Radiation safety at the PRIMA facility: a review of shielding solutions and personnel dose assessment

The Neutral Beam Test Facility (NBTF), also called PRIMA (acronym for Padova Research on ITER Megavolt Accelerator) consists of two separate development test beds currently under construction in Padua (Italy): the ion source test facility, named SPIDER (Source for Production of Ion of Deuterium Extracted from Rf plasma) and the megavolt test facility, called MITICA (acronym for Megavolt ITER Injector & Concept Advancement). Both injectors accelerate negative deuterium and hydrogen ions with a maximum energy of 1 MeV for MITICA and 100 keV for SPIDER, and a maximum beam current of 40 A for both the experiments. Accelerated ions are stopped on a.CuCrZr alloy calorimeter where an intense neutron field is generated following to D-D and D-T reactions. In the present paper, a systematic review of the radiation safety analysis for both injectors is presented. The shielding design is described, including special shielding solutions planned with the aim of allowing personnel access for inspection and maintenance of the injector. In addition, the major results regarding the activation level of the corrosion products (ACPs) generated inside the cooling loops are reported, as a function of the neutron yield, time campaign and pulse duration. Furthermore, personnel classification, tritium inventory and dispersion are also presented and discussed in the paper. Our analysis indicates that the radiation safety system for the PRIMA facility is appropriate in maintaining the individual doses for workers and population well below the Italian (and internationally stated) regulatory limits. Tritium diffusion from the facility is not an issue from the radiation protection point of view. Even considering the most conservative scenario, the dose rate per year to the reference group (population) is about three orders of magnitude lower than the dose limit. Furthermore, our results indicate that for SPIDER, even considering the maximum workload, the contribution of ACPs to external exposure is negligible. On the other hand, due to the higher neutron yield, ACPs produced on MITICA cooling loops may significantly contribute to external exposure of workers. Radiation shielding and radiation protection criteria were assessed in order to meet the Italian regulatory limit for non radiation workers (1 mSv/yr). Our analysis and project evaluations confirm that this limit is never exceeded during operating phases of the injectors.