Site risk analysis for nuclear installations

Currently, multi-unit risks have not typically been adequately accounted for in risk assessments, since the licensing is based on unit-specific probabilistic safety assessment (PSA) with focus on a reactor accident. NKS-R project SITRON (SITe Risk Of Nuclear installations) has searched for practical approaches for Nordic utilities to assess the site level risk. Starting point of SITRON work has been the fact that the Nordic utilities already have good unit-specific PSAs. Therefore, the question is what additional efforts are needed to obtain a site level risk assessment. Practically, it means two tasks: 1) to identify relevant inter-unit dependences, and 2) to quantify the site level risk. Inter-unit dependences consist of multi-unit initiating events, shared systems, structures and components, dependences in human actions, inter-unit common cause failures, and plant operating state combinations. SITRON provides guidance how to perform the identification of dependences and how to select relevant dependences for quantification (screening). Quantification of site risk can be performed quite straightforwardly, given that the quality of the single-unit PSAs is sufficient. SITRON project has also included a survey on the role of Emergency Response Organisation (ERO), often referred to as the Technical Support Centre (TSC) in accident management. Based on responses from four plants in Finland and Sweden, SITRON has investigated different implementations of EROs with respect to possible impact on operational decisions in severe accident and multi-unit scenarios. The human role in severe accidents differs markedly: new decision makers (ERO and TSC rather than main control room); different instructions (guidelines rather than procedures); different decisions (involving trade-offs, novel actions, and strategies contrary to conventional knowledge); inter-unit influences; unreliability of instrumentation; and long time windows for actions.