CHF EXPERIMENTS UNDER STEADY-STATE AND TRANSIENT CONDITIONS FOR TIGHTLATTICE CORE WITH NONUNIFORM AXIAL POWER DISTRIBUTION

Steady-state and transient critical heat flux (CHF) experiments were p erformed using triangular pitched 7-rod assemblies with non-uniform ax ial power distributions under the maximum pressure of 15.5 MPa. The on set of steady-state CHF was predicted within the uncertainty of 10% wi th the KfK correlation using the local flow conditions calculated by t he subchannel analysis code COBRA-IV-T. On the other hand, various mec hanistic CHF models did not agree with the steady-state CHF data. The transient CHFs under the conditions of flow reduction, power increase or flow and power simultaneous variation were predicted with the quasi -steady-state method within approximately the same uncertainty as the steady-state CHF experiments. The predictive capability did not depend on the transient speed within 30%/s of the flow reduction rate and wi thin 120%/s of the power increase rate. It was also revealed that ther e exists large CHF margins under the thermal-hydraulic conditions simu lating the locked rotor accident and the control rod cluster ejection accident of the double-flat-core type high conversion pressurized wate r reactor (HCPWR).