Laboratory tests of a modified 3He detector for use with startup instrumentation

Boron trifluoride (BF3) detectors are currently used in all CANDU stations as startup instrumentation (SUI) detectors for monitoring neutron flux during extended outages and startups. Experience at some CANDU stations has shown that some models of BF3 detectors degrade quickly, even in moderate neutron and gamma fields. Degradation and life expectancy tests for five models of BF3 detectors from different manufacturers were performed at Chalk River Laboratories (CRL) to investigate the problem. The test results reveal that most BF3 detectors have low neutron and gamma durability, and some exhibit an undesirable time-dependent degradation followed by recovery. As a result of this finding, other detector options including a modified helium (3He) detector described herein were investigated. Modified 3He detectors were procured from an established supplier and were found to perform without degradation in neutron and gamma fields. (author)