The use of in situ gamma spectroscopy to study radionuclides contributing to dose rate at 540 MWe pressurised heavy water reactors

Tarapur Atomic Power Station Unit-3 and 4 are twin reactors of 540 MWe capacity each. Unit-4 and Unit-3 operated for about 1030 and 910 effective full power days (EFPD) respectively. With the reactor operation, radiation field on reactor system equipments mainly on PHT system, Moderator system and spent fuel transfer system increases due to deposition of fission and activation product. These dose rates significantly contribute to the external exposure and stations collective dose in maintenance activities during reactor outages. In situ gamma spectroscopy has been successfully used at TAPS 3 and 4 operating nuclear facility to identify the radionuclide contributing to the dose rates for incorporating the corrective measures to control these sources and limit the exposures to ALARA. In situ gamma spectroscopy offers advantages over the traditional method of extracting a representative sample, transporting it to a laboratory, and then preparing the sample for counting. Some samples are physically difficult to obtain (material inside pipes, tanks, strainers, filters, very radioactive samples like resin beads, pressurized cover gases, heavy water sample collection). Since in situ spectroscopy is a non-contact process, and since the sample doesn't need to be physically extracted, these problems are minimized. In situ spectroscopy can give near-instantaneous results, and therefore allow prompt decisions to be made while the equipment is in the field. The availability of nuclide-specific information rather than just gross count or dose-rate information can allow better decisions to be made by the plant Health Physicist and plant management to define the optimum amount of personnel protection for the job. Reliable knowledge of exactly what radio nuclides are present, where they are located, will allow the job to be planned better. This better knowledge should lead to a safer operation, lower dose, lower risk of things going wrong, lower cost, and a quicker finish. Today, the availability of Ge detectors with high resolution and low background makes the analysis of the spectrum much easier. High purity Ge, detectors now can be stored at room temperature, although they must be operated at low temperature. This paper highlights the use of In-situ Gamma spectroscopy using portable hand held Coaxial HP(Ge) detector with high quality digital Multi Channel Analyzer (MCA) to study the radio nuclides in the nuclear systems which are contributing to the station collective exposures after about 1050 EFPDs of reactor operation. (author)