Use of a Gain Stabilized NaI(T1) Gamma Ray Detection System to Determine the Power Shape in Light Water Reactor Cores

A data acquisition system which consists of a NaI(T1) detector, associated electronics and a disc-based minicomputer has been used to obtain accurate relative gamma ray intensities over a large range of count rates (up to ~250 kHz) from light water reactor fuels. The system uses a green light-emitting diode optically coupled to the photomultiplier face to insure gain stabilization over a variation of count rates and environmental conditions, and to determine empirically the pileup and dead-time losses. The energy resolution of NaI(T1) is adequate for this application because only the well-isolated 1.6 MeV gamma from La-140 is used to relate fission product activity to power. Measurements have been performed on irradiated fuel during refueling outages at both boiling and pressurized water reactor (BWR and PWR) nuclear power plants.