Probabilistic method for determining optimal ROP detector locations in the CANDU-6 reactor

In order to maximize the regional overpower protection (ROP) trip margin under any condition in the CANada Deuterium Uranium (CANDU) reactors, several methods have been devised and applied at the sites, such as steam generator cleaning and adjustor rod lock-in. However, the operating margins obtained from these techniques are calculated based on the current locations of the fixed in-core ROP detectors. There is a possibility for increasing the trip margin if insignificant ROP detectors are removed and new critical detectors are added at different positions that do not conflict with the current locations. In fact, AECL had proposed such a deterministic detector layout optimization (DLO) technique to minimize the number of ROP detectors in 1998. However, this deterministic approach could not propose the best or optimized three-channel solution for each shutdown system, which has the maximum trip setpoint. Recently, in order to overcome the defect of the current DLO method, KEPRI adopted a probabilistic approach to determine the ROP detector location and incorporated it in the ROP design code, ROVER-K. To verify the applicability of the new method, the optimal ROP shutdown system was obtained for an initial and aged core condition, respectively, based on only the current 58 detector locations. The results show that the total number of ROP detectors decreases from 58 to 46 or 49 and the small TSP gain is obtained simultaneously based on the re-arranged best three safety channels. Therefore, the new method can be used to select the best ROP detector locations and also to estimate the optimal ROP TSP for an aged CANDU reactor to recover the operation margin.