An Offsite Dose Assessment

Abstract

A set of hypothetical Beyond Design Basis Accidents (BDBAs) at the Darlington Nuclear Generating Station were analyzed to estimate the off-site dose consequences of each sequence. This study used the code MAAP5-CANDU v5.00a to model accident progression and predict a release source term, and the code WinMACCS v3.11.2 to model atmospheric dispersion and estimate off-site doses. The results showed that for a 4-unit Station Blackout (SBO) event, if no mitigating action is taken, the predicated dose at the site boundary is above the 100 mSv evacuation criterion as prescribed in the Provincial Nuclear Emergency Response Plan (PNERP). However, if likely mitigating actions are credited, such as shield tank inventory makeup or containment Air Cooling Unit (ACU) restoration, the predicted dose at the site boundary is below the 100 mSv evacuation criterion. Further, if no on-site mitigating action is taken and only off-site protective measures are modelled, a significantly lower public dose is predicted compared to the unmitigated case without off-site protective measures. Overall, this study confirms the effectiveness of likely on-site mitigating actions as well as off-site protective measures in reducing public dose.