Simulation of CANDU Bundle Cross-Sectional Averaged Actual Flow Quality and Void in One-Dimensional Two-Phase Flow Models

Abstract

This paper presents the theoretical and empirical background for the representation of CANDU bundle sub-channel void distribution effects in One-Dimensional Two-Phase flow. Due to the non-uniform velocity profile in the sub-channels and to the non-uniform fuel element power distribution in the bundle, the cross-sectional average flow quality is quite different than the thermodynamic quality in a bundle. This is clearly seen in CANDU fuel bundle full-scale tests which show transition to two-phase pressure drop well before the onset of bulk boiling based on thermodynamic quality. Models have therefore been proposed to represent the bundle cross sectional averaged void-quality relationship in one dimensional two-phase flow. The focus of this paper is the application of the Leung sub-cooled boiling model in steady state and transient simulations as implemented in NUCIRC and SOPHT-G2. The Leung model predicts the onset of significant void in a fuel bundle string and computes the bundle actual flow quality using the Saha-Zuber model developed for uniformly heated tubes. The paper shows the importance of representing actual quality and void for predictions of two-phase pressure drop, CHF and void reactivity feedback.