Application of dynamic probabilistic risk assessment techniques for uncertainty quantification in generation IV reactors

Demonstration of a practical approach is presented by which dynamic probabilistic risk/safety assessment (PRA/PSA) techniques are used to augment the uncertainty quantification process for Generation IV reactors. While the traditional PRA approach using event- and fault-trees is mostly concerned with aleatory uncertainties, dynamic PRA/PSA techniques allow simultaneous consideration of both epistemic and aleatory uncertainties in a phenomenologically and stochastically consistent manner. Epistemic uncertainties are expected to be particularly important in the PRA/PSA of Generation IV reactors. A dynamic PRA/PSA methodology is coupled with a reactor safety code to enable a more efficient and yet accurate evaluation of epistemic uncertainties which appear in the analysis of Generation IV reactors. Key uncertainties are identified for a postulated Loss of Forced Circulation event in a Pebble Bed Modular Reactor. The effects of these uncertainties on a selected key Figure of Merit, the stored energy in the core materials, are quantified. As a key result, the analyses show that the shape of the cumulative distribution function for each event tree is influenced by the aleatory uncertainties while the variations in the magnitudes of the cumulative distribution functions are determined by the epistemic uncertainties.