Improving High-Temperature Measurements in Nuclear Reactors with Mo/Nb Thermocouples

Abstract

Many irradiation experiments performed in research reactors are used to assess the effects of nuclear radiations on material or fuel sample properties, and are therefore a crucial stage in most qualification and innovation studies regarding nuclear technologies. However, monitoring these experiments requires accurate and reliable instrumentation. Among all measurement systems implemented in irradiation devices, temperature-and more particularly high-temperature (above 1000 • C)-is a major parameter for future experiments related, for example, to the Generation IV International Forum (GIF) Program or the International Thermonuclear Experimental Reactor (ITER) Project. In this context, the French Commissariat à l'Energie Atomique (CEA) develops and qualifies innovative in-pile instrumentation for its irradiation experiments in current and future research reactors. Logically, a significant part of these research and development programs concerns the improvement of in-pile hightemperature measurements. This article describes the development and qualification of innovative high-temperature thermocouples specifically designed for in-pile applications. This key study has been achieved with technical contributions from the Thermocoax Company. This new kind of thermocouple is based on molybdenum and niobium thermoelements, which remain nearly unchanged by thermal neutron flux even under harsh nuclear environments, whereas typical high-temperature thermocouples such as Type C or Type S are altered by significant drifts caused by material transmutations under the same conditions. This improvement has a significant impact on the temperature measurement capabilities for future irradiation experiments. Details of the successive stages of this development are given, including the results of prototype qualification tests and the manufacturing process.

Metrology for New Generation Nuclear Power Plants–MetroFission

“…These thermocouples are showing negligible neutronic drift (low neutron activation) [7]. The thermocouples were identified, constructed, and studied with respect to their reference functions, stability, and thermoelectric homogeneity.…”

MetroFission: New high-temperature references and sensors for the nuclear industry

“…The overall objective of this task is to investigate the metrological characteristics of these sensors and the possible means for enhancing their stability and ensuring their traceability to the ITS-90. Thermocoax (France), one of the world leading suppliers of thermocouples, has recently collaborated with CEA (Commissariat à l'énergie atomique et aux aux énergies alternatives, France) to develop and characterise specific Mo/Nb thermocouples with dopants aimed at the stabilisation of the Seebeck coefficient of the wires [8]. This research led to a promising version of Mo/Nb thermocouples which is currently under study in the frame of this task.…”

Advanced Measuring (Instrumentation) Methods for Nuclear Installations: A Review

“…The design of such a small bulk detector has been focused on good background rejection and high detection efficiency. The advanced qualification of innovative high-temperature thermocouples specifically for in-pile applications was designed by Villard et al [9]. This new kind of thermocouple was based on niobium thermoelements and molybdenum, which persisted nearly unchanged by thermal neutron flux even though under harsh nuclear environments, whereas Type C or Type S as typical high-temperature thermocouples is changed by significant drifts caused by material deformations under the same conditions.…”