Reactor Core Analysis at Low Flow Condition Using THALES Subchannel Code

摘要

During the operation of PWR (Pressurized Water Reactor) nuclear power plant, the minimum DNBR (Departure from Nucleate Boiling Ratio) should be maintained within the Tech. Spec. LCO (Limiting Conditions for Operation) to satisfy the acceptance criteria during AOO (Anticipated Operational Occurrence) and event condition. For the DNBR analysis, the calculation of the flow and energy fields is required. Some events may experience the low flow condition if the LOOP (Loss of Offsite Power) is occurred. The reactor core analysis at low flow condition is classified into two ways. One is for the power generation of the reactor core and the other is for DNBR calculation of hot node. Generally, the flow analysis for core power generation is more difficult. The representative event which experienced the low flow condition is SLB (Steam Line Break) event. During the SLB event with LOOP, the reactor core is tripped and all RCPs (Reactor Coolant Pump) are stopped. As a result, the flow rate and pressure in reactor core decrease and the peaking factor is relatively high compared to a nominal operating condition. For the prediction of state in the PWR reactor core, the reactor core analysis at low flow condition is performed using the subchannel code THALES (Thermal Hydraulic AnaLyzer for Enhanced Simulation of core) developed by KEPCO Nuclear Fuel. Since the subchannel analysis code was developed to solve the reactor core flow field at the high axial flow rate and pressure condition, the convergence at the low flow condition such as SLB is very difficult. Therefore, most of subchannel analysis codes fail to obtain the converged flow field at very low flow and high power, which has resulted to insufficient research of 3 dimensional analysis of SLB event. This paper will show why we have a trouble to obtain the converged result for the reactor core analysis at low flow condition. And we show the high distortion characteristics of the flow and energy fields at the low flow condition by using subchannel analysis code. Through the core flow analysis at low flow condition, it is verified that subchannel code THALES can predict reasonable results despite of the extreme condition and that the distortion of the flow and energy field is huge.