Patent Number: 051026155
Section: summary

BRIEF SUMMARY OF THE INVENTION The invention is directed to a container for storing, transporting and disposing radioactive material comprising a vessel and cap each having walls with a core of radioactive shielding material enveloped and isolated within a continuous metal lining. In the operation of nuclear power stations spent fuel is generally stored in short term storage on the power station site until such radioactive materials have decayed to a state where long term storage or disposal is desirable due to space limitations as material accumulates, and transport to locations off-site does not entail unacceptable risks. To this end containers have been developed for storing and transporting radioactive material encased within a shell of radioactive shielding material such as concrete for example. Although lead has superior shielding capabilities, due to its high weight, toxicity, and cost, other preferable shielding materials have been developed including high density concrete. To provide corrosion and leakage resistance as well as structural strength to the shell of shielding material, conventional containers often include inner or outer metal liners. Conventional containers having a core of lead are described in the following U.S. Pat. Nos. 3,229,096 to Bonilla et al; 2,514,909 to Strickland; and 4,666,659 to Lusk et al. A hollow concrete-shielded steel outer walled container with a steel inner liner is described in U.S. Pat. No. 3,448,859 to Hall et al. for use in association with liquid wastes. Conventional containers which utilize lead as a shielding material suffer from disadvantages when compared to concrete shielded containers since lead is relatively expensive. Lead is toxic and therefore requires more careful handling during construction. Lead has a low melting point and low structural strength which are disadvantageous due to the heat generated by radioactive decay. Lead also has significantly different thermal expansion characteristics compared to the associated composite metal liners used, requiring the designs to incorporate means to allow for differential expansion. Concrete as a shielding material is preferred therefore since it is less costly and is easily prepared and handled. Concrete also has similar thermal expansion characteristics to steel enabling the use of steel liners and internal reinforcing bars, without the necessity of accommodating differential expansion. However, concrete typically contains pockets of water that has not reacted with the cement powder and therefore concrete is a relatively porous material. Concrete also cracks under thermal or other stresses and upon impact. As a result therefore concrete often allows contaminated fluid to migrate through it reducing its effectiveness as a radioactive shield. Spent fuel elements are often stored under water in short term storage pools within the power station. To minimize the risk of contamination, loading of spent fuel into long term storage containers is preferably carried out under water within the short term storage pools. The water within such pools contains radioactive material and therefore containers with concrete shielding material which is exposed to such contaminated water during loading are unsuitable since the shielding material may become permeated with contaminated water through the cracks and pores of the concrete. Conventional containers generally comprise a vessel with a central cavity to house the radioactive material and a cap which is bolted to the vessel to seal the cavity. Although bolted caps may be preferred if repeated access is desired, bolted caps and associated flexible gasket seals are often unreliable in the long term due to gasket and bolt corrosion. Bolted caps are also difficult to install since an evenly distributed compressive force is required to seal the flexible gaskets. Frequent inspection is required to ensure initial sealing and maintenance of the seal when bolted caps are used, increasing the associated costs and risks. The present invention relates to a novel container for storing and transporting radioactive material which overcomes the above disadvantages of conventional containers. In accordance with the invention is provided a container for storing and transporting radioactive material comprising: a vessel, having an upwardly open cavity for accommodating said radioactive material, said vessel having walls with a core of concrete shielding material enveloped and isolated within a continuous metal lining; and a cap, covering the top surface of said vessel sealing said cavity, said cap having a core of concrete shielding material enveloped and isolated within a continuous metal lining, the lower outer peripheral edge of said cap being continuously welded to the upper outer peripheral edge of said vessel.