ADVANTG shielding analysis for closure operations in an open-mode repository

Open-mode repository concepts could require worker entry into access drifts after placement of fuel casks in order to perform activities related to backfill, plug emplacement, routine maintenance, or performance confirmation. An ideal emplacement-drift shielding configuration would minimize dose to workers while maximizing airflow through the emplacement drifts. This paper presents a preliminary investigation of the feasibility and effectiveness of radiation shielding concepts that could facilitate worker operations in an open-mode repository. The repository model for this study includes pressurized-water reactor fuel assemblies (60 GWd/MTU burnup, 40 year post-irradiation cooldown) in packages of 32 assemblies. Halloysite clay is selected as the host medium; our results suggest that the dose rate in the access drift is not strongly affected by this choice. The closest fuel packages are 5 meters from dosimetry voxels in the access drift. The unshielded dose to workers in the access drift is 73.7 rem/hour. Prior work suggests that open-mode repository concepts similar to this one may require 15 m/s of ventilation airflow through each emplacement drift. Shielding concepts considered here include partial concrete plugs, labyrinthine shields, and stainless steel "photon attenuator" grids. Maximum dose to workers in the access drift was estimated for each shielding concept using MCNP5 with variance reduction parameters generated by ADVANTG. Because airflow through the shielding is important for open-mode repositories, a semi-empirical estimate of the head loss due to each shielding configuration was also calculated. Most configurations achieve the airflow target with pressure drop near 2.4 Pa. Shielding performance varies widely among the proposed configurations. Of the considered configurations, a labyrinthine shield with a smoothly curving airflow channel offers the best shielding performance. In this configuration, a 1-cm lead lining of the airflow channel reduces the dose rate below 1 mrem/hour.