Analysis of a LOFA in EU DEMO Breeding Blanket

In the next decades, in order to address climate change, the electricity generation sector needs to definitely abandon fossil fuels and rely on sustainable and carbon-free energy sources. In view of this, fusion energy represents, in the long-term, a clean and predictable energy source, useful to handle and balance the well-known intermittency of the wind and solar electricity production. In the framework of the European roadmap for the realization of fusion energy, a key step is the realization of the EU DEMO fusion reactor, the first fusion device to produce electricity and to exploit a closed-fuel cycle, demonstrating tritium self-sufficiency. To produce the tritium fuel needed for the fusion reactions, DEMO will be the first fusion reactor to have a Breeding Blanket (BB); the BB, in fact, contains lithium-based materials (called breeding materials) which, interacting with the neutron flux coming from the plasma, produce the tritium needed to sustain the fusion reactions. Among the concepts proposed during the years for the EU DEMO BB, one of the most promising ones is the Helium Cooled Pebble Bed (HCPB). This concept involves the use of gaseous helium at 80 bars as a coolant, ceramic pebbles made by a mixture of Li4SiO4 and Li2TiO3 as breeding material, beryllium ceramic pebbles as Neutron Multiplying Material (NMM), and the EUROFER97 low-activation stainless steel as structural material. Since the BB is directly exposed to the heat load coming from the plasma, its cooling is of huge importance to avoid excessive overheating of the solid structures which can lead to structural failure; moreover, the cooling scheme must be optimized to maximize the heat removal and, hence, deliver more heat to the Power Conversion System (PCS). The thermal-hydraulic design of the BB concepts must be studied under both operational and accidental conditions. Being the design of EU DEMO reactor at a pre-conceptual stage, preliminary studies using numerical tools are needed to give useful feedbacks for future detailed studies and experiments. In this work a model of the of the Primary Heat Transfer System (PHTS) of the HCPB BB is developed, starting from the already existing GETTHEM code developed at Politecnico di Torino, to perform parametric analyses regarding the acci-dental scenario of a Loss of Flow Accident (LOFA). The model, written with the Modelica modelling language, aims to be a system-level, fast-running tool useful to simulate the global thermal-hydraulic behavior of the HCPB BB under the above-mentioned accidental scenario, with a particular focus on the temperatures reached inside the solid structures of the First Wall (FW), as it is the region directly exposed to the plasma. Three different initiating events are considered for the onset of a LOFA: (i) the complete loss of the circulating power of the cooling loop, which leads to the most severe scenario, (ii) the failure of one of the two circulators feeding the cooling loop, and (iii) the obstruction of a single FW channel. Moreover, each scenario has been tested with and without the intervention of the emergency plasma shutdown system, which, if activated soon enough, has shown that it can effectively mitigate the consequences of the accident.