CRITICAL HEAT-FLUX EXPERIMENTS UNDER STEADY-STATE AND TRANSIENT CONDITIONS AND VISUALIZATION OF CHF PHENOMENON WITH NEUTRON RADIOGRAPHY

Steady-state and transient critical heat flux (CHF) experiments were p erformed using triangular-pitch 7-rod assemblies with non-uniform powe r distributions under the maximum pressure of 15.5 MPa. The onset of s teady-state CHF was predicted within an uncertainty of 10% with the Kf K correlation using local flow conditions calculated by the subchannel analysis code COBRA-IV-I. However, existing mechanistic CHF models di d not agree with the steady-state CHF data. The transient CHFs under t he conditions of flow reduction, power increase or flow and power simu ltaneous variation were predicted with the quasi-steady-state method w ithin approximately the same uncertainty as the steady-state cases. In order to clarify the CHF phenomenon, a real-time neutron radiography technique was used to visualize boiling flow inside a stainless-steel shroud under a pressure of 1.0 MPa. Various two-phase flow patterns an d flow behaviors just before and after the onset of CHF were observed by this method.