Probabilistic Basis and Assessment Methodology For Effectiveness of Protecting Nuclear Materials

For nuclear facilities, a key approach for defeating insider activities includes procedural measures for protecting critical materials, specifically material control and accountability (MC&A) operations. The work presented here describes recent developments for a new method to incorporate MC&A protection elements within the existing probabilistic vulnerability assessment (VA) methodology to estimate the probability of effectiveness (PE) for insider threats. MC&A activities, from monitoring to inventory measurements, provide information about target materials and define security elements useful against insider threats. Activities that discourage insiders provide many, often reoccurring opportunities to determine the status of critical items, including detection of missing materials. Previous developments for elements of the method are reviewed, including the object-based state machine paradigm whereby an insider theft scenario races against MC&A activities that can move a facility from a normal state to a heightened alert state having additional detection opportunities, the definition of possible timing distributions, and the use of probabilistic convolution. The latest method development furthers the coupling of the object-based paradigm with nuclear plant probabilistic risk assessment (PRA) methods to incorporate the evaluation of MC&A elements in the existing VA methodology. These include the use of event sequence diagrams (ESDs) and human error probabilities (HEPs) for detection of missing material. The combination of the elements in the method provides a probabilistic basis for applying this method for determining the effectiveness of protecting nuclear materials against insider threats. Information from ongoing analyses to demonstrate the method and determine an effectiveness measure for MC&A activities is also discussed. Along with the PE for the physical protection system (PPS) determined in existing VA analyses, the overall result is an integrated effectiveness measure of a protection system that addresses outsider and insider threats.