The measurement of sedimentary source terms (SSTs) in the primary circuit of a pressurized water reactor is important for radiological protection and collective radiation exposure control. In situ gamma spectrometry technology is the uniquely suitable nondestructive method for SST measurement, which gains strong interest from staff members of nuclear power plants. In this work, we provide a comprehensive introduction and discussion on in situ gamma spectrometry technology, including the basic principles, the development status, and the key technologies, with the aim to provide reference and guidance for future research.

Specifically, the key technologies of in situ gamma spectrometry technology, including the detector, collimator, and efficiency calibration method, are analyzed and discussed. The detector is generally based on a high-purity germanium (HPGe) or CdZnTe (CZT) detector with the ability to recognize different nuclides from in situ gamma spectra. Variable apertures are recommended for the collimator adapting to different dose rates. The combined source-less efficiency calibration method, which combines Monte Carlo simulations and numerical integration, is the most suitable for in situ measurement of SSTs.

The existing problems and future trends are also analyzed and discussed. CZT detectors with large-size crystals should be developed to increase the detection efficiency for high-energy gamma rays. The in situ measurement system should be operating automatically to reduce the workload and exposure of the plant staff. The accuracy of the calibration remains a problem in practical applications. Attentions should be paid to the accuracy of the field condition modeling.

In addition to SSTs surveys during shutdown, in situ gamma spectrometry technology can be used for long-term online monitoring of SSTs, which is very helpful for the study of SST control. The problem is how to distinguish the SSTs from the source terms in the coolant, which is quite challenging and needs more study.