Scram speed assessment for transient analysis in Chinshan BWR-4 Plant

Control rods perform very important function during normal operation including anticipated operational occurrences. By controlling reactivity changes, the control rods must ensure that the fuel design limits are not exceeded. For the Chinshan BWR-4 units in Taiwan, the Improved Technical Specification (ITS) of this plant specifies the Surveillance Requirement of Control Rod Scram Times, which may be too conservative for safety analysis. This paper provides a method to determine an analytical scram speed used in transient analysis. It can be used to establish the Core Operating Limits Report limits which ensure the fuel mechanical integrity during the most limiting anticipated transients. In addition, a method is proposed to generate the equivalent scram reactivity by tuning a slower insertion rate without considering the control rods configuration. Based on the proposed method, the bounding and analytical scram speed are determined. A benchmark analysis for the typical pressurization transient is performed with the different scram speeds. The delta Critical Power Ratio (ΔCPR) is increased by 12% compared with the case using the Technical Specification scram speed. Therefore, this analytical scram speed can be used in reload analyses to obtain conservative results.