Analyses of LOCAs in the primary heat transfer system of the Helium cooled pebble bed blanket concept

The future introduction of fusion power plants requires also the demonstration that all the radiological risks, in term of potential hazards to the staff, the population and the environment, are below the limits established by national authorities in both normal and accidental conditions. As for LWRs, one of the most challenging accidents is the Loss of Coolant Accident (LOCA), which causes the depressurization of the Primary Heat Transfer System (PHTS) and the pressurization of the confinement structures and components, as the Vacuum Vessel (VV), the Expansion Volume (EV), and the Tokamak Building (TB). Hence, a detailed analysis on the break area and on the location of the rupture should be executed to demonstrate that the confinement barriers are able to withstand the pressure peak within design limits and the residual cooling capabilities of the PHTS are sufficient to remove the decay heat coming from the in-vessel components. Nonetheless, basing on the results of these analyses, several improvements can be introduced in the systems design in order to improve the overall safety performances of the plant itself. For this purpose several Ex-vessel and In-vessel LOCA analyses for the Primary Heat Transfer System (PHTS) of the DEMO Helium Cooled Pebble Bed (HPCB) blanket concept have been conducted. In particular, two PHTS designs were investigated, the first developed in the 1992 which is actually considered the reference design for the HCPB blanket concept, and a new one (alternative design) developed basing on the preliminary safety analyses results of the reference design. The aim of the work is to compare the performances of the two designs basing on the pressure values within the confinement barriers and the release of He inventory from the PHTS. Finally, taking into account the results of these analyses, considerations are formulated on the design criteria of the confinement barriers.