Shielding analysis and optimization of IFMIF-DONES test cell floor bolt penetrations using MCNP and advanced variance reduction techniques

Abstract

This study presents a shielding analysis and safety assessment for the Test Cell (TC) floor of the IFMIF-DONES (International Fusion Material Irradiation Facility-DEMO Oriented NEutron Source), focusing on the impact of mounting bolt penetrations and associated gaps. These penetrations, designed to allow thermal expansion and installation tolerances, pose potential risks of neutron and gamma radiation streaming, which must be mitigated to ensure worker safety and system integrity during operation and shutdown phases. A detailed computational model was developed by converting CAD geometry into MCNP input using the McCAD tool. Neutron transport calculations were performed using the MCNP Monte Carlo code with the McDeLicious extension to define the deuterium-lithium neutron source. Variance reduction techniques, implemented through ADVANTG, were used to enhance simulation efficiency and reduce statistical uncertainty. Shutdown dose rates (SDDR) were evaluated using the R2Smesh method coupled with FISPACT-2010 for activation analysis at cooling times of 1 h and 1 week. Results demonstrate that the neutron flux remains effectively shielded by the mounting bolts, with only a modest increase observed in photon flux near bolt gaps. Dose rate maps confirm no significant radiation leakage, and SDDR calculations reveal a reduction of less than 1 % over a one-week cooling period