Temporal evolution of activation products in Zircaloy - 4 guide tube and estimation of its external gamma dose rate for decommissioning activities

The dismantling of structural objects during decommissioning of nuclear facilities needs radioactive source characterization for the planning of decommissioning strategies in compliance with the ALARA (as low as reasonably achievable) principle. The sources may arise from neutron activation of the structural components in the reactor core as well as contamination due to the radioactive release from the fuel occurred during normal operation or unplanned events in a nuclear power plant (NPP). In a pressurized heavy water reactor (PHWR), various incore components are predominantly made of either zircaloy - 2 or 4. The nuclides present as impurity in the zircaloy, play a crucial role in the activity inventory due to neutron activation of those nuclides, which in turn determine the external gamma dose rate. The activity of the activation products depend on the neutron flux seen by the component, duration of irradiation and cooling period, half-lives of the daughter products and amount of the impurities present in the structural components. To illustrate this, a guide tube made up of zircaloy – 4 has been considered. A guide tube assembly is a part of the primary shut down system (PSS) which guides the movement of absorber elements in the upward and downward direction in the calandria. This study has identified and quantified the activity inventory in a guide tube at the end of the operation of the reactor using the ORIGEN2 code, and then estimated the associated external gamma dose-rate using the FLUKA Monte Carlo code. The findings will help the management of radioactive waste, cost optimization and collective dose budgeting during the decommissioning stage of a typical PHWR.