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

Abstract

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.

Overview and Discussion of the OECD/NRC Benchmark Based on NUPEC PWR Subchannel and Bundle Tests

“…FLICA-OVAP and SUBCHANFLOW [13] predictions show dependence on the radial power shape for both DNB power and DNB time. MATRA [14] seems to give reasonable predictions of the DNB time but significantly overpredicts the DNB power.…”

Development of a Single-Phase, Transient, Subchannel Code, within the MOOSE Multi-Physics Computational Framework

“…The reason for the additional mixing effect is thought to be the special mixing vanes that the NUPEC facility incorporates in its design. Specifically, the temperature gradient appearing in the experimental data was attributed to the thermal mixing in the diagonal direction of the test bundle, which may be caused by the alignment of mixing vanes mounted in the spacer grids [40]. This is the physical reason behind the need to use an increased value of β = 0.08.…”

“…So clearly β had to to be adjusted to a much higher value. That value was adopted from [40] and produces code predictions that are in a much better agreement with the experimental results.…”

“…The experimental results exhibit a non symmetric distribution that we cannot capture with a constant value of β. There is a temperature gradient towards the corner due to an additional mixing effect, which may reduce the exit temperature differences between subchannels and finally lead to an increase of the value of the optimum β [40].…”