I've been involved in measurements of plant since I started as a young research officer in the CEGB in 1979. Most of my work has used high resolution gamma ray spectrometry as the tool of choice, despite the extra complication of cooling the business end. (See "Philosophy & Ramblings" for some of the reasons for using HRGS).
Things measured have been quite diverse, and include:
A long series of in-situ measurements in AGR primary coolant circuits covering over thirty years of operation.
Measurements in the primary circuits of PWRs (in Spain and Sizewell B) in dose rates of approaching 100 mSv/h
Measurements in fuel cooling ponds, most recently at Chapelcross, using low resolution spectrometry and dose rate probes.
Management of many measurements in Magnox decommissioning plant to determine inventories
The estimation of inventories usually involves determination of the
unscattered flux, use of a conceptual model of activity distribution,
and a radiation transport calculation to turn the results into Bq. The
conceptual model may be confirmed by separate measurements, which at
their simplest may be a dose rate measurement combined with a shielding
calculation incorporating build-up. Alternative methods may require
invasive measurements (such as coring) to determine an activity