Abstract
Tremendous work has been done in the Light Water Reactor (LWR) Modelling and Simulation (M&S) uncertainty quantification (UQ) within the framework of the Organization for Economic Cooperation and Development (OECD)/Nuclear Energy Agency (NEA) LWR Uncertainty Analysis in Modelling (UAM) benchmark, which aims to investigate the uncertainty propagation in all M&S stages of the LWRs and to guide uncertainty and sensitivity analysis methodology development. The Best-Estimate Plus Uncertainty (BEPU) methodologies have been developed and implemented within the framework of the LWR UAM benchmark to provide a realistic predictive simulation capability without compromising the safety margins. This paper describes the current status of the methodological development, assessment, and integration of the BEPU methodology to facilitate the multiscale, multiphysics LWR core analysis. The comparative analysis of the results in the stand-alone multiscale neutronics phase (Phase I) is first reported for understanding the general trend of the uncertainty of core parameters due to the nuclear data uncertainty. It was found that the predicted uncertainty of the system eigenvalue is highly dependent on the choice of the covariance libraries used in the UQ process and is less sensitive to the solution method, nuclear data library, and UQ method. High-to-Low (Hi2Lo) model information approaches for multiscale M&S are introduced for the core single physics phase (Phase II). In this phase, the other physics (fuel and moderator), providing feedback to neutronics M&S in a LWR core, and time-dependent phenomena are considered. Phase II is focused on uncertainty propagation in single physics models which are components of the LWR core coupled multiphysics calculations. The paper discusses the link and interactions between Phase II to the multiphysics core and system phase (Phase III), that is, the link between uncertainty propagation in single physics on local scale and multiphysics uncertainty propagation on the core scale. Particularly, the consistency in uncertainty assessment between higher-fidelity models implemented in fuel performance codes and the rather simplified models implemented in thermal-hydraulics codes, to be used for coupling with neutronics in Phase III is presented. Similarly, the uncertainty quantification on thermal-hydraulic models is established on a relatively small scale, while these results will be used in Phase III at the core scale, sometimes with different codes or models. Lastly, the up-to-date UQ method for the coupled multiphysics core calculation in Phase III is presented, focusing on the core equilibrium cycle depletion calculation with associated uncertainties.
Highlights
The accuracy of the computational codes has been improving with the use of more realistic models and hypotheses as well as increased computation resources. e best-estimate approach, typically accompanied with confidence bounds obtained from uncertainty analysis, is regarded in both nuclear industries and regulation as an acceptable alternative to the traditional conservative regulation approach. e Organization for Economic Cooperation and Development (OECD)/Nuclear Energy Agency (NEA) benchmark for Uncertainty Analysis in Modelling
More detailed analyses were performed to determine the correlation between each of the calculation parameters. e uncertainty of k-inf and the results indicate that the choice of the covariance library strongly impacts the Relative Standard Deviation (RSD) of k-inf, as shown in Figure 4, while other parameters have limited influence. e average RSD of k-inf calculated using the SCALE 6.0/SCALE 6.1 covariance libraries is below 0.5% (0.47% and 0.48%, resp.), while the value corresponding to SCALE 6.2 is 0.54%
Work is paper summarizes the current status and outcome of the development of the Best-Estimate Plus Uncertainty (BEPU) framework for the multiscale, multiphysics Light Water Reactor (LWR) core analysis under the guidelines of the LWR Uncertainty Analysis in Modelling (UAM) benchmark, primarily focusing on the Pressurized Water Reactor (PWR) cases based on the TMI-1 reactor. e stand-alone neutronics exercises in Phase I are mainly concerned with the propagation of input uncertainties through the standard multistep LWR simulation procedure to key core parameters, such as the multiplication factor
Summary
3. Uncertainty Propagation in Reactor Physics Simulations e first phase of the LWR UAM benchmark [1] is dedicated to the stand-alone multiscale neutronics M&S and carried out in three steps, each corresponding to one of the steps of the standard LWR simulation approach: cell physics (to produce multigroup microscopic cross section libraries), lattice physics (to derive multigroup homogenized macroscopic cross section sets), and core physics (to assess fullcore performance parameters). It is convenient to use the PT method to compute the sensitivity coefficients of output variables with respect to nuclear data as compared with the sampling approach, making it possible to determine the most influential nuclidereaction pair to the predicted k-inf uncertainties by sorting them from greatest to least variance fraction.
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