ACCELERATION OF SHUTDOWN DOSE RATE MONTE CARLO CALCULATIONS USING THE MULTI-STEP CADIS HYBRID METHOD

摘要

Shutdown dose rate (SDDR) analysis requires (1) a neutron transport calculation to estimate space- and energy-dependent neutron fluxes, (2) an activation calculation to compute the distribution of radionuclide inventories and the associated photon sources, and (3) a photon transport calculation to estimate the final SDDR. In some applications, accurate full-scale Monte Carlo (MC) SDDR simulations are needed for immensely large systems that involve massive amounts of shielding materials with complex geometric arrangements. However, these simulations are impractical because accurate calculation of space- and energy-dependent neutron fluxes in these systems is difficult with the MC method even if global variance reduction techniques are used. This paper describes the Multi-Step Consistent Adjoint Driven Importance Sampling (MS-CADIS) hybrid MC/deterministic methodology that accelerates multi-step MC shielding calculations. MS-CADIS speeds up the SDDR neutron MC calculation using an importance function that represents the neutron importance to the final SDDR. Using a simplified example, preliminary results showed that the MS-CADIS method enhanced the efficiency of the SDDR neutron MC calculation by a factor of 550 compared to standard global variance reduction techniques, and that the efficiency enhancement compared to analog MC is higher than a factor of 10,000.