Migration of Radionuclides through Rock Mass: Centrifuge Modelling
Radioactive waste is a bye-product generated from various activities such as operation of the nuclear fuel cycle, the use of radioactive isotopes in medical diagnosis and treatment, industrial and research applications of radioactive materials, and nuclear weapons testing. Radioactive wastes occur in solid, liquid and gaseous forms and contain different amounts and types of radioactive isotopes. These wastes emit heat and radiation which have to be isolated from the biosphere until these have died down. Radioactive waste is kept away until decay has reduced the radioactivity to an acceptable level. Safe disposal of radioactive wastes is a matter of concern and research world-wide. To isolate radioactive waste from the biosphere several proposals have been made, but with each proposal came an array of objections. Table 1 presents the proposed methods of disposal of long lived radioactive waste with the current status. It is widely agreed that the preferred option of radioactive waste disposal is to process the waste into stable solid forms and place these in an underground repository in a suitable geological formation. Radioactive wastes are usually subdivided into two main categories: low level wastes (LLW) and high level waste (HLW). This subdivision is based on an estimation of long term health hazard posed by the waste, which is related to its content of long lived radioisotopes. LLW can be regarded as a material that must be isolated from the biosphere for 100 to 1000 years, after which it becomes innocuous. HLW, on the other hand, must be isolated for 105 to 106 years. In order to dispose HLW safely, a site should be chosen which can isolate HLW for the required length of time. For this, a proposed site must satisfy certain criteria. This criteria has been dealt with in chapter 2. In order to assess the risks and to understand the basis on which the above criteria is set, it is necessary to look into various processes of the migration of radionuclides.
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