Analysis of Flow Streaming and Ex-Core Detector Shadowing Effects Due to Mechanical Shim Operation for AP1000 Nuclear Power Plants

AP1000 nuclear power plants use Mechanical Shim（MSHIM） operation. At present, few studies have been done on impacts to ex-core operation parameters from MSHIM operation. Control rod insertion and cold leg temperature reduction processes were simulated, and flow streaming and ex-core detector shadowing effects due to MSHIM operation for AP1000 plants were analyzed in this paper. The results show that: M banks can obviously influence the flow streaming and ex-core detector shadowing effects, while AO bank has negligible influence to these effects; cold leg temperature reduction results in obvious ex-core detector shadowing effect which is insensitive to the control rod position. This paper proposed that following control rod positions should be included in related startup tests: 130, 14,-66,-116,-296 and-436 steps. During full power operation, tests at-436 steps could be executed at 130 steps to avoid a low power margin.