Comparison among MELCOR 1.8.2 fusion version and MELCOR 2.1 during normal and accidental scenarios for the Primary Heat Transfer System of the DEMO Helium Cooled Pebble Bed blanket

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 Light Water Reactors (LWR), 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, several analyses should be executed to demonstrate that the confinement barriers are able to withstand the accident 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. In LWRs these analyses are commonly executed employing "Severe Accident" codes, which are mainly integral codes able to simulate the incidental scenario from the initiating event to the release of radionuclides outside the containment. In principle, the same codes could be also employed for fusion reactors, but due to the intrinsic and deeper differences among the two reactor types it cannot be assured the quality and the correctness of the results obtained. For this purpose, some codes have been expanded to cope also with fusion reactors and their specific phenomena. One of the codes which have undergone this adaptation process is MELCOR, but unfortunately the only version developed under a quality assurance program is the old 1.8.2, which is no longer maintained. On the contrary, for LWRs, the latest MELCOR version available is 2.1, which was also expanded by SNL (Sandia National Laboratories) to cope with HTGRs (High Temperature Gas Reactor). Hence, this latest available MELCOR version should be also capable to treat, in a basic manner, the main phenomena occurring in the helium-cooled blanket concepts of DEMO. For this purpose, several analyses during normal and accidental (Ex-Vessel LOCAs) conditions have been executed considering the Primary Heat Transfer System (PHTS) of the DEMO Helium Cooled Pebble Bed (HPCB) blanket concept. The aim of these analyses is to compare the results obtained with MELCOR 1.8.2 fusion version vs. the MELCOR 2.1 standard version, in order to highlight the differences among the results on the main thermal-hydraulic parameters. In particular, the heat transfer coefficients predicted among the heat structures and the control volumes are investigated, being one of the main indicator of the differences among the two code versions