The measurement of temperatures above 1000 °C is both difficult and yet vital for the success of a wide range of industrial processes, including nuclear fuel production and essential nuclear safety testing. In the nuclear sector, measurements can be required up to 2500 °C and the nuclear waste often needs long term, continual and reliable observation.
The harsh environment which is synonymous with the nuclear industry makes it difficult, if not impossible, to use traditional temperature sensors. Conventional sensors undergo mechanical and chemical changes which cause calibration drift and errors in measurement, and frequent replacement is not possible.
Using temperature sensors based on fundamental thermometry, like a Johnson noise thermometer, avoids this problem. The main development challenge is to extract the tiny Johnson noise signal from ambient electrical noise influences using compact electronics.
Monitoring of the UK’s nuclear waste inventory can be improved by periodic non-contact temperature measurements. Identification of ‘hot spots’, where high radioactivity exists is critical. Thermal imaging can remotely measure the surface temperature, provided the surface emissivity is known. This is often not the case. Imaging phosphor thermometry can be used to determine the surface temperature at a number of key locations independently of the emissivity. By combining thermal and phosphor imaging, it is possible to measure surface temperature accurately.
Practical Johnson noise thermometer
NPL’s joint project with Metrosol to develop a practical Johnson noise thermometer has now resulted in a working prototype which can measure temperature with an uncertainty of the order of ±1 °C in just a few seconds. Thus, these particularly harsh environments can now be monitored, while avoiding errors in measurement due to calibration drift. The system is being finalised and further trials are planned before entering the commercial market.
Fibre thermometry for high gamma environments
Conventional electrical sensors, such as thermocouples and resistance thermometers, are degraded when exposed to gamma or neutron radiation. Phosphor tipped fibre-optic (non-electrical) sensors have the potential to overcome these challenges. The temperature can be determined optically, where the phosphor is excited with light and the characteristics of its emission (intensity ratio or decay time) used to determine the temperature. However, optical fibres degrade in the nuclear environment. To address this, hollow core (photonic) fibres are used, where the reduction in light transmission with radiation damage is significantly less than traditional solid core fibres. Additionally, provided the fibre transmission remains constant during the measurement ( < 1 s), the phosphor derived temperature is independent of fibre attenuation. These sensors can potentially be used to monitor nuclear storage drums over an extended period of time, up to many years.
The temperature of nuclear waste containers indicates the level of both radio and chemical activity contained within them. This must be carefully monitored to ensure that control and management of the waste packages and storage facility is maintained. NPL is working to improve thermal observations such as relating thermal images of the container vent that would allow a confident estimate to be made of the internal container temperature using non-contact techniques.