Radiant energy – Radiation controlling means – Shielded receptacles for radioactive sources
Patent
1987-09-05
1989-09-19
Anderson, Bruce C.
Radiant energy
Radiation controlling means
Shielded receptacles for radioactive sources
2505181, 376272, G21F 500
Patent
active
048684007
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to casks for the transportation and storage of radioactive materials. More specifically the invention relates to an improvement in cask design to reduce the weight and increase the payload, yet protect the handlers of the cask from the radiation emitted by the radioactive material being transported or stored.
BACKGROUND ART
Radioactive material must be stored and transported in containers that not only provide adequate interior capacity and structural integrity, but also shield those persons who handle the containers from the radioactivity of the material contained. For the type of radioactive material that is known as spent nuclear fuel, considerable shielding is necessary.
Spent nuclear fuel, and certain other types of radioactive waste, emits a great deal of gamma radiation and neutrons. Shielding gamma radiation is accomplished by making the container or spent fuel cask sufficiently massive. Higher density materials allow thinner-walled casks; lower density materials lead to thicker-walled casks.
Spent nuclear fuel also emits neutrons which are a form of radiation. Neutrons can be shielded best with certain kinds of material, notably those materials with a large number of hydrogen atoms.
In addition to capacity, structural integrity, and shielding, spent nuclear fuel casks must allow for the dissipation of heat energy from the radioactive decay taking place in the fuel. This decay heat can cause internal stresses when dissimilar materials are heated or when too great a heat gradient is established across a material used in cask construction.
There are two other considerations that limit the cask designer: weight and cost of the cask. For a cask that travels over the road, state highway restrictions disfavor vehicles weighing more than 80,000 pounds. Furthermore, greater cask weight can make loading and unloading more difficult.
The cost of materials and fabrication is a significant component of the costs of spent nuclear fuel storage and transportation. Another component is the number of casks needed for a given quantity of fuel. If cask payloads are small, more casks are needed to store and transport that quantity of spent nuclear fuel.
Thus, the cask designer is faced with competing choices of shape and materials for strength and shielding both gamma radiation and neutrons, while keeping weight and cost as low as possible.
Casks of various configurations have been constructed in the prior art to serve as containers for the transportation and storage of radioactive materials. Typically, they are cylindrical or square containers of cement or a dense metal such as steel or cast iron, having an inner cavity in which radioactive material being transported or stored is placed. These containers have heavy lids and trunnions to which lifting cables may be attached. The cask bodies are frequently composed of concentric layers of materials that provide structural strength and shielding. Until the present invention, the prior art has not successfully made any breakthrough in increasing payload without corresponding increases in weight. As the preferred embodiment of the present invention is the novel deployment of depleted uranium as shielding material in the form of pipes having polyethylene cores or in the form of rods, dispersed in a highly flexible pattern of bore holes around the periphery of a ductile iron cask, the discussion of the relevant art will be primarily directed to the prior use of uranium as a shielding material. Tungsten or other dense metal rods, as well as polyethylene rods, are viewed as alternative or additional shielding materials.
The use of shielding of multilayer construction was first noted in the radiation shield of Zinn in 1955 (U.S. Pat. No. 2,716,705). His shield comprised one or more layers of a neutron absorbing or shielding material and one or more layers of a gamma and neutron shielding material. Concrete, paraffin and steel shot were his protective materials. The Pat. No. of Trudeau et al. of 1973 (U.S. Pat. No. 3,732,427) illustrate
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Anderson Robert T.
Barnhart Victor J.
Anderson Bruce C.
Chem-Nuclear Systems Inc.
Reynolds Benoni O.
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