Reliable surface thermometry of special nuclear material (SNM) containers is essential for their long-term monitoring. For example, an excessively hot container indicates higher than anticipated nuclear activity, which would prompt a number of interventions that may include re-canning of the material or a change in the storage environment. Additionally, thermometry can be used to inform thermal and corrosion models and assist in risk assessment activities. Traditionally used surface thermometry techniques such as infrared thermometers and sprung loaded thermocouples suffer from large uncertainties . For example infrared thermometers require prior knowledge of the surface emissivity and an understanding of background thermal radiation; whilst sprung loaded thermocouples are prone to large errors due to their perturbative nature when placed on the surface.
As opposed to the currently used approaches phosphor thermometry offers the potential for long-term, low uncertainty surface thermometry of SNM containers.
Phosphor thermometry requires a thin strip of thermographic phosphor to be applied to the outside of the SNM containers, which can be interrogated periodically (either in-situ or ex-store), by optical means. The phosphorescent properties of the phosphor, during or immediately after excitation (with blue or UV light) can be used to determine the temperature with a low uncertainty that does not depend on the emissivity of the surface or the presence of background thermal radiation and no physical contact with the surface is required. Since phosphor thermometry has not been used in this environment before, NPL undertook a programme of work to assess its suitability. This included the development of a phosphor/binder compatible with the can construction material, calibration and testing of a phosphor thermometer prototype instrument, and assessment of any degradation in the performance of the coating following radiation exposure and how this will effect the temperature measurements.
NPL demonstrated that phosphor thermometry can provide surface temperature measurements over the required range from 10 °C to 200 °C with a standard uncertainty of less than 1 °C, which is 5 to 10 times less than previous approaches. Gamma and neutron radiation exposure tests of the phosphor coated samples, equivalent to up to 150 years exposure, did not show any change in the performance of the coating as a thermometer. Work is now underway to develop a practical phosphor thermometer that can be deployed and tested in, first a non-radioactive test rig at Sellafield, and later rolled out to active stores. Work to address the long-term objective of in-situ monitoring of a new storage container for SNM material, the so-called 100 year can, is currently ongoing at NPL/Sellafield. This innovative storage system will allow for the on-going compliance, safety and futureproofing of SNM containers.