Safety Assessment of Bore-Hole Repositories for Sealed Radiation Sources Disposal
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M.I. OJOVAN, A.V. GUSKOV, L.B. PROZOROV, A.E. ARUSTAMOV, P.P. POLUEKTOV, B.B. SEREBRYAKOV. Scientific and Industrial Association "Radon", The 7-th Rostovsky Lane, 2/14 Moscow, 119121, Russia, [email protected]. ABSTRACT Bore-hole repositories (BHR) are considered to be promising for disposal of HLW and spent sealed radiation sources (SRS). A safety assessment of BHR disposal of SRS was performed using geologic environmental analysis, available parameters for BHR and SRS design and radionuclide inventory. The probabilistic calculations take into account some data uncertainties and variability. The results showed that there is practically no release of shortlived radionuclides into the environment for about 1000 years. This is completely due to the very low corrosion rate of the lead matrix in which the SRS are encapsulated. Various models were applied for more detailed numeric simulation of the repository temperature, radiation fields, and transport of released radionuclides in the geosphere. Ultra-conservative scenarios were chosen for these models. The worst case comprises both breaching of all engineered barriers and flooding of the disposal site plus eventual failure of an imperfect SRS immobilization matrix with some sources partly exposed by the breached matrix. For this extreme case, the maximum dose was found to be not higher than 55 - 75 jiSv/y. INTRODUCTION Experience in operation of shallow ground BHR for SRS and inspection of their status have indicated the possibility that high-power ionising radiation fields could accelerate corrosion of engineered barriers, allowing radionuclides to penetrate into underground water and the surrounding soil [1]. A technological scheme to immobilize SRS in a metallic matrix directly in the BHR was therefore developed that increased the SRS BHR safety considerably. This scheme allows the use of a conventional scheme of SRS BHR disposal followed by in-place immobilisation of the SRS to provide an additional barrier - a metal layer - between the sources and environment. This decreases the potential release of
radionuclides and enhances disposal safety [2]. After sufficient radionuclide decay (requiring about 500 - 1000 years), the metallic blocks containing the SRS will be removed and melted for matrix material reutilisation. To estimate the applicability of the SRS immobilisation technology in BHR one should evaluate the long term safety of the repository based on the most likely event scenarios. SIMULATION For modelling we used a number of computer codes that model each scenario for potential release of radionuclides from the disposal site and transport into the environment. We considered many possible scenarios, among them the most important were: "* Lateral spreading of radionuclides; "* Transport of radionuclides in perched water tables due to flow into the repository; "* Transport of radionuclides into perched water tables due to flow of water into the repository and heat convection of this water caused by radionuclide decay heating of the repository; "• Transport of rad
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