Range Extension From -7.0 to -7.2 Decades for Flux Monitoring Using Reactor Regulating System (RRS) Ion Chambers at Bruce Power

Abstract

In CANDU reactors, neutron flux must be monitored at all power levels. Operation of the permanent ion chamber systems at low power is limited by background gamma radiation which causes a small current to flow in the ion chamber. This residual current becomes a significant portion of the total current during outages.The low end of the ion chamber range was originally -7.0 decades (10^-7 full power) when the current due to neutrons was estimated to equal the residual current. Start-up instrumentation was required at this point.A range extension to -7.2 decades has been supported for Bruce Power RRS ion chambers by showing from outage and approach to critical data and theoretical calculations that the neutron component of the current and residual current become equal at about -7.4 decades such that SUI installation may now be avoided for many outages with significant reduction in dose to maintenance staff.