Modelling LOFA scenario for WCLL BB concept using MELCOR1.8.5 code

In the framework of the EUROfusion activities for the DEMO project, the evaluation of the Breeding Blanket (BB) performance and its ancillary systems is a compulsory activity. The European liquid BB concept, named Water Cooled Lithium Lead (WCLL), makes use of the liquid metal alloy PbLi as a tritium breeder/neutron multiplier and water as a coolant. A dedicated ancillary system guarantees the recirculation of the PbLi between the BBs and the tritium extraction/purification system. The BBs and the ancillary system must operate safely and reliably under the designed conditions, while accidental scenarios have to be identified for risk assessment and to define safety plans. In the present work an analysis of the In-Board Loss Of Flow Accident (IB LOFA) in the PbLi ancillary loop of the WCLL BB is conducted. This numerical analysis is performed by employing the fully integrated code MELCOR1.8.5, the release for fusion application. A LOFA scenario is considered by assuming the electromagnetic pump in the IB BB loop, which provides forced circulation to the PbLi, stops working. The nodalisation and numerical models are performed by applying geometries and design parameters using the 2020 WCLL PbLi design. The analysis is performed to develop a suitable numerical model, which simulates transients by studying several factors such as: tritium mobilization and transport inside the loop, fast plasma shutdown leading to pressure and temperature transients, and maximum temperatures inside the First Wall (FW). The preliminarily results demonstrate the maximum temperature inside the FW does not exceed the defined temperature thresholds for Tungsten and EUROFER (1000 °C and 500 °C, respectively). Furthermore, the pressure transient is appropriately accommodated within the ancillary loop. The initial tritium inventory in breeding modules is relocated and accrued in the tritium extraction system's free volume. This work is the first step in the safety analyses of the PbLi ancillary system aiming to support the R&D works carried out in the framework of the DEMO project.