Fission Chambers for CANDU® SDS Neutronic Trip Applications

Abstract

The two CANDU® shutdown systems (SDS) use the same type of neutronic instrumentation to monitor the thermal neutron flux and generate the fastest safety trip to terminate the chain reaction, the Log Rate trip. The Log Rate trips on both shutdown systems employ uncompensated ionization chambers. A proven neutronic instrumentation system, which can also be used for Log Rate trips, is also commercially available: the fission chamber based fluxmonitoring system. The fission chamber neutron flux-monitoring system is relatively more sensitive to neutrons (leading to a stronger signal), and wider-range (covering the start-up region as well). It is designed to function in high gamma radiation fields and does not need massive lead shielding. This paper discusses the fission chamber based flux monitoring system and provides a comparison with the ion chamber based flux-monitoring system to show that both neutron fluxmonitoring systems can perform similar functions. The comparison highlights the unique and salient features of both neutron flux-monitoring systems, with a view towards the option of extending its use for reactor start-ups.