Review of radiation exposure of workers at Indian NPPs

The collective radiation dose at the nuclear power plants in India has been showing downward trend as a result of dose control efforts guided by the operating experience and the periodic regulatory reviews. However, there is a need for further optimization and control of the exposures in view of the international scenario. The radiation exposure to the workers is caused mainly by (i) enhanced concentration of specific radionuclides in the plant systems and (ii) task related exposures which arise from specific tasks. It is observed that a few radionuclides are responsible for major part of the exposures. The important radionuclides significantly contributing to occupational exposure in the plant were identified on the basis of the contribution of these sources to the annual collective dose. The plant data on radionuclide concentration in the primary heat transport and moderator systems were examined to evaluate their contribution to the personnel exposures. About 70-80 % of the annual collective dose in the NPPs is expended in shutdown maintenance (biennial + poison shutdowns) jobs and the remaining dose is accumulated from normal operational activities. Active particles of cobalt and antimony in the circuits have been responsible for a significant share of shutdown dose across all nuclear power plants. About 20-40 % of the annual collective dose in PHWR plants in the country arises from exposure to tritium. The internal exposure due to tritium continues to be a concern in spite of the control measures such as dedicated ventilation system, D2O vapor recovery system and several administrative controls. Source control measures, especially by ensuring low cobalt content material (typically <0.01%) on all PHT and moderator boundary components, avoiding antimony bearing components in reactor systems and reducing the tritium concentration in the plant systems is the desired course of action. (author)