Long Term out-of-pile Thermocouple Tests in Conditions Representative for Nuclear Gas-cooled High Temperature Reactors

Abstract

During irradiation tests at high temperature failure of commercial Inconel 600 sheathed thermocouples is commonly encountered. As instrumentation, in particular thermocouples are considered safety-relevant both for irradiation tests and for commercial reactors, JRC and THERMOCOAX joined forces to solve this issue by performing out-of-pile tests with thermocouples mimicking the environment encountered by the HTR in-core instrumentation. The objective was to screen innovative sheathed thermocouples which would consecutively be tested under irradiation. Two such screening tests have been performed in high temperature environment (i.e. temperature in the range 1100 - 11500C) with purposely contaminated helium atmosphere (mainly CH4, CO, CO2, O2 impurities) representative for High Temperature Reactor carburizing atmospheres. The first set of thermocouples embedded in graphite (mainly conventional N type thermocouples and thermocouples with innovative sheaths) was tested in a dedicated furnace at THERMOCOAX lab with helium flushing. The second out-of-pile test in Petten with a partly different set of thermocouples replicated the original test for comparison. Performance indicators such as thermocouples thermal drift, insulation resistance measurements and loop resistance measurements were monitored. Through these long-term screening tests several effects were investigated: niobium sleeves, bending, diameter, sheath composition as well as the chemical environment. SEM examinations were performed to analyze local damage (bending zone, sheath). The present paper describes the two experimentations, sums up data collected during these tests in terms of thermocouple behavior and describes further work, in particular to select suitable thermocouples equipped with fixed point mini cells for qualification under irradiation.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen