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

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 safetysystems that will mitigate the consequences of an undesired temperature excursion and to preventcomponent failure. This issue is exacerbated in new reactor designs that use liquid metals, such as forexample a molten lead-bismuth eutectic, as coolant. Unlike water cooled reactors that need to operateat high pressure to raise the boiling point of water, liquid metal cooled reactors can operate at hightemperatures whilst keeping the pressure at lower levels. In this paper we demonstrate the use of opticalfibre sensors to measure the temperature distribution in a lead-bismuth eutectic cooled installation andwe derive functional input e.g. the temperature control system or other systems that rely on accuratetemperature actuation. This first-of-a-kind experiment demonstrates the potential of optical fibre basedinstrumentation in these environments. We focus on measuring the surface temperature of the individualfuel rods in the fuel assembly, but the technique can also be applied to other components or sections ofthe installation. We show that these surface temperatures can be experimentally measured with limitedintervention 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 of30 mK. With these sensors, we assess the condition of the test section containing the fuel assembly duringdifferent steps of the operation of the facility, including the heating and verification of the vacuum of theloop as well as the filling and draining of the LBE loop. We also identify a simulated electrical shut-downand heating circuit failure in the primed installation.