Criticality alarm system in Rokkasho reprocessing plant

Criticality Alarm System (CAS) in Rokkasho Reprocessing Plant (RRP) is designed with consideration to the delayed criticality in addition to the prompt criticality. This CAS is designed to have 3 detectors and to judge by 2 out of 3 logic whether the trip signals from these 3 detectors show the criticality or not, so that it might prevent any false alarm and function loss. In RRP, the equipments to be watched by the CAS are inside of thick shields, there is difference of effective thickness of the shields. So, in case of a delayed criticality which has not so many fission rate but long doubling time, the exposure rate around the detectors is low and risen slowly, so that the threshold should be low but duration for simultaneity judgement of 3 detectors should not be so long, because there is the sufficient difference of duration until each detector activates. Therefore, in RRP, the detectors of the CAS for the delayed criticality mentioned above is designed to have lower possibility of a false alarm by reduction of noise suitably by means of exclusion of the narrow pulse regarded as a noise pulse so as to have high reliability.