Thermal Hydraulic Analysis of the 80 MWth Pb–Bi cooled XADS with modified RELAP5/MOD3.3 code

Abstract

The Experimental Accelerator-driven System (XADS) is a proposed 80 MWth Lead-Bismuth cooled pool type facility coupling a proton accelerator and a sub-critical fission core by means of a spallation target. A cascade fission reaction is sustained by the spallation neutrons. The facility primarily aims to demonstrate the operability of the whole complex and, subsequently, also the capability to reduce the inventory of Pu, Minor Actinides and selected long-lived fission products. The paper, after a brief description of the LBE-cooled XADS concept, focuses on the assessment of its response to the changing of core power which is caused by doubling and cutting the external neutron and loss of flow events affecting both the whole core and a single fuel assembly while the former are originated from the postulated malfunction or loss of the peculiar gas injection system employed to enhance primary coolant circulation, the latter may derive from significant flow blockages at a fuel assembly inlet. Through the whole research, the RELAP5/MOD3.3 code which is modified to fit the sub-critical fission core is used. The results of the performed analyses show the excellent natural circulation capability of the system and the safety performance leaded by negative feedback, specifically in an accident scenario with the assumed simultaneous failure to trip the proton beam, and highlight the indeed peculiar heat removal mechanisms from a fuel assembly in case the associated coolant flowrate drops to very small values.