PSI Methodologies for Nuclear Data Uncertainty Propagation with CASMO-5M and MCNPX: Results for OECD/NEA UAM Benchmark Phase I

Capabilities for uncertainty quantification (UQ) with respect to nuclear data have been developed at PSI in the recent years and applied to the UAM benchmark. The guiding principle for the PSI UQ development has been to implement nonintrusive “black box” UQ techniques in state-of-the-art, production-quality codes used already for routine analyses. Two complimentary UQ techniques have been developed thus far: (i) direct perturbation (DP) and (ii) stochastic sampling (SS). The DP technique is, first and foremost, a robust and versatile sensitivity coefficient calculation, applicable to all types of input and output. Using standard uncertainty propagation, the sensitivity coefficients are folded with variance/covariance matrices (VCMs) leading to a local first-order UQ method. The complementary SS technique samples uncertain inputs according to their joint probability distributions and provides a global, all-order UQ method. This paper describes both DP and SS implemented in the lattice physics code CASMO-5MX (a special PSI-modified version of CASMO-5M) and a preliminary SS technique implemented in MCNPX, routinely used in criticality safety and fluence analyses. Results are presented for the UAM benchmark exercises I-1 (cell) and I-2 (assembly). 1. Introduction The OECD/NEA benchmark for uncertainty analysis in modeling (UAM) was launched a few years ago to promote the development, assessment, and integration of comprehensive uncertainty quantification (UQ) methods in best-estimate multiphysics coupled simulations of LWRs during normal as well as transient conditions [1]. Although very ambitious by nature (due to the complexity of the task to treat all potential sources of uncertainties), the benchmark has nevertheless achieved one of its first objectives, namely, to constitute a major (if not the main) international framework to drive forward the development of methodologies for the propagation of nuclear data uncertainties in reactor simulations. This topic was proposed as the first phase of the benchmark, and since research in precisely this area was at the same time being launched within the STARS project [2] at the Paul Scherrer Institut (PSI), participation to this benchmark was considered as a timely and highly valuable opportunity to complement the development and assessment of the PSI methods. In that context, two parallel lines of development were in fact initiated at PSI. On the one hand, the development of a UQ methodology for the propagation of neutronic uncertainties in the deterministic CASMO/SIMULATE/SIMULATE-3？K chain of reactor analysis