Development and assessment of the code for reactor outages thermal hydraulic and safety analysis. Midloop operation with loss of RHR

Although only a few percent of residual power is remained during plant outages, the associated risk to experience core uncovery and correspondent fuel overheating has been identified to be relatively high, particularly during the midloop operation (MLO) in PWR. However, the current available tools to analyze the system behavior, such as RELAP5, RETRAN, etc., can not easily perform the right job. A medium-sized program aiming at the reactor outages simulation and evaluation, such as MLO with loss of PHR, has been developed. All important thermal hydraulic processes involved during MLO with loss of RHR will be properly simulated by this newly developed ROSE code. Instead of using classical nodal momentum approach, two-region approach with modified two-fluid and piston models will be the theoretical basis of the new program to analyze the NSSS during plant outages. To verify the analytical model in the first step, posttest calculation against the integral midloop experiments with loss of RHR were performed. The paper also demonstrates the excellent simulation capacity of the ROSE code against IIST test data. (author)