 510 and 520 becomes heated and rises. In general, air in gap 510 will rise and leave the chamber via lid 430, while air in gap 520 will leave via the vents 72. In the event of a blockage in either of the vents 172 or 436, the communication slots 532 in the liners 450 allow air in gap 510 to leave via vent 172 or air in gap 520 to leave via lid 430. The heat absorbed by the concrete will cause its temperature to rise. Some of this heat is removed by air which flows through further ducts 700, see Figure 2, which are cast into the upper and lower members. The outlines of three of these ducts 700 are indicated by dashed lines in Figure 3. As shown in Figure 7, air 701 flows through the ducts 700 because they communicate with the inlet and outlet ducts 170, 172.

The surface temperature of the canisters 400 will normally be sufficiently high that, in the absence of liner 450, heat radiated from the canister would cause the temperature of the concrete 460 to rise to a level which could give rise to long-term damage of the concrete. Liner 450 prevents the temperature of wall 460 from becoming excessive, both by providing a shield from direct thermal radiation and by increasing the surface area from which heat may be transferred to the air by convection. This reduction in concrete temperature allows conventional concrete to be used without the need for special strengthening members.

It will be seen that the vault cooling system is driven by the natural buoyancy of heated air, with the stored irradiated fuel providing the source of heat. The cooling system air flow is a function of the decay heat of the stored fuel: the hotter the fuel, the greater the buoyancy of heated air, the faster the air flow, the greater the cooling effect of the air. This arrangement thus provides a self-regulating, inherently reliable, cooling system with diversity of flow paths in the event of accidental blockage.

Canister loading and unloading operations will now be described.

The loading operation starts when a transfer cask and canister arrives at the vault shown in Figures 1 to 9 on the back of a tow trailer. The canister transfer cask is removed from the trailer using the overhead crane and lifted onto the vault storage location. The cover plate from the storage location has been previously removed by the crane. Shield doors on the base of the transfer cask are opened to permit the canister to be lowered out of the transfer cask into the vault storage position by the overhead crane. The vault shielding integrity is completed by the integral shield plug that is part of the canister lid. The transfer cask shield doors are closed and the transfer cask is removed from the vault. After the vault cover plate is replaced the storage transfer operation is complete.

When the canister has to be removed from the vault it can be placed directly into a transportation cask without having to return to a pool facility. A stand is located adjacent to the vault and this is used to support the transport cask in a vertical position ready to receive the canister. An adapter ring is docked on top of the transport cask to permit it to interface with the vault transfer cask. The canister is removed from the vault using the overhead crane and transfer cask which is then docked onto the transportation cask. The canister can then be transferred directly into the transportation cask from the transfer cask. After the transfer cask has been removed, the lid to the transportation cask is fitted and the cask loaded onto a trailer for off-site shipment.

While the embodiment of Figures 1 to 9 provides storage for eight canisters, the invention can be applied to greater or lesser numbers.

Figure 10 corresponds with Figure 3, and shows an arrangement for four canisters. Compared with the first embodiment this uses the same end units 110, 130 and 160 as the first embodiment and only requires the provision of a special side wall in place of the side wall 150.

Figure 11 corresponds with Figure 3, and shows an arrangement for use with six canisters. This arrangement uses two sets of end units 110, 130 and end walls 160 and only one set of centre units 120, 140. It only requires the provision of a special side wall in place of side wall 150.

It will be seen that, compared with prior art arrangement, vaults in accordance with the invention can be manufactured off-site in prefabricated form, only requiring assembly on site.

The invention also makes it possible to construct a range of vaults of different sizes using a number of components common to all designs, only a minimal number of components being adapted exclusively to the particular design. A number of modification are possible within the scope of the invention.

Although in the described embodiments the flanges and cap are steel, any other suitable material, metal or non-metal, may be used. It is only necessary that the materials be capable of withstanding relatively high temperatures and low levels of neutron radiation without deterioration, and provide adequate radiation shielding.

The vault may be used to store any suitable heat-emitting radioactive material in canisters, whether spent nuclear fuel or high level waste, and is not restricted to use with any one type of material.

If the installation is such that it can be ensured that the exit vents for hot air cannot become blocked, the communication slots at the top of the chamber affording communication between the inner and outer annular spaces may be omitted. This will ensure that the hotter air from the inner annular chamber never comes into contact with concrete at all.

Whereas the above described embodiments all relate to storage vaults whose canisters, liners and chambers are conveniently circular in cross-section for manufacture and assembly, other feasible section shapes are within the scope of the invention, such as elliptical, polygonal, or even rectangular and square. However, it is believed that circular section canisters, liners and chambers are preferable, at least for optimum integrity of the finished structure, and because the flows of air in the air flow spaces 510, 520 (Figure 5) may be more evenly distributed over the surfaces 402, 452, 454 and 460 if the air flow spaces are annular.

Claims

CLAIMS:

1. A dry storage vault for heat-emitting radioactive material disposed within a canister (400), the vault comprising a concrete body having a chamber to accommodate the canister and means for air cooling the canister by convection, the vault further comprising: a liner (450) arranged within the chamber to define a first air flow space (520) between and coextensive with an inner wall (460) of the chamber and an outer wall (454) of the liner, the liner being arranged to accommodate the canister therein with a clearance between an inner wall (452) of the liner and an outer wall (402) of the canister, the clearance defining a second air flow space (510) between and coextensive with the inner wall of the liner and the outer wall of the canister, air inlet means (170, 470, 530) )at a lower end of the chamber communicating with the first and second air flow spaces at their lower ends, and air outlet means (436, 475, 172) at an upper end of the chamber communicating with the first and second air flow spaces at their upper ends.

2. A dry storage vault as claimed in claim 1 in which the liner is provided with a number of apertures at its upper end to afford communication between the first and second air flow spaces.

3. A dry storage vault as claimed in claim 1 or 2 in which the concrete body is provided with cooling duct means arranged to cool the concrete body.

4. A dry storage vault as claimed in claim 3 in which the cooling duct means communicates with the air inlet and air outlet means.

5. A dry storage vault as claimed in any of claims 1 to 4 in which the air outlet means comprises a first air outlet connected to the first air flow space and a second air outlet connected to the second air flow space.

6. A dry storage vault as claimed in claim 5 in which the second air outlet means comprises duct work arranged between the second air flow space and the second air outlet, in which the duct work and outlet comprise heat-resistant members.

7. A vault as claimed in claim 6 in which the heat-resistant members comprise metal members.

8. A vault as claimed in any preceding claim in which the vault comprises a plurality of pre-cast concrete members.

9. A vault as claimed in claim 8 comprising baffle means arranged to cover at least one joint between abutting members to inhibit escape of radiation.

10. A vault as claimed in claim 8 in which mating surfaces between abutting members are shaped to provide a baffle to inhibit escape of radiation.

11. A vault as claimed in any one of claims 8, 9, or 10 in which at least one joint between abutting members is provided with a void arranged to be filled with concrete during assembly to secure the abutting members together.

12. A vault as claimed in any preceding claim comprising a plurality of chambers.

13. A method of constructing a vault as claimed in any preceding claim comprising the steps of prefabricating individual concrete components, and assembling said prefabricated concrete components on-site.