Low drift type N thermocouples in out-of-pile advanced gas reactor mock-up test: Metallurgical analysis

Abstract

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. As part of a collaboration between Idaho National Laboratory (INL) and the University of Cambridge a variety of Type N thermocouples have been exposed at INL in an Advanced Gas Reactor mock-up test at 1150°C for 2000 h, 1200°C for 2000 h, 1250°C for 200 h and 1300°C for 200 h, and later analysed metallurgically at the University of Cambridge.The use of electron microscopy allows to identify the metallurgical changes occurring in the thermocouples during high temperature exposure and correlate the time dependent thermocouple drift with the microscopic changes experienced by the thermoelements of different thermocouple designs.In this paper conventional Inconel 600 sheathed type N thermocouples and a type N using a customized sheath developed at the University of Cambridge have been investigated. The rationale for the superior performance of the type N using a customized sheath developed at the University of Cambridge is explained in comparison with the behavior of conventional type N Inconel600 sheathed thermocouples. I. THERMOCOUPLES FOR NUCLEAR APPLICATIONSHERMOCOUPLES at high temperatures experience drift, a time dependent departure of the thermocouple signal from the real temperature of the environment. Thermocouple drift introduces a time dependent error in the temperature reading of the sensor.Metallurgical modifications occurring along the length of the thermocouple are responsible for drift. In nuclear applications thermocouples experience not only high temperatures, responsible for temperature activated microstructural or compositional changes, but also neutron irradiation, which produces:Atomic displacements generate dislocation loops and voids into the irradiated materials.With sustained irradiation the dislocation loops density increases. Macroscopically the effects are equivalent to cold work: increased hardening, reduced te...

Nuclear instrumentation and measurement: a review based on the ANIMMA conferences

“…(INL) [112] has shown the superiority of the performance of the Cambridge design with respect to the standard construction of type N thermocouples: after 2060 h at 1157°C, the special sheath Cambridge thermocouples had drifted an average of only 4°C, and after an additional 2000 h at 1207°C, the total drift was about 15°C. The metallurgical analysis presented in [113] shows that, at 1300°C, Cr and Fe and also Mn and Al are transferred from the Inconel sheath to the Nicrosil and Nisil thermoelements, while the Cr contamination to Nisil (the most responsible for the drift) is much reduced with the Cambridge special sheath, and the Fe and Mn contaminations to Nisil have disappeared.…”