Inhalation hazard of radioactive aerosols from dismantling in-core structural components in a pressurized heavy water reactor during decommissioning

Abstract Decommissioning of pressurized heavy water reactors (PHWRs) is a complex process involving safe shutdown, dismantling, decontamination, and disposal of reactor materials, while adhering to stringent international safety standards. This process is typically executed under three options: Immediate dismantling, deferred dismantling, and entombment. In India, the deferred decommissioning approach is preferred, involving a 50-year safe enclosure period to allow significant reduction in radiation from activated components, such as cobalt-60. This study focuses on the final stage of decommissioning, specifically the cutting of highly activated pressure tubes (PTs) and calandria tubes (CTs). We assess inhalation doses from radioactive aerosols generated during the cutting process using the ORIGEN2 code for material activation and the FLUKA code for external dose rate simulations. The study calculates airborne activity concentrations of radionuclides and estimates inhalation doses assuming no protective measures. Results indicate that inhalation doses are substantially lower compared to external doses, with internal doses contributing ≤1% for PT and CT. External dose rates decrease over time, with a more rapid reduction for CT due to differing radionuclide compositions. Notably, Nb-94 in PT contributes significantly to long-term activity, whereas Co-60 and Zr-93 are more influential for CT. Detailed dose rates, radionuclide activity levels, and inhalation dose contributions, emphasize that while external exposure is predominant, internal exposure from inhaled radionuclides also poses risks. The study underscores the necessity for protective measures, including shielding and remote handling, to mitigate both external and internal radiation exposures during decommissioning. Comprehensive safety planning is essential to ensure the protection of workers and the environment.