Mechanistic Dryout Predictions in Uncrept and Crept CANDU Pressure Tubes with MEFISTO-T Subchannel Analysis Code

Abstract

In support of core thermal-hydraulic code development and applicability demonstration, OECD/NEA has organized a benchmark on CANDU fuel thermal-hydraulic (CTH). Specifications have been released based on full-scale rod bundle heat transfer experiments in a 28-element fuel assembly, covering both CANDU and BWR design operations. The benchmark database includes a single set of axial pressure drop (single-phase and two-phase) and critical power tests (non-blind phase) released for code calibration purposes. Boundary conditions for the remaining part of the database were released without measurement data (blind phase). The tested geometries include CANDU pressure tubes under both uncrept and crept (i.e. ageing) conditions, as well as various bearing pad heights. The crept geometry creates a significant bypass on the upper side of the fuel lattice and associated crossflows which are notoriously challenging to capture by simulation codes. The provided benchmark database was simulated with Westinghouse subchannel analysis code MEFISTO-T using a two-phase three-field approach of annular two-phase flow. Code calibration parameters related to form loss and drop deposition enhancement due to structural components (endplates and spacer pads) were adjusted based on the limited non-blind database. The axial pressure drop was accurately predicted, including under two-phase conditions. Furthermore, the radial position of dryout was correctly predicted by the code for the provided case, however downstream of the measured position. After calibration, all blind cases were simulated. Further assessments of code prediction performances will be performed after release of the benchmark data.