Safety analysis of near surface nuclear waste repository constituting an aquifer in proximity

Abstract

Probabilistic safety assessment involves the calculation of risk associated with possible radioisotope contamination of Groundwater. The overall scenario and the consequences that may occur on the failure of the external barriers are identified using an Event Tree analysis to be the multi-barrier failure due to rainwater intrusion into the near-surface disposal site and leaching out of radionuclides into groundwater which is extracted as drinking water by critical groups present around the repository. A program is developed in Python 3.0 for such a scenario to assess necessary parameters such as radioactive release rate, groundwater radionuclide concentration, dose ingested by members and total risk associated with each radioisotope. The initiation is with the assessment of the failure density function of the multi-barrier system operating in redundancy, using the failure density functions. The radionuclide concentrations of the near-surface disposal facility are necessary to be calculated to know the probable release rate of each radionuclide into groundwater. The concentration depletion with Spatio-temporal variance is also considered a factor for radioisotope concentration variation in groundwater. The concentration of radionuclides in groundwater when ingested at a rate of 2.2 L/day helps us to calibrate the Radiation dose to humans through this pathway by using the Ingestion dose coefficient for each isotope fixed by International Atomic Energy Agency [IAEA]. The final risk assessment is accomplished by multiplying the radiation dose ingested by members of the critical group with a risk factor for fatal or non-cancers and critical hereditary effects by the International Commission on Radiological Protection. In the present work, a subjective analysis is done to calibrate the dosage of radiation, of five radioisotopes (3H, 60Co, 59Ni, 90Sr, 129I) for disposal. Radioactive Iodine delivered maximum risk despite a low concentration in the inventory of radioactive waste. However, much lower than the permissible dosage suggested by ICRP i.e.,1 mSv in a scenario where an exclusion zone is considered to be existing, and a drinking water well is established at a distance of 1.6 km from the repository and the risk is computed over a period of 150 years since the multi-barrier failure.