Transient Boiling of Sodium in a Seven-Pin Bundle under Loss-of-Flow Conditions

Abstract

Transient sodium boiling experiments have been conducted in an electrically heated 7-pin bundle under loss-of-flow conditions. Each run was made by reducing or stopping the inlet flow at constant heater power. There was no strong effect of temperature ramp rate on the incipient-boiling (IB) wall superheat and the data were scattered with values as high as ~190°C in the range of the present experimental conditions. The observed coolant voidings, except in some high superheat runs, were initially limited in the central subchannel because of the steep temperature gradient in the bundle, and then spread slowly. In order to describe these voiding processes, a two-dimensional voiding model was required. The initial expulsion acceleration of liquid was higher with higher IB wall superheat. The measured values were lower than the theoretical prediction from the one-dimensional liquid column expulsion analysis. The bulk pressure rises at the initial vaporization were less than the vapor pressure corresponding to t...