The temperature of the special nuclear material (SNM) containers at Sellafield Ltd, a large multi-function nuclear site on the coast of Cumbria, England, is a key indicator of the state of their health. Temperature data is used to provide early warning of overheating and alert to potential container degradation. The temperature of each container is assessed regularly by a robotic ’port profiler’. This device moves along the row of containers, using an infrared thermometer and a spring-loaded thermocouple to perform temperature measurements at various positions along the outer surface of the container. These measurements are an integral part of monitoring and decision-making for the containers in the store, but their accuracy had not been completely evaluated.
Sellafield Ltd approached NPL to perform an end-to-end uncertainty analysis for both thermometry methods. This included rigorous assessment of the uncertainty of every part of the temperature measurement chain, from the transducer through to the readout device. To determine the individual uncertainty contributions involved, both theoretical calculations, such as modelling the effect of reflected thermal radiation, and experimental measurements, such as studying the effect of thermocouple contact resistance on measured temperature, were used. The uncertainty of the temperature measurement was assessed in line with the widely used techniques given by the ISO JCGM 100:2008 Guide to the Expression of Uncertainty in Measurement.
NPL’s rigorous assessment of measurement uncertainties enabled Sellafield Ltd to identify which contributions to the uncertainty were the most impactful. This allowed NPL to devise further investigations to reduce the uncertainty of temperature measurement of the containers, in particular, to begin investigating phosphor thermometry as a low uncertainty alternative approach. Conversely, it also showed which contributions were unimportant and did not warrant the resources for further improvements. The work also provided a framework for the rigorous assessment of the overall temperature measurement uncertainty of the SNM container thermometry. This will facilitate decision-making going forward, maintain the integrity of the SNM containers and ultimately improve the safety of the nuclear material stores.