In CANDU reactors, three instrumentation systems are provided to measure reactor thermal neutron flux over the full power range of the reactor (Knoll, 2000). Start-up instrumentation covers the eight-decade range from 10-14 to 10-6 of full power; the ion chamber system extends from 10-7 to 1.5 of full power, and the in-core flux detector system provides accurate spatial measurement in the uppermost decade of power (10% to 120% of full power). The fuel channel temperature monitoring system is provided for channel flow verification and for power mapping validation. The self-powered in-core flux detectors are installed in flux detector assemblies to measure local flux in the regions associated with the liquid zone controllers. The flux mapping system uses vanadium detectors distributed throughout the core to provide point measurements of the flux. The fast, approximate estimate of reactor power is obtained by either taking the median ion chamber signal (at powers below 5% of full power) or the average of the in-core inconel flux detectors (above 15% of full power) or a mixture of both (5% to 15% of full power).

2. Several advanced power measuring and monitoring systems

The power range channels of nuclear reactors are linear, which cover only one decade, so they do not show any response during the startup and intermediate range of the reactor operation. So, there is no prior indication of the channels during startup and intermediate operating ranges in case of failure of the detectors or any other electronic fault in the channel. Some new reliable instrument channels for power measurement will be studied in this section.