The effect of increased CRA trip insertion times for TMI

In recent years, testing of control rod assembly (CRA) drop times at TMI has resulted in a few rods that have failed to meet the Technical Specification (TS) acceptance criteria of 1.66 seconds to 3/4 inserted. Crud deposition was determined to be the cause of the slow rod insertion times. Corrective actions included increasing lithium concentration and increasing the frequency and extent of exercising the control rod drive mechanisms. However, after one cycle of operation, it was determined that these measures were not fully successful in retarding the crud buildup. Consequently, the safety significance of rods potentially having a longer drop time than the TS limit was evaluated. The analyses in Chapter 14 of the TMI FSAR demonstrate the ability of the plant to mitigate the consequences of postulated accidents without undue hazard to the health and safety of the public. To determine the safety consequences of the longer rod drop times, a reanalysis of some limiting accidents had to be done using the RETRAN, RELAP5 and TRAC computer codes. The safety evaluation concluded that a 3.0 second rod drop time would be acceptable because all of the event acceptance criteria were met. A permanent resolution of the problem is the replacement of the existing thermal barriers with new open flow path thermal barriers. Thermal barriers on half the CRAs at TMI have been replaced to date