Neutronics analysis for fusion DEMO reactor design

This paper presents neutronics analysis mainly focused on key design issues for self-sufficient tritium production based on the conceptual design study carried out for a fusion DEMO reactor in the past several years, which includes new findings regarding the design methodology of breeding blanket. Self-sufficient production of tritium is one of the most critical requirements for fusion reactors. As a practical matter, tritium production in the reactors depends on the design of the overall configuration of in-vessel components as well as breeding blanket design including materials selection. We considered a fusion DEMO reactor with a major radius of about 8 m and fusion output of 1.5 GW with breeding blanket consisting of a mixed bed of Li₂TiO₃ and Be₁₂Ti pebbles. The coolant was assumed to be pressurized water in the condition of 15.5 MPa and 290-325°C. Considering a lot of design requirements such as plasma positional control and maintainability of in-vessel components, a reasonable dimension of breeding blanket was determined. The net tritium breeding ratio (TBR) was estimated to be 1.15 with a three-dimensional analysis with the MCNP-5 with nuclear library of FENDL-2.1, satisfying a self-sufficient supply of tritium (net TBR ≥ 1.05). Throughout the research, we found that tritium breeding capability (i.e., local TBR) of breeding blanket is less dependent on the arrangement of cooling pipe in the blanket when the neutron wall loading is lower than about 1.5 MW/m² which is met in the DEMO considered. The result suggests that tolerance for the installation of cooling pipes in each blanket module will not be a critical matter. In addition, we found that a gap of about 0.02 m between neighboring blanket modules has little impact on the gross TBR. The result is favorable in terms of the access of remote handling equipment for maintenance and the installation tolerance of blanket modules. Based on the neutronics analysis, the latest design of breeding blanket and the other in-vessel components is presented.