A new methodology for improving nuclear instrument system intermediate range's reliability when elevating reactor power after an unplanned reactor shutdown - 14112

The purpose of this paper is to introduce a new methodology for more accurate calibration of intermediate range(IR) channels for ex-core detectors used in Westinghouse Electric Company (WEC) type of nuclear power plants when elevating reactor power after an unexpected reactor trip occurs. Although power range (PR) channels have been calibrated every three months, we only implemented one calibration for IR channels related safety interlocks, such as high flux shutdown and rod insertion stop under an existing method, nuclear instrument system pre-alignment procedure, developed by WEC prior to initial criticality of the reload cycle. In case increasing the reactor power after accidental reactor trip happened during any specific cycle, we may confront discrepancy problems between PR and IR because plants have no official procedure to adjust IR interlocks' current value except before the initial calibration. The main reason for this issue is that the plant doesn't do any action to enhance IR's linearity matched with PR as time goes by because WEC type of nuclear power plants depend on only PRs to monitor reactor power during power operation. We have studied a new methodology to solve the matter using available resources we can easily obtain within a plant sector. We have already known neutron flux and relative assembly power at the specific fuel assembly through INCORE Flux Mapping. By using the data, we are able to finally introduce a new method for IR calibration considering core depletion. When an unplanned reactor shutdown breaks out during a fuel cycle, following two types of data is required to apply the new method for adjusting IR interlocks. (1) Relative assembly power of peripheral assemblies located near IR detectors A. from the first INCORE Flux Mapping output after arriving at reactor full power B. from most recently performed INCORE Flux Mapping output before an untimely reactor shutdown occurred (2) The initial IR interlocks' current value on this fuel cycle We can get Relative Power Factor from data (1) and also calculate the final intermediate IR interlock current value by multiplying (1) by (2). To evaluate validity of this methodology, we have collected data for IR current values related to IR interlocks in plant digital computer system in case of unexpected reactor trips in several plants data and compare situations with and without new calibration method like case 1 and case 2 below. (3) Case 1: IR current values have been left not calibrated without any action. (4) Case 2: It assumed we calibrated IR current values according to the new method. We found that when we used the new calibration methodology, deviation between predicted values and measured values decreased than not to calibrate IR. Significant benefits for using the new calibration method are as follows: (5) By reducing imbalance between PR and IR and improving reliability of IR at low reactor power, we can operate and monitor the plant more accurately and safely. (6) The plant can prevent IR interlocks from functioning with unmatched with PRs when reactor power goes up and down after unforeseen reactor shutdown. (authors)