Fusion energy systems present increasingly significant computational challenges as they grow in size and complexity. Once constructed, ITER will be a full-size nuclear facility with highly complicated structures and support systems, with an array of scientific equipment in close proximity to the neutron-emitting deuterium-tritium plasma. Characterization of shutdown dose rate (SDDR) distributions caused by the neutron activation of these structures is important to the final design and full-power operation of the device. This work summarizes the theoretical basis and parallel implementation of the Multi-Step Consistent Adjoint-Driven Importance Sampling (MS-CADIS) method designed specifically for highly efficient execution of multistep activation problems. Fusion SDDR benchmark problems have been solved with these new tools, and the results have been compared to experimental and other computational results to establish their validation basis.