Examination of dynamic response of detectors for criticality accident alarm systems at Tokai Reprocessing Plant

The Tokai reprocessing plant is so designed as to prevent the occurrence of any criticality accident during the operation including the handling, storage and transport of fissile materials in the plant. High reliable alarm systems against criticality accidents are urgently necessary to enable workers to evacuate immediately from the plant in a rare accident. The present systems were manufactured by SEIN Corp. in France, and the quality was guaranteed by CEA. The installation and maintenance of the systems have been performed by Toshiba Corp. The systems consist of 12 detectors which can detect gamma or neutron radiation resulting from critical excursion, a 623 BJ data processing module, an evacuation alarm sender and the warning equipment of 65 flash light boxes and 18 horn blowers. One area is monitored with a set of three criticality detectors of the same type, and alarm sounds when two out of three detectors detect the signals exceeding the threshold limit within the coincidence time of 500 msec. The configuration of gamma detectors, the test on dose rate linearity, alarm output characteristics and in-pile performance, and the results are reported. The time constant of the detectors was determined, and the detectors satisfactorily worked in the exposure test. (Kako, I.)