Detection of faulty fuel elements may be achieved by receipt of high count alarms or high differential counts. Once any of these signals are received, the rate-of-rise of count rate must be investigated from the analysis of previous readings.

The level of the signal and the rate at which the signal is rising dictates the type of action that the op­erator takes. This varies from placing the channel on single sampling, where the count rate doubles in about 24 hours to immediate reactor trip where the count rate is rising and doubling at a rate of one hour or less, or the count exceeds an absolute limit set usually by the Operating Rules.

The absolute high limits and the maximum rate — of-rise of counts is specified for each BCD system and related to the course of action that the operator must take.

Each course of action for an identified abnormal set of readings on the BCD system will be contained in the Plant Operating Instructions.

In normal operation, frequent checks are carried out on the equipment to ascertain that it is perform­ing correctly and, at less frequent intervals, to ensure its sensitivity to fuel leaks is adequate. The latter may be done by using purposely-contaminated fuel or by the use of probes contaminated to give a measure of sensitivity.