Accuracy and Uncertainty Analysis of PSBT Benchmark Exercises Using a Subchannel Code MATRA

In the framework of the OECD/NRC PSBT benchmark, the subchannel grade void distribution data and DNB data were assessed by a subchannel code, MATRA. The prediction accuracy and uncertainty of the zone-averaged void fraction at the central region of the 5 × 5 test bundle were evaluated for the steady-state and transient benchmark data. Optimum values of the turbulent mixing parameter were evaluated for the subchannel exit temperature distribution benchmark. The influence of the mixing vanes on the subchannel flow distribution was investigated through a CFD analysis. In addition, a regionwise turbulent mixing model was examined to account for the nonhomogeneous mixing characteristics caused by the vane effect. The steady-state DNB benchmark data with uniform and nonuniform axial power shapes were evaluated by employing various DNB prediction models: EPRI bundle CHF correlation, AECL-IPPE 1995 CHF lookup table, and representative mechanistic DNB models such as a sublayer dryout model and a bubble crowding model. The DNBR prediction uncertainties for various DNB models were evaluated from a Monte-Carlo simulation for a selected steady-state condition. 1. Introduction The critical heat flux (CHF) is a parameter of great importance, which constrains the thermal power capability of a light water nuclear reactor (LWR). It is usually predicted by a local parameter CHF correlation accompanied with an appropriate thermal-hydraulic field analysis code to obtain the local subchannel grade conditions in the fuel assembly. For this purpose, the subchannel approach has been widely adopted in the design calculation of an LWR core since it provides reasonably accurate results on the flow and enthalpy distributions in rod bundles with a pertinent computing time. The OECD/NRC PWR Subchannel and Bundle Tests (PSBT) benchmark was organized on the basis of the NUPEC database. The purposes of the benchmark are the encouragement to develop a theoretically based microscopic approach as well as a comparison of currently available computational approaches. The benchmark consists of two separate phases: a void distribution benchmark and DNB benchmark. Subchannel-grade void distribution data was employed for validation of a subchannel analysis code under steady-state and transient conditions. The DNB benchmark provided subchannel fluid temperature data, which can be used to determine the turbulent mixing parameter for a subchannel code. The steady-state and transient DNB data can be used to evaluate and improve the currently available DNB prediction models in PWR bundles. The NUPEC