On the shielding of the tokamak coolant systems in the EU-DEMO

Abstract The conceptual development of the EUropean DEMOnstration fusion reactor (EU-DEMO) envisages a series of modifications aimed at improving the design of the reactor and its plant systems and triggering iterative calculation procedures for an organic development of the project itself. The present work fits into this context, focusing on a critical evaluation of the shielding performances of the tokamak building structures surrounding the primary coolant system for the water-cooled breeding blanket concept. As yardstick to measure the cooling loop shielding features the assessment was based around the water activation product which generally drives the size of the secondary shield walls in water-cooled fusion reactors, namely the 16 N isotope, and the numerical analyses were centered onto the 2022 configuration of the Primary Heat Transport System (PHTS) for the Water Cooled Lithium Lead Breeding Blanket (BB) concept. Research activity was articulated in two main phases. A first stage where the following quantities were assessed: (i) production rates of 16 N within the BB cooling water and (ii) spatial distribution of 16 N across the entire cooling loop. A final phase where photonic calculations were performed to evaluate the spatial distribution of the absorbed dose rate in the neighborhood of the cooling loop. The preparatory phase was necessary to set up the volumetric gamma source in the different regions of the primary circuit. Distribution of 16 N within the PHTS was obtained through a dedicated 1-D transport analysis, while particle transport simulations have followed a computational approach based on the Monte Carlo method adopting the Monte Carlo N-Particle code (MCNP5-1.60). Results highlighted some critical issue and shortcomings in the layout of the PHTS as well as in the approach followed to develop the cooling loop shielding. Models, assumptions and analysis results are herein reported and a critical discussion on needed design modifications is presented.