Neutron irradiation and temperature effects on induced voltages in various mineral insulated cables and ceramic-coated wires

U-shaped single copper and stainless steel core mineral insulated cables (with outer diameter 1.0 and 1.5 mm), dual core mineral insulated cables (also with copper and stainless steel cores) and three types of ceramic-coated wires have been irradiated in the BR2 research reactor up to a maximum thermal neutron fluence of 1020 n/cm2. The cables were guided through a 25 cm long double wall tube to create significant temperature gradients during the irradiation or during reference measurements in zero radiation field using electrical heating. The induced core-to-core voltages, which were continuously measured, scale with the temperature gradients. For all single-core cables and for the wires they increased significantly during the irradiation, while for the dual core cables the thermoelectrical effects from the two legs of the core wires cancel almost completely. No significant effect of the core wire diameter was observed. The increase in thermoelectric sensitivity of the copper core cables was found to scale with the thermal neutron fluence and could be attributed unambiguously to partial transmutation from copper to nickel. The stainless steel core cables showed a fast sensitivity increase at low fluences, but saturating at a moderate level, depending on the local irradiation temperature.