Assessing the Conservatism in EPZ Determined on Plume Centerline Dose: A CALPUFF-Based Method Used in Level 3 PSA

Abstract

Emergency preparedness is the final level of the defense in depth in nuclear safety. Emergency planning zone (EPZ), the basis for emergency preparedness, is usually determined based on centerline doses, with the hypothetical individual located directly under plume path throughout the exposure. This conservative assumption with no credit taken for wind shifts could overestimate projected doses and accordingly provide a margin in EPZ. However, over-conservatism does no good to emergency preparedness, but brings in unnecessary fear among the public. To assess the conservatism in plume EPZ determined on centerline doses, we propose here a best estimate approach based on CALPUFF dispersion model to account for wind shifts in Level 3 probability safety assessment (PSA). Doses from CALPUFF-based method in real atmospheric conditions are then compared in detail with centerline doses from PAVAN and MACCS, on a case-by-case basis. Results of this study demonstrate that neglect of wind shifts in dose assessment could result in unrealistic Level 3 PSA results and significant conservatism in plume EPZ, especially when wind direction changes frequently. These results highlight the importance of characterizing real atmospheric conditions and treating wind shift in dose assessment for dividing plume EPZ. Despite its preliminary nature, this study makes an interesting attempt to integrate a realistic assessment of offsite consequences in nuclear emergency planning and preparedness, which could benefit to develop a reasonable conservative emergency plan to better protect the public from radiological releases, and contribute to building public trust in nuclear safety.