Low drift type N thermocouples for nuclear applications

Thermocouples are the most commonly used sensors for temperature measurement in nuclear reactors. They are crucial for the control of current nuclear reactors and for the development of GEN IV reactors. In nuclear applications thermocouples are strongly affected by intense neutron fluxes. As a result of the interaction with neutrons, the thermoelements of the thermocouples undergo transmutation, which produces a time dependent change in composition and, as a consequence, a time dependent drift of the thermocouple signal. Thermocouple drift can be very significant for in-pile temperature measurements and may render the temperature sensors unreliable after exposure to nuclear radiation for relatively short times compared to the life required for temperature sensors in nuclear applications. Previous experiences with type K thermocouples in nuclear reactors have shown that they are affected by neutron irradiation only to a limited extent. Similarly type N thermocouples are expected to be only slightly affected by neutron fluxes. Currently the use of Nickel based thermocouples is limited to temperatures lower than 1000°C due to drift related to phenomena other than nuclear irradiation. In this work, undertaken as part of the European project METROFISSION, the drift of type N thermocouples has been investigated in the temperature range 600-1300°C. The approach of this study is based on the attempt to separate the contributions of each thermoelement to drift. In order to identify the dominant thermoelement for drift, the contributions of both positive (NP) and negative (NN) thermoelements to the total drift of 3.2mm diameter MIMS thermocouples have been measured in each drift test using a pure Pt thermoelement as a reference. Conventional Inconel600 sheathed type N thermocouples have been compared with type N thermocouples sheathed in a new alloy. At temperatures higher than 1000°C conventional Inconel600 sheathed type N thermocouples can experience a drift of several degrees, while in the new sheath the drift of type N thermocouples is limited to about ±1.5°C. At the highest temperatures the negative thermoelement in conventional Inconel600 sheathed thermocouples produces the largest contribution to the total type N drift. Metallurgical analysis has shown that the composition of type N thermoelements changes with time due to contamination with elements transferred from the sheath: this phenomenon is less pronounced for type N thermoelements in the new sheath. This work demonstrates that the selection of appropriate alloys for the sheath can allow to use Nickel based thermocouples up to 1300°C for nuclear applications.