Traditional devices for measuring reactor flux and power are grouped into two categories: excore instrumentation and incore instrumentation. There are two popular radiation measuring devices: fission detectors and ion chambers. These devices have measured reactor flux and power reliably over the last 40 years.

Ion chambers, located outside the core (excore), can detect thermal neutron flux that is directly proportional to the fission rate and reactor power. Fission chamber flux detection devices, on the other hand, have a wider range than ion chambers and are more sensitive to neutrons. Both devices are typically used in traditional large PWRs. The same technology is being considered for iPWRs.10

iPWRs, however, may be able to take advantage of recent size reductions in fission chambers, and recent accuracy improvements. For example, micro-pocket fission detectors have been fabricated and tested as incore flux monitors in the 250 KW TRIGA reactor at Kansas State University. These micro-fission detectors have shown high performance with radiation hardness to neutrons, gamma rays, and charged radiation products.11

Additionally, another emerging technology in the nuclear power monitoring field is gamma thermometers. Although approved for US nuclear use as a local range power monitor since 1982, they have not been deployed widely in US PWRs. Based on the principle of temperature difference in a thermocouple type junction which is proportional to incident gamma flux, gamma thermometers may provide an alternative option for power monitoring in the future, offering possible improvements over traditional methods in incore time response, calibration, and size.12

Although iPWRs provide the opportunity for the use of new technologies for power and flux measurement, the incorporation of new technologies will require increased development time to design and qualify new devices for this important measurement.