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Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)

Michael G. Stamatelatos
Michael G. Stamatelatos
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Harold S. Blackman
Harold S. Blackman
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ASME Press
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In the PSA models of accident sequences, the designed response of operators and managers leads to a safe system state when a sequence of hardware systems and barriers operate successfully and the operators perform a set of required actions at the appropriate points in scenarios. Unsafe system states of Internal Event PSA are reached due to a combination of hardware failures, omissions of required actions at selected points of scenarios and/or commissions of unrelated actions because of wrong cognitive process or incorrect execution. The first generation HRA methods are focused basically on the errors of omission. Unsafe system states of External Event PSA are also reached through a combination of external hazards (earthquake, flood, fire, hurricane, tsunami etc.), hardware failures, omissions of required and/or commission of unrelated actions. However, under higher stress of hardware, barrier materials and humans, the operators and managers can erroneously perform an action that will aggravate the scenario at any point of its evolution — errors of commission. Errors of commission often involve an error of omission as well; by doing an unrelated action the person may fail to do the necessary action. The identification and treatment of potentially significant hazards, human errors are of particular importance during the external event accident conditions. Other issues that should be addressed are modelling and quantification of the human errors in the external event PSA.

The paper presents an investigation into the applicability and consistency of three HRA techniques for calculation of post-accident Human Error Probabilities (HEPs) in the external event PSA, namely: Human Error Assessment and Reduction Technique (HEART); Holistic Decision Tree (HDT); and Performance Evaluation of Teamwork (PET). Different external event training scenarios based on standard (benchmark) external event PSA model assumptions are designed and their detailed boundary conditions are described. The HEPs of post-accident main control room critical operator's actions are calculated by means of each investigated technique. For each inconsistency identified, a counter measure and recommendations are proposed.

The designed study also includes:

1) Validation and verification of three HRA methods (HEART, HDT and PET) for the external event PSA;

2) Development and implementation of an advanced Data Collection System (based on the CREDIT system) for processing the records of each observer during crew debriefing sessions;

3) Improvement of the existing Simulator and Audio-Video System of Full-Scope Simulator of WWER-1000 (FSS-1000) monitoring, introducing sensor control equipment for human stress/behavior processes registration and its integration to the HRA methodology;

4) Determination of specific and general counter-measures and recommendations in a checklist guide that HRA practitioners can use to minimize bias in the HEP assessments.

The improved methodologies and proposed experimental design should be used for system pilot implementation. This activity should include modeling of external events training scenarios. These scenarios will be used for all operator and team performances (Unit 5 and 6 of Kozloduy NPP) during the regular continuous training exercises on the FSS-1000.

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