AUTHOR=Leclaire Nicolas , Bess John Darrell 

TITLE=Comparison of two methods for assessment of the rod positioning uncertainty and consequences on the evaluation of correlation factors

JOURNAL=Frontiers in Energy Research

VOLUME=Volume 10 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2022.1058750

DOI=10.3389/fenrg.2022.1058750

ISSN=2296-598X

ABSTRACT=For many years now, the aim of the international community has been to provide documented critical experiments with a “best estimate” evaluation of the experimental uncertainties. Many systems are optimized such that minor variations in their fuel pitch are insignificant.  For some parameters such as the positioning of UO2 rods within under-moderated-square-pitched lattices, the choice of the method to propagate the experimental uncertainties can have a strong influence on the overall evaluated uncertainty. The aim of the paper is to compare a formerly used method to propagate this uncertainty in terms of keff to those provided in ISO Uncertainty Guidelines.  The paper briefly describes the tight-packed lattice experimental program performed at the Valduc Research Centre, which is at the origin of these concerns.  Then it proposes a simplified model on which to apply simulations of rod positioning to be performed with MORET 5 Monte Carlo code using the Prométhée tool.  Finally, it suggests an interpretation to the observed discrepancies and a comparison with repeatability/reproducibility experiments allowing determination of such uncertainty.  Results demonstrate that use of Monte Carlo methodologies provide more realistic uncertainty estimates in fuel pitch.  The current comparisons use light water reactor systems, which is directly relevant to some small modular reactor designs.  However, accurate prediction and estimate of uncertainties in pitch for advanced reactor systems is also relevant.  The application of unrealistic uncertainty analysis methods can incur larger margins in advanced reactor design, safety, and operation than are necessary.