DEMO Divertor preliminary safety assessment

Abstract

European DEMOnstration plant is currently finalizing a pre-conceptual design phase. A carefully selected set of safety analyses are contextually being performed on DEMO systems in order to assess its integrated performance and capability to achieve expected targets while keeping in a safe operation domain. A key in-vessel system is the Divertor, in charge of exhausting a major part of the plasma ions thermal power in a region far from plasma core to control plasma pollution. To accomplish this mission a modular design approach was adopted based on two main sub-systems: the divertor cassette and plasma facing units (PFUs). Each cassette supports two vertical plasma facing units, one inboard and one outboard, intercepting the main part of the particle loads from the plasma. An assembly of 3 of such divertor cassettes and related PFUs is then foreseen for each of DEMO 16 sectors, hence resulting into 48 cassettes. Two independent heat transfer systems for cassettes and plasma facing units respectively provide heat sink function and connect the Divertor to DEMO Balance of Plant.An overview of the preliminary safety analyses performed on such system is here proposed. At first, a safety-perspective overview of current design is proposed. In particular, moving from the divertor system functions, specific component failure modes and related possible consequences, a set of relevant postulated initiating events are identified.Then, aiming at investigating whether accident consequences remain within accepted safe domain, two accidents from this set were selected to be analysed deterministically by means of MELCOR code. In particular, an in-vessel Loss of Coolant Accident initiated by a large rupture of the divertor PFU cooling pipes and a Loss of Flow Accident (LOFA) initiated by heat transfer system (HTS) pump seizure have been analysed. The nodalization includes divertor PFU and related HTS among other in-vessel components and safety devices (e.g., Vacuum Vessel Pressure Suppression System) devoted to Vacuum Vessel pressurization transient control or Fast Plasma Shutdown System. Parametric analyses were also performed to investigate possible impact of the design solutions (e.g., implementation of isolation valves in Divertor loop) on the accident consequences.