Investigation of the level of airborne contamination in the Pebble Bed Modular Reactor(PBMR) building or specific room due to design base leakrate of 0.1% of helium inventory

Abstract

The fission products release into reactor core and primary circuit lead to airborne contamination as a result of helium that leaks from the system into the room. The airborne activity was calculated by deriving and solving a differential equation considering room ventilation and a twin room ventilation to check the effects of ventilation system or migration of airborne activity to the other rooms. The HVAC system was optimized by considering and comparing the activity calculated for the three HVAC system models (i.e. recirculation with and without filter and no recirculation of air or once through model) and by increasing the air change rate. The activity calculated for each nuclide ~as used to calculate the annual dose received by a worker spending 2000 working hours per year in the room. It was found that the dose received for the three HVAC models differs. The recirculation of air without filter was the most contributing in terms of dose followed by recirculation of air with filter model then no recirculation model was the least because there is no activity recirculated back into the room. The filters used for recirculation of air with filter model are radioactively contaminated, but the calculated filter activity was found to be low and can be disposed as low level waste or can be continuously used with an efficiency test regularly performed on the filters.