Temperature monitoring using fibre optic sensors in a lead-bismuth eutectic cooled nuclear fuel assembly

• We demonstrate the use of optical fibre sensors in lead-bismuth cooled installations.
• In this first of a kind experiment, we focus on temperature measurements of fuel rods
• We acquire the surface temperature with a resolution of 30 mK.
• We asses the condition of the installation during different steps of the operation.

In-core temperature measurements are crucial to assess the condition of nuclear reactor components. The sensors that measure temperature must respond adequately in order, for example, to actuate safety systems that will mitigate the consequences of an undesired temperature excursion and to prevent component failure. This issue is exacerbated in new reactor designs that use liquid metals, such as for example a molten lead-bismuth eutectic, as coolant. Unlike water cooled reactors that need to operate at high pressure to raise the boiling point of water, liquid metal cooled reactors can operate at high temperatures whilst keeping the pressure at lower levels. In this paper we demonstrate the use of optical fibre sensors to measure the temperature distribution in a lead-bismuth eutectic cooled installation and we derive functional input e.g. the temperature control system or other systems that rely on accurate temperature actuation. This first-of-a-kind experiment demonstrates the potential of optical fibre based instrumentation in these environments. We focus on measuring the surface temperature of the individual fuel rods in the fuel assembly, but the technique can also be applied to other components or sections of the installation. We show that these surface temperatures can be experimentally measured with limited intervention on the fuel pin owing to the small geometry and fundamental properties of the optical fibres. The unique properties of the fibre sensors allowed acquiring the surface temperatures with a resolution of 30 mK. With these sensors, we assess the condition of the test section containing the fuel assembly during different steps of the operation of the facility, including the heating and verification of the vacuum of the loop as well as the filling and draining of the LBE loop. We also identify a simulated electrical shut-down and heating circuit failure in the primed installation.