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

The dismantling of structural objects during the decommissioning of nuclear facilities needs radioactive source characterisation 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 pressurised heavy water reactor (PHWR), various in-core components are predominantly made of either zircaloy-2 or 4. The nuclides present as impurities in the zircaloy, playing 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 the 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 optimisation and collective dose budgeting during the decommissioning stage of a typical PHWR.

Dose distribution to a random walker moving in a two-dimensional surface around a radioactive source.

BACKGROUND: Modeling of dose distribution of randomly moving population around a radioactive source is a complex problem. OBJECTIVE: The objective is to develop a model and solution techniques to estimate radiation absorbed dose to the population randomly moving around a radioactive source. METHODS: The problem is formulated using a second-order partial differential equation; different moments of the dose distribution function are defined related to physically realizable quantities, and solutions are obtained using standard moments methods. Alternatively, numerical simulations are performed to estimate the radiation doses using Monte Carlo approach for individual positions and random motions of the people around the source. RESULTS: A good agreement is found between average doses obtained from moments method and numerical simulations. A typical application of this model to different exposure conditions shows that the average dose is highly dependent on the population density. The study results show that average dose decreases with increase in the population density and movement area of random walker. SIGNIFICANCE: This mathematical model can be used as a rapid assessment tool by the emergency planners in resource optimization by providing quick estimates of likely exposures for triage and emergency response.

DOSE ASSESSMENT FOR ATMOSPHERIC DISCHARGE OF LONG-LIVED RADIONUCLIDES IN NUCLEAR POWER PLANT DECOMMISSIONING.

Decommissioning of nuclear power plants is a multistage process involving complex operations like radiological characterization, decontamination and dismantling of plant equipment, demolition of structures, and processing and disposal of waste. Radioactive effluents released into the environment may result in exposure of population through various exposure pathways. The present study estimates the public dose due to atmospheric discharge of important radionuclides during proposed decommissioning activities of Indian Pressurized Heavy Water Reactors. This study shows that major dose contributing radionuclides are 60Co followed by 94Nb, 134Cs, 154Eu, 152Eu, 133Ba, 99Tc, 93Mo and 41Ca. It is found that infant dose is higher than adult dose and major fraction of total dose (~98%) is through ground shine and ingestion; other pathways such as inhalation and plume shine contribute only a small fraction. This study will be helpful in carrying out radiological impact assessment for decommissioning operations which is an important regulatory requirement.