Probabilistic Safety Assessment of twin-unit nuclear sites: Methodological elements

highlights
• A practical approach to fulfill the requirements of the PSA existing standards
• A comprehensive treatment of initiating events and common systems of twin-unit site
• A practical procedure of Human Reliability for site core meltdown risk assessment

When assessing the risk related to Nuclear Power Plants in terms of impacts on the population health and on the environment, multi units issues should be taken into account. Generally speaking, to date mainly models relating to a single unit have been developed by operators. The purpose of this paper is to present possible solutions or methodological options, suggested by EDF (Electricity of France) R&D, in order to switch from a risk assessment for the unit to a risk assessment for the site. The case of a site with two units is addressed here. A review of practices and standards showed that the specific aim of a PSA at site level was to deal with the dependencies existing between the units on that site. The risk calculation for the site is therefore proposed for six configurations resulting from the combination of two types of scenarios and three types of systems which are defined. The treatment of CCF events and the adaption of the assessment of the Human Errors Probabilities to the case of multiple units are also addressed in this paper. The proposed approach is illustrated using a simplified case inspired by the EDF 900 MWe units level 1 PSA model.