GB2199279A

GB2199279A - Storage of heat generating materials

Info

Publication number: GB2199279A
Application number: GB08631012A
Authority: GB
Inventors: Norman Bradley; Colm O'tallamhain
Original assignee: National Nuclear Corp Ltd
Current assignee: National Nuclear Corp Ltd
Priority date: 1986-12-30
Filing date: 1986-12-30
Publication date: 1988-07-06
Also published as: GB8631012D0

Abstract

Heat-generating material, such as radioactive waste is stored above ground level in steel-lined bore holes (12) in a block (16) of concrete. A system of ducting (24, 26, 27) provides a natural circulation path for fluid between an external air-cooled condenser (28) and evaporation ducting sections (18) which run alongside the bore holes (12). The ducting sections (18) are directly connected to the bore liner (20) or are linked to the bore liner (20) by thermally conductive elements such as fins (22) so that there is a heat flow path of low thermal resistance, provided by the fluid circulation p@ and the thermally conductive elements, between the bores and atmosphere. The fluid is one which changes phase from liquid to gas at temperatures which do not deleteriously affect the concrete, e.g. 80 DEG C. The fluid may be water/steam @ sub-atmospheric conditions. <IMAGE>

Description

Storage of heat-generating materials This invention relates to an installation for the storage of heat-generating materials such as radioactive waste materials from nuclear facilities. Spent fuel or heat generating waste from nuclear facilities requires to be stored and cooled for many decades before it is suitable for ultimate disposal.

The basic requirements of an acceptable storage system are: containment to prevent release of active 'species to the environment; shielding against nuclear radiation, and some means for limiting the temperature of the stored product.

For long-term storage, any heat removal system employed should preferably be passive, eg not reliant on external power supplies A number of different systems, both wet and dry have been developed to meet these requirements, one such system, known as the dry-well, stores the product in steel-lined holes in the ground, utilising the earth both as the shielding material and as a heat sink. Whilst it is a simple passive storage system, it has a number of potential draw-backs.

a. to ensure sufficient capacity to dissipate the heat the holes need to be widely spaced, resulting in poor space utilitation.

b. a dry soil is needed to prevent corrosion of the steel liner and avoids unacceptable migration of leached products should the liner fail; and c. the thermal rating of each hole is low because of the relatively low thermal conductivity of the earth.

According to the present invention there is provided an installation for the storage of heat-generating materials comprising a body of shielding material having at least one bore therein for reception of said heat-generating material, means for circulating a fluid through the body of shielding material so that the fluid over part of its path of flow is in heat exchange relation with the contents of said at least one bore to the extent that the fluid can absorb sufficient heat to undergo evaporation, said circulating means including condenser means for removing heat from said fluid in a part of the recirculatory flow path remote from said at least one bore.

Preferably means is provided for preferentially conducting heat from the contents of said at least one bore to the circulating fluid so that the shielding material itself is not relied upon as the primary heat conduction path between the bore and the circulating fluid. Thus, the heat conducting means, in conjunction with the circulating fluid, serves to provide a heat conducting path separate from the shielding material and of substantially lower thermal resistance.

The circulating means is advantageously of the passive type in which the heat absorbed by the fluid promotes circulatory flow of the fluid without the need for external power supplies.

Preferably the circulating means comprises ducting which in part extends through said body of shielding material and, in part, is external to said body and the external ducting may connect with the condenser means.

The ducting within the body of shielding material may extend generally lengthwise of said at least one borf and may comprise a number of branch sections each extending generally lengthwise of the bore and located a spaced intervals around the circumference of the bore.

The or each bore may be provided with a liner of suitable metal such as steel and the ducting is preferably thermally coupled to the liner either by virtue of direct contact between the ducting and the liner or by virtue of thermal conducting elements, such as fins, bridging the space between the ducting and the liner, the thermal conducting elements having a substantially greater thermal conductivity than the surrounding shielding material.

The fluid is conveniently one which changes phase from liquid to gas at a temperature below that at which the integrity of the shielding material is deleteriously affected. In one presently preferred example of the invention, the shielding material is concrete which has a tendency to spall when subjected to temperatures approaching or exceeding 100 c. In this event, the arrangement is preferably such that the fluid changes to the gaseous phase at temperatures below 1000C, more preferably at temperatures of about 80 C or less.

The temperature at which the fluid changes to the gaseous phase may be determined by control of pressure so that, for example, water may be employed as the working fluid but in sub-atmospheric conditions so that the water can convert to steam at temperatures of the order of 800C.

The body of shielding material is preferably located at a level at which it cannot be permeated by ground water thereby isolating the contents of the bore or bores from potential migration through the ground water system.

To promote further understanding of the invention, one embodiment will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is diagrammatic view of an installation in accordance with the invention; Figures 2 is a horizontal sectional view of part of the installation; and Figure 3 is a cross-sectional view showing the ducting and bore hole liners linked by thermally conductive fins.

The installation illustrated in Figures 1 to 3 serves to store heat-generating waste product in the form of for example spent nuclear fuel or radioactive waste material in a concrete structure above ground level and, by providing a separate heat path to atmosphere of low thermal resistance, avoids high concrete and product temperatures.

A heat removal system is employed involving the use of a fluid which evaporates and condenses at a temperature which is acceptable to the concrete, yet high enough to dissipate the heat to atmosphere. The resulting change of phase of the fluid provides.a sufficiently large density change to ensure a passive natural circulation heat removal system which by virtue of the latent heat of the fluid provides a means of removing large amounts of heat at substantially constant temperature.

As shown, the product 10 to be stored is placed in a number (four illustrated) of steel lined holes 12 in a block of concrete 14, each hole 12 being sealed with a plug 16 after it has been filled, preferably by welding.

A number of cooling pipes 18, preferably vertically oriented, are arranged around each hole 12 (see Figures 2 and 3), either directly attached to the liner 20 or spaced a short distance from the liner 20 and connected to it by means of steel fins 22 (see Figure 3). The tops and bottoms of the pipes are connected together into header pipes 24, 26, the latter being led out of the concrete to external air cooled condensers 28 (via pipes 27). A short stack 30 above each condenser provides sufficient head to drive cooling air entering via inlets 32 over the condensers by natural draught.

The cooling pipes 18, pipes 27 and headers 24, 26 may conduct water/steam at sub-atmospheric pressure so that the water is converted to steam at temperatures below 1000C. Other fluids which undergo the phase change, liquid to gas, at temperatures below 1000c (either at atmospheric or sub-atmospheric pressure) may be employed instead of water.

The installation described above thus provides a system for storing heat generating nuclear waste products which a. thermally bypasses the shielding material (14), allowing the use of ordinary concrete as the shielding material; b. provides a passive heat removal system by utilising a fluid which boils at a temperature acceptable to ordinary concrete; c. isolates the waste product from heat sink, thus preventing the release of active species to the environment through the cooling system; d. isolates the product from potential migration through the ground water system; e. by using a high density fluid and utilising its latent heat to provide high thermal transport capacity, limits the volume of fluid penetrating the shielding concrete, and in consequence minimises the required shielding volume; and f. provides a compact storage system of modular design particularly suitable for storing small product quantities.

Claims

1. An installation for the storage of heat-generating materials comprising a body of shielding material having at least one bore therein for reception of said heat-generating material, means for circulating a fluid through the body of shielding material so that the fluid over part of its path of flow is in heat exchange relation with the contents of said at least one bore to the extent that the fluid can absorb sufficient heat to undergo evaporation, said circulating means including condenser means for removing heat from said fluid in a part of the recirculatory flow path remote from said at least one bore.

2. An installation as claimed in claim 1 including means for preferentially conducting heat from the contents of said at least one bore to the circulating fluid whereby the shielding material itself is not relie upon as the primary heat conduction path between the bor and the circulating fluid.

3. An installation as claimed in Claim 1 in which the heat conducting means, in conjunction with the circulating fluid, serves to provide a heat conducting path separate from the shielding material and of substantially lower thermal resistance.

4. An installation as claimed in Claim 1, 2 or 3 in which the circulating means is of the passive type in which the heat absorbed by the fluid promotes circulator flow of the fluid without the need for external power supplies.

5. An installation as claimed in any one of the preceding claims in which the circulating means comprises ducting which in part extends through said body of shielding material and, in part, is external to said body and the external ducting is connected with the condenser means.

6. An installation as claimed in Claim 5 in which the ducting within the-body of shielding material extends generally lengthwise of said at least one bore and comprises a number of branch sections each extending generally lengthwise of the bore and located at spaced intervals around the circumference of the bore.

7. An installation as claimed in any one of Claims 1-6 in which the or each bores provided with a metal liner.

8. An installation as claimed in Claim 7 in which the ducting is thermally coupled to the liner by direct contact between the ducting and the liner.

9. An installation as claimed in Claim 7 in which the ducting is thermally coupled to the liner by virtue of thermal conducting elements bridging the space between the ducting and the liner, the thermal conducting elements having a substantially greater thermal conductivity than the surrounding shielding material.

10. An installation as claimed in any one of Claims 1-9 in which the fluid is one which changes phase from liquid to gas at a temperature below that at which the integrity of the shielding material is adVersely affected.

11. An installation as claimed in Claim 10 in which the shielding material comprises concrete.

12. An installation as claimed in Claim 10 or 11 in which the arrangement is such that the fluid changes to the gaseous phase at temperatures below 1000C, more preferably at temperatures of about 800C or less.

13. An installation as claimed in Claim 10, 11 or 12 in which the temperature at which the fluid changes to the gaseous phase is determined by control of pressure.

14. An installation as claimed in Claim 13 in which water is employed as the working fluid in sub-atmospheric conditions so that the water can convert to steam at temperatures of the order of 800C or less.

15. An installation as claimed in any one of Claims 1-14 in which the body of shielding material is located at a level at which it cannot be permeated by ground water thereby isolating the contents of the bore or bores from potential migration through the ground water system.

16. An installation for the storage of heat-generating materials substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

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