Identification of Errors of Commission for a Swiss Nuclear Power Plant: application of the CESA method

This paper presents the first results of a study of Errors Of Commission (EOCs) for a Boiling Water Reactor (BWR), the Leibstadt nuclear power plant in Switzerland. EOCs refer to inappropriate, undesired actions potentially carried out by the plant operators in the course of an accident and that may aggravate the accident scenario. EOCs contrast with Errors Of Omission (EOOs), which relate to the omissions of required actions – typically included in the Probabilistic Safety Assessment (PSA) and treated by current Human Reliability Analysis (HRA) methods. One of the difficulties connected with the treatment of EOCs is that, in principle, there is a very large number of possible actions with the potential to aggravate accident scenarios (i.e. the number of potential EOCs) in a plant. This poses challenges for a systematic identification of EOCs to be included in a PSA. To cope with this issue, the Commission Error Search and Assessment (CESA) method was developed. It aims to identify and analyze EOC events, prioritized based on their potential risk-relevance. This paper presents the application to the BWR of the CESA method to identify a first set of candidate EOC events. The following stage will be an initial judgment on the plausibility of these candidate events – some events may not be considered credible and therefore not carried forward in the analysis. The subsequent steps will be to analyze, quantify and include in the PSA the retained EOCs. It is one of the first EOC analyses ever done for BWRs. tified EOCs, to discuss corresponding countermeasures. To address the outstanding issues associated with EOCs as risk contributors, the Risk and Human Reliability Group at the Paul Scherrer Institute developed the Commission Error Search and Assessment (CESA) method. Its aims are to identify and analyze EOC events, prioritized based on their potential riskrelevance. Following up a pilot study for a Pressurized Water Reactor (PWR) (Reer et at. 2004), a recent study was carried out to identify EOC scenarios and quantify their risk significance for another nuclear power plant, a Boiling Water Reactor (BWR), the Leibstadt nuclear power plant in Switzerland. The scope of the study includes EOCs occurring during accidents initiated during full-power operation, by internal initiating events (IE). It is one of the first EOC analyses ever done for BWRs – two other examples are in Torralba & Muñoz (2002) and Ruiz-Sanchez & Nelson (2009). The first aim of the study is to contribute to the understanding of EOC scenarios and their significance for BWRs, although the results are of course plant-specific. The second aim is to evaluate the CESA method when applied to BWRs, and if needed, identify possible improvements. This paper presents the work carried out to identify candidate EOC events for the target plant. These are actions that may disable PSA-important plant systems. At this stage, neither the likelihood of these actions nor specific reasons motivating these actions are addressed. The plausibility of these events will subsequently be analyzed to identify the set of EOCs to be finally included in the plant PSA. This paper is organized as follows. The next section will summarize the CESA method (for details, cf. Reer & Dang (2007) and Reer et al. (2004). Section 3 briefly describes the power plant studied in this work, with emphasis on the emergency and heat removal systems. The application of the CESA method for candidate EOC identification is presented in Section 4 (this corresponds to CESA steps 1 and 2). Conclusions are given at the end.