Determining the irradiation temperature of witness samples of VVER-1000 AND -440 reactor vessel steel

The most convenient methods for measuring the temperature in nuclear reactors are enumerated in this paper, specifically thermocouples, fusible metal inserts, and diamond probes. Their advantages and disadvantages are listed and data are presented on their use for measuring the irradiation temperatures of containers with witness samples of steel in VVER-1000 and -440 reactor vessels. It is found that the sample temperature in these assemblies in the VVER-1000 is 300 ± 2°C, as measured by fusible inserts, while thermocouples in chains in the VVER-440 indicated 270°C. The advantage of the method employing diamond probes is their small size, but the range of possible direct measurements for this technique is restricted to a maximum neutron fluence of 1018 cm−2 (for neutron energies above 0.5 MeV).