GB2157222A - Containment for heat generating noxious materials - Google Patents

Abstract

A containment for a container enclosing heat generating dangerous materials such as radioactive waste materials consists of at least two concentric walls 2-4 which are spaced from one another so as to form therebetween a chamber 6-7 which is filled with a meltable material, preferably a metal or metal alloy which provides for good heat conductivity for the heat generated within the container. All but the innermost walls have discharge openings 8 adapted to discharge the material from said chambers when the chamber walls reach temperatures higher than the melting temperature of the material in the chambers, for example, during a fire. Release of the molten material from the chamber provides for gaps of low heat conductivity which prevents heat from the outside to be conducted into the interior of the containment. <IMAGE>

Description

SPECIFICATION Containment for heat generating noxious materials Background of the invention The invention relates to a containment for heat generating noxious materials such as radioactive waste, especially for a transport container enclosing such material which, under normal conditions, permits removal of the heat generated but also prevents the supply of excessive heat to the material within the container from without.

During transport, highly radioactive materials (HAW ingots) are enclosed in containers which must permit disposal of the heat generated in the radioactive material. Highly radioactive material generates a substantial amount of decay heat which may cause a substantial temperature increase, possibly causing damage to the material in the container. Solidified waste materials (HAW) for example, should not achieve temperatures of more than 450"C. It is therefore important that the transport container have walls of relatively good heat conductivity so as to keep the temperature of the material in the container relatively low.

On the other hand, the containment must not permit heat to enter, so that, for example, when exposed to flames of 800"C during an accident, the material is not heated to dangerous levels for at least half an hour.

Under these circumstances, the containment should be thermally insulated, that is, its walls should have very low heat conductivity so as to avoid heating of the material to dangerous temperature levels from the outside.

In most prior art arrangements, containers for the transport of highly radioactivity heat generating materials include walls of highly heat resistant insulating materials (for example, asbestos, sand) resulting in relatively high temperatures within the container, although at tolerable levels.

There is also a container with walls of one-way insulation material by the company TN (Transnuklear), which material is a cement including crystalline waterthat is released above a certain temperature, which release causes a reduction of its heat conductivity.

However, the first-mentioned materials have the disadvantage that the temperature of the material within the container is relatively high during normal use, and the cement insulation of company TN is not usable in connection with HAW-transport containers since the release of the crystalline water occurs too early, that is, at relatively low temperatures.

It is the object of the present invention to provide a containment for heat producing, temperature-sensitive materials with walls of relatively high heat conductivity below a predetermined temperature threshold and of low heat conductivity above such temperature threshold so that heating of the materials within the container from without is inhibited when the temperature rises above the given threshold.

Summary ofthe invention In orderto easily conduct heat to the outside under normal circumstances but prevent heat input through the container in an accidental high-temperature environment, the present containment for heat generating dangerous materials has at least two containment wall structures arranged within one another with a space formed between the walls and a meltable material disposed in the space, which meltable material provides for good heat conductivity through said space under normal conditions.However, when the containment walls are heated, for example, during a fire, above the melting point of the meltable material, the material, when molten, is discharged through openings formed in the containment walls such that the space between the containment walls becomes highly insulatory, thereby preventing heat to be conducted into the containment interior from without.

With this arrangement, the thermal insulation of the containment increases dramatically when the melting temperature threshold of the selected meltable material is reached and this material becomes liquid and drains off the space in the containment wall. It is advantageous that the temperature threshold is selectable in a wide temperature range since meltable materials are available over a large range.

It is also advantageous that the meltable material, generally a metal, has a high heat conductivity which provides for excellent heat transfer during normal use of the containment, much better than, for example, asbestos, whereas, in a high temperature environment (in a fire), the insulation of the containment walls becomes better than asbestos. It is also pointed out that the melting of the meltable material consumes a considerable amount of heat.

With the containment according to the invention, the temperature of a heat producing HAW ingot in a single-unit transport container is reduced by about 40"C. Taking into consideration that, generally, a temperature of 450"C is considered critical and must not be exceeded, and that, with asbestos insulation, a temperature of 400"C is reached under normal conditions, it will be appreciated that, with the present invention, a considerably increased safety margin with regard to the critical temperature is obtained. Of even larger importance is the concept of the present invention in the design of multiple-unit transport containers since their internal temperature is even more difficult to control as the relationship between volume and surface is less advantageous.

The invention will be explained in greater detail on the basis of two schematic Figures 1 and 2 in connection with a table given in the specification.

As shown in Figure 1, the containment comprises, around a container 1, containment walls 2,3 and 4 arranged at a distance d from one another. The innermost wall 2 is disposed adjacent the shielding through which the heat generated by the radioactive content 5 of the container 1 flows. The spaces 6 and 7 between the walls 2 and 3, and 3 and 4 are filled with a meltable material. The walls 3 and 4 (not the innermost wall 2) have discharge openings 8 providing for passages from spaces 6 and 7 to the outside so that molten material may flow out of spaces 7 and 6 during a fire, for example. For later calculations, the temperatures of the walls 2,3 and 4 are to beT1, T2 and T3.

The containment with one-way insulating structure is to provide good heat conductivity below a predetermined temperature threshold while providing for a dramatic reduction of the heat conductivity when the predetermined temperature threshold is passed and the material in the spaces 6 and 7 is melted and discharged, providing for air gaps 9 and 10 (Figure 2).

When the spaces 6 and 7 are filled with the meltable material of high heat conductivity as shown in Figure 1, the containment has excellent heat transfer characteristics. However, when the meltable material is melted, as during a fire, it flows out of the spaces 6 and 7 through the dishcarge openings 8. After discharge of the molten material, heat transfer through the containment wall is limited essentially to radiation heat for which the size of the distance d between the walls 2,3 and 4 is of little import.

The heat transfer of the arrangement according to Figure 1 is given by the equation: T1-T2 = X T2-T3 (1) s d s d and, in the arrangement according to Figure 2 by the equation:

It can be seen that the emissivity E of the walls 2,3 and 4 is of substantial import to the equation (2), that is, it greatly affects the heat conductivity of the emptied containment walls. Walls with low emissivity, that is, highly reflective walls, greatly reduce the heat transfer of the emptied containment wall. The following table gives heat transfer numbers for various arrangements. The outside temperature is assumed to be 800"C while the temperature within the container is assumed to be 1 50"C.

Space d or Insulation Thickness [m] 0.01 0.01 0.01 0.01 0.01 Inside Temperature ["C] 150 150 150 150 150 Outside Temperature [ C] 200 300 400 600 800 Insulation Materials Heat Transfer Numbers Asbestos 20.5 21 21.5 22.5 23 Sand 34 35 37 40 42 Ml (E1=1) without 21 29 39 69 113 1W (E2=1) Ml (E1=0.1) without 1.1 1.5 2.1 3.7 5.9 1W (E2=0.1) Ml (E1=1) with(E2=1 10 14 19 35 57 1W (E3=1) Ml (E1=0.1) with (E2=0.1) 0.54 0.75 1.0 1.8 3.0 1W (E3=0.1) MI = one-way melt insulation IW = intermediate wall Various materials may be utilized as meltable materials. Metal alloys with melting points as low as 20"C are available from company "Bleiwerk Goslar KG". Alloys with any desired melting point above this temperature may be formed. In the temperature range of 20" to 150 alone there are 14 alloys readily available whose melting points are evenly distributed over the given range. However, not only alloys but, in principle, also other materials, such as salts with low melting points, may be utilized although such salts do not have the high heat conductivity A of the metals.

With regard to the number of shielding walls 3, 4, it is pointed out that the insulating effect increases with the number of walls, though at a decreasing rate since, at lower temperatures, heat is transmitted between adjacent walls to a greater degree by convection than by heattransferthrough the gas in the spaces between the walls 2,3 and 4. With two or three walls, this portion of heat transfer is negligible. The number of walls to be selected depends on the given conditions and is limited by available manufacturing techniques. For a HAW-transport containment in accordance with present requirements, a double wall structure with a single meltable material chamber would provide sufficient insulation.If the transport containment is to be insulated so as to be able to withstand a longer fire, for example, in a mine storage (for example, for several hours), it may be necessary to provide two meltable material chambers with mirrored walls.

The distances d between the walls 2,3,4 have to be so selected that the molten material can flow out of the chambers 6,7 in spite of its adhesion to the chamber walls. After discharge of the molten material from the chambers 6, 7, the distance d between the chamber walls has no particular importance with respect to the insulation effect as long as the heat is mainly transferred by radiation. In practice, A HAW-transport containment will have a distance d between the walls of 3 mm to 1 cm. Tests have shown that, in this range, no problems should be encountered with the discharge of the molten material.

The openings 8 for the discharge of the molten material from the containment should be distributed in such a manner that molten material may flow out also if the melting takes place only in limited areas, that is, when only sections of the containment walls are overheated. The appropriate distance and arrangement of the discharge openings depends on certain conditions assumed to occur during a fire, for example. For a HAW-transport containment presently developed, one discharge opening per 0.1 m2 containment surface is provided. The discharge passages are selected large enough to insure proper discharge of the molten material. They should have a diameter of preferably 5-10 mm although tests have shown that even gaps of 1 mm or less permit the molten material to flow out of the containment.

Claims (8)

1. A containment for a container enclosing heat generating dangerous materials, especially radioactive waste materials, adapted to dissipate the heat generated by the material contained therein and also adapted to restrain heat from without to be conducted into the interior of said containment, said containment comprising at least two containment walls arranged within one another with a space formed between said walls, and a meltable material disposed in said space, with all but the innermost wall having discharge openings for releasing the meltable material from the space between said walls when the temperature of the containment walls reaches the melting temperature of said meltable material.

2. A containment according to claim 1, wherein said walls are disposed at such a distance from one another that release of said meltable material, when molten, is not inhibited by the given adhesion forces.

3. A containment according to claim 1, wherein said discharge openings are sized so as to permit uninhibited discharge of the molten material from the spaces between said walls.

4. A containment according to claim 1, wherein the surfaces of the walls defining said space are mirrored.

5. A containment according to claim 1, wherein said meltable material is a metal.

6. A containment according to claim 1, wherein said meltable material is a metal alloy.

7. A containment according to claim 1, wherein said meltable material is a salt.

8. A containment for heat generating noxious materials substantially as herein described with reference to and as illustrated in the accompanying drawings.