Abstract— During irradiation tests at high temperature, failure of commercial Inconel 600 sheathed thermocouples is commonly encountered. To understand and remediate this problem, out-of-pile tests were performed with thermocouples in carburizing atmospheres which can be assumed to be at least locally representative for High Temperature Reactors. The objective was to screen those thermocouples which would consecutively be used under irradiation. Two such screening tests have been performed with a set of thermocouples embedded in graphite (mainly conventional Type N thermocouples and thermocouples with innovative sheaths) in a dedicated furnace with helium flushing. Performance indicators such as thermal drift, insulation and loop resistance were monitored and compared to those from conventional Type N thermocouples. Several parameters were investigated: niobium sleeves, bending, thickness, sheath composition, temperature as well as the chemical environment. After the tests, Scanning Electron Microscopy (SEM) examinations were performed to analyze possible local damage in wires and in the sheath. The present paper describes the two experiments, summarizes results and outlines further work, in particular to further analyze the findings and to select suitable thermocouples for qualification under irradiation I. INTRODUCTION HE main objective of High Temperature Reactor (HTR) fuel irradiation tests in the High Flux Reactor Petten is to check the feasibility of low coated fuel particle failure at high temperatures and high burn-up [1][2][4]. To qualify this fuel type, the irradiation campaigns (typically > 2 years at high temperature exposure) must demonstrate effective fission product retention under normal and off-normal operating conditions. Manuscript submitted May 20, 2011. M. Laurie is with the European Commission, Joint Research Centre, Institute for Energy, P.O. Box 2, NL-1755 ZG Petten (telephone: +31 224 565062, e- mail: mathias.laurie@ec.europa.eu). S. Fourrez is with THERMOCOAX SAS, BP 26, Planquivon, F-61438 Flers Cedex, FR (telephone: +33 233 628119, e-mail: stephane.fourrez@thermocoax.com). M. A. Fütterer is with the European Commission, Joint Research Centre, Institute for Energy, P.O. Box 2, NL-1755 ZG Petten (telephone: +31 224 565158, e-mail: michael.fuetterer@jrc.nl). J. M. Lapetite is with the European Commission, Joint Research Centre, Institute for Energy, P.O. Box 2, NL-1755 ZG Petten (telephone: +31 224 565159, e-mail: jean-marc.lapetite@ec.europa.eu). In a pebble irradiation rig, in-pile instrumentation including thermocouples, gamma scan wires and neutron fluence detector sets has to find place in this narrow environment. In such a rig, graphite structures are employed to hold the fuel in place and to conduct the produced heat radially to the outside. Real reactors use large amounts of graphite as moderator and reflector material. The reaction of traces of water, trapped in the graphite or imported by the purge gas, with the carbonaceous material in the fuel element may lead to appearance of reaction products such as CO, CO 2 , CH 4 and H 2 in the irradiation rig. Humidity is low in this environment, but impurities (5 – 10 vppm) in the purge gas (a He/Ne mix) may lead to reactions with the metallic sheath of the thermocouples. This chemical contamination may react with the built-in instrumentation and lead to severe corrosion or carburization with thermocouples in contact with graphite. Indeed, massive failures of Inconel 600 sheathed thermocouples have been encountered during the HFR-EU1 irradiation test. Measurements showed significant drops in insulation values on most thermocouples. In order to prevent this situation in the future, two out-of- pile tests were performed with a set of chosen thermocouples mimicking as much as possible the environment encountered by the in-core instrumentation. During these long-term screening tests in representative conditions, the effects of several parameters were investigated: niobium sleeves, bending, thickness, sheath composition, temperature as well as the chemical environment. The sets of thermocouples embedded in graphite were tested in a dedicated furnace with 2.4 l/h helium flushing under 0.15 MPa at 1373 K and 1423 K. The last part of the tests was performed with a purposely contaminated helium atmosphere representative of the atmosphere inside an irradiation rig with an initial content of moisture assumed from degassing of graphite. II. IN-PILE TESTS IN THE HFR A. Background The HFR is a multi-purpose research reactor which uses light water as coolant and moderator. The normal operating power is 45 MW. Special irradiation rigs have been developed and are regularly used for testing fuel samples under conditions which are representative of the current conditions of the power plant[2][3]. A typical HTR fuel irradiation test is Long Term out-of-pile Thermocouple Tests in Conditions Representative for Nuclear Gas-cooled High Temperature Reactors M. Laurie, S. Fourrez, M. A. Fütterer, J. M. Lapetite, T 978-1-4577-0927-2/11/$26.00 ©2011 IEEE