Fire safe radioactive source container

Abstract

A fire-safe container for storing radioactive sources, comprises a thermally insulating outer layer 3 to reduce heat transfer into the container, and an inner layer forming a base 6 of high-density ma

Description

This invention relates to a device for storing small radioactive sources safely, even in the event of a fire. It serves two functions: in normal circumstances it will shield an individual from harmful ionising radiation, in the event of a fire it will ensure no radioactive material is released from the container. Small sources of radioactive isotopes emitting gamma photons, beta particles, and alpha particles are widely used throughout academia, medicine, and wider industry. Most source containers currently consist of a small lead box, or a lead box with an inner container of plastic. Lead is a high density and high atomic number material; consequently, it will strongly attenuate gamma radiation. Accordingly, lead is used to shield gamma emitting radioisotopes. A lead box with an inner container of plastic is used to shield beta emitting radioisotopes. When a beta particle is slowed down rapidly, for instance when it interacts with a high density, high atomic number material such as lead, it will emit Bremsstrahlung radiation - essentially x-rays. The solution is to first slow them down slowly, in this case with plastic, and secondly, attenuate the x-rays that were produced when they were slowed down, in this case with lead. These solutions work well at normal temperatures. However, in the event of a fire within the facility that houses the radioactive material these solutions will fail. The temperatures of normal housefires range from 500°C to over l,000°C. The melting point of lead is 327°C, and the melting / burning point of most plastics is lower. Additionally, the melting and boiling point of some radioisotopes is low, depending upon the element in question. In the event of a fire the radioactive isotopes may melt, boil, and escape their containers. Even in the case of radioactive isotopes with a high melting point, the lead container will melt; no longer providing the required shielding of harmful ionising radiation. This results in radioactive contamination to property and the environment, constituting an external radiation hazard. This contamination may also be inhaled, constituting an internal hazard. This is a dangerous, costly, and harmful result from a reasonably foreseeable incident. To overcome these problems, the present invention proposes a composite radioactive source container that, when exposed to the heat of a fire, will immobilise radioactive material and seal it within the container. The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows the complete Fire Safe Radioactive Source Container with lid fitted. Figure 2 shows the complete Fire Safe Radioactive Source Container with lid elevated. Figure 3 shows the Fire Safe Radioactive Source Container after high temperatures have melted the contents of the container, with the exception of the radioactive source, as designed, and the radioactive source is immobilised. In figure 1, the complete Fire Safe Radioactive Source Container is shown, with a pressure regulating value 1, contained within a tungsten lid 2, a thermally insulating ceramic outer layer 3 containing a base of lead 6, and a middle of low melting point glass 4 (melting point below that of lead, for example Vaneetect <VS-1305B>). The position of the radioactive source is also shown 5. The radioactive source does not constitute any part of this invention and is shown purely to highlight its location when stored within the Fire Safe Radioactive Source Container and method of immobilisation. In figure 2, the Fire Safe Radioactive Source Container is shown with the tungsten lid 1 elevated above the rest of the container. In figure 3, the Fire Safe Radioactive Source Container is shown after high temperatures have melted the low melting point glass 4, and the lead 6. The radioactive source 5 can now be seen immobilised between the layer of glass 4 and lead 6. The ceramic outer layer decreases the rate of heat transfer into the container and provides a thermally robust casing. The base of lead acts as a high density and high atomic number material that will strongly attenuate gamma radiation. The middle of low melting point glass acts as a chemically inert low density and low atomic mass number material that will slow beta particles down; reducing the quantity of x-rays they produce. Additionally, in the event of a fire, the low melting point glass will melt. Glass consists mostly of an amorphous collection of silicon oxide molecules with a density around 3 g / cm3. Radioactive sources are typically metals, which have a greater density than glass; consequently, the radioactive source will sink under the molten glass. As the fire continues to heat the container the lead will melt. Lead has a density of 11.34 g / cm3, which is greater than most radioactive sources. Thus, the radioactive material will be trapped between molten lead and molten glass, fully isolating it from the environment. Finally, the top of the container will be made of tungsten. Tungsten, like lead, has a high atomic number and high density, meaning it will strongly attenuate gamma radiation; however, unlike lead, it has a melting point of 3,422°C. This exceeds the temperatures reached in normal fires and so will not suffer thermally induced material degradation. After the fire subsides, and the temperatures begin to fall, the lead will initially solidify, then the glass will solidify. This is because the low melting point glass will have a melting point below that of lead. If higher melting point glass were used, the glass would solidify and then the lead would solidify, the thermal contraction of the lead on the solid glass would crack the glass and the radioactive material would not be encapsulated. Additionally, as the temperature increases, the air inside the container will thermally expand, pushing molten glass between the ceramic outer layer 2 and the tungsten lid 1. Excess pressure will be relieved by the one-way pressure regulating value 1, which ensures excessive pressure cannot build up inside the container in exceptionally hot fires, and further facilitates the creation of a vacuum after cooling. As the fire subsides and the temperature drops the glass will solidify forming a seal around the ceramic outer layer 2 and the tungsten lid 1. As the temperature continues to drop the air inside the container will continue to cool, thermally contracting, forming a partial vacuum inside the container. Consequently, the radioactive source will be trapped in a layer of glass, followed by a vacuum, followed by a glass seal. Furthermore, the radioactive source will be surrounded by either lead, or tungsten. Both lead and tungsten strongly attenuate gamma photons, x-rays, beta particles, and alpha particles.

Claims

1. A fire-safe container for storing one or more radioactive sources, comprising:o a thermally insulating ceramic outer layer configured to reduce heat transfer into the container;o an inner radiation shielding layer composed of lead or bismuth, arranged within the outer layer, configured to attenuate gamma radiation;o a glass layer positioned between the radiation shielding layer and an upper tungsten layer, the glass layer having a melting point lower than the melting point of the radiation shielding layer, configured to melt during exposure to temperatures reached in fires, thereby encapsulating the radioactive sources when the container is in use;o the upper tungsten layer forming the top of the container, configured to maintain structural integrity and provide radiation shielding at temperatures encountered in fires;o a pressure regulating one-way valve integrated into the tungsten upper layer to relieve pressure from the container.

2. A method of immobilising radioactive sources within the container according to claim 1, comprising:o positioning the radioactive sources within the container;o sequentially melting the glass layer followed by the radiation shielding layer upon exposure to fire temperatures;o encapsulating radioactive sources between the molten glass and molten radiation shielding material;o maintaining the structural integrity of the tungsten upper layer to prevent escape of radioactive contamination upon exposure to fire;o creating a vacuum seal upon cooling, further immobilising and isolating the radioactive sources.

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