Uncertainty and Sensitivity of Neutron Kinetic Parameters in the Dynamic Response of a PWR Rod Ejection Accident Coupled Simulation

Abstract

In nuclear safety analysis, it is very important to be able to simulate the different transients that can occur in a nuclear power plant with a very high accuracy. Although the best estimate codes can simulate the transients and provide realistic system responses, the use of nonexact models, together with assumptions and estimations, is a source of uncertainties which must be properly evaluated. This paper describes a Rod Ejection Accident (REA) simulated using the coupled code RELAP5/PARCSv2.7 with a perturbation on the cross-sectional sets in order to determine the uncertainties in the macroscopic neutronic information. The procedure to perform the uncertainty and sensitivity (U&S) analysis is a sampling-based method which is easy to implement and allows different procedures for the sensitivity analyses despite its high computational time. DAKOTA-Jaguar software package is the selected toolkit for the U&S analysis presented in this paper. The size of the sampling is determined by applying the Wilks’ formula for double tolerance limits with a 95% of uncertainty and with 95% of statistical confidence for the output variables. Each sample has a corresponding set of perturbations that will modify the cross-sectional sets used by PARCS. Finally, the intervals of tolerance of the output variables will be obtained by the use of nonparametric statistical methods.

Uncertainties propagation in the UAM numerical rod ejection benchmark

“…Although such kind of items was discussed on many workshops and conferences, we could not find many openly available relevant publications. One may mention (C. Mesado, et al, 2012) where authors computed uncertainties for the rod ejection transient presuming some feedback using Latin Hypercube and simple sampling techniques.…”

Uncertainty Quantification and Sensitivity Analysis for Cross Sections and Thermohydraulic Parameters in Lattice and Core Physics Codes. Methodology for Cross Section Library Generation and Application to PWR and BWR.

“…More accurate model response variance is obtained using LHS (Hernández-Solís 2012). More information can be found about LHS sampling method in Swiler and Wyss 2004. These sampling techniques are well studied, for example Hernández-Solís 2012, Strydom 2013 andMesado et al 2012 compare SRS and LHS sampling techniques. These studies show that LHS has a better domain coverage, however, significant differences are not found.…”

“…Finally, the uncertainty propagated is reflected in PARCS output parameters (multiplication factor -k eff -, axial power peak -P z -and peak node location -N z -). Even though in this thesis the methodology to propagate the cross section uncertainty is shown without thermohydraulic coupling, there is not any limitation in the methodology that prevents the core physics code to be coupled with a thermohydraulic code, as it was proved by the same thesis' author in Mesado et al 2012. The default perturbation library provided with the module SAMPLER is used to propagate the uncertainty through SCALE.…”

“…As explained in Mesado et al 2012, it is known that the fraction of delayed neutron precursors (and their decay constants) play an important role in the output variable variance. Especially, for assemblies near the end of cycle, delayed neutrons play an important role in the time evolution of the neutron flux, and so the uncertainty in β i and λ i will have a greater effect on the output parameters.…”

“…As explained in Mesado et al 2012, two different approximations are used. First approximation, or maximum response approach, calculates the U&S analysis only at time step where the maximum absolute response is found.…”