The Availability of Neutron Channels and Power Range Monitoring Systems with In Situ Detection of Channel Degradation

Abstract

The availability of power range monitoring systems (PRMSs) is important to reliable and safe operation of nuclear plants, since the primary functions of PRMSs are to provide control signals and generate a trip signal if the neutron flux level exceeds preset values during operation. The PRMS can be inspected for degraded modes of neutron channel failure with conventional methods during the time the plant is shut down. Recently, techniques have been developed for in situ inspection of neutron flux channels. The effect of in situ surveillance of PRMS channels on the channel and system availability is investigated as a function of the probability of detecting the degraded channels and the frequency of inspection. The PRMS and its subsystems are modeled as M-out-of-N systems with identical and statistically independent three-state units. It is shown that the single channel unavailability can be appreciably decreased (4 to 10 day/yr) using in situ surveillance techniques. The improvement in PRMS availability in pressurized water reactors, however, is predicted to be small (<1.5 h/yr) because of channel redundancy. The effect of these techniques on PRMS availability in boiling water reactors is virtually unobservable.