Phosphor thermometry for nuclear decommissioning and waste storage

Reliable surface thermometry of stored nuclear waste containers is essential for both health monitoring and corrosion modelling. In this paper we investigate the feasibility of using phosphor thermometry for long-term temperature monitoring of nuclear waste containers and storage racks.

Nuclear Engineering and Design 375 (2021) 111091 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Phosphor thermometry for nuclear decommissioning and waste storage Alberto Sposito a, Edward Heaps a, Gavin Sutton a, *, Graham Machin a, Robert Bernard b, Sandra Clarke b a b National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom Sellafield Ltd., Seascale, Cumbria CA20 1PG, United Kingdom A R T I C L E I N F O A B S T R A C T Keywords: Phosphor thermometry Nuclear waste storage Reliable surface thermometry of stored nuclear waste containers is essential for both health monitoring and corrosion modelling In this paper we investigate the feasibility of using phosphor thermometry for long-term temperature monitoring of nuclear waste containers and storage racks Two strands of research were conduct­ ed Firstly, two thermographic phosphors (ruby and manganese-doped magnesium fluorogermanate [MFG]), mixed with six different binders were coated onto stainless steel (316L) substrates and exposed for one month to gamma radiation (γ-ray) and sodium hydroxide (NaOH) solution Secondly, a hybrid fibre-optic phosphor thermometer, capable of measuring these samples, was constructed and tested The instrument can measure temperatures using both the luminescence decay time and intensity ratio techniques Following γ-ray and NaOH exposure, no significant degradation in phosphor appearance or performance was observed The MFG/silicone binder combination gave the best results The hybrid phosphor thermometer demonstrated for MFG/silicone binder that: • the intensity ratio technique gave the lowest measurement standard deviation (σ(T) < 0.1 ◦ C) but suffered minor drift with thermal cycling • the decay time technique (650 nm emission) did not show any drift and was capable of a mea­ surement standard deviation of σ(T) ≤ 0.3 ◦ C Introduction Reliable measurement of the surface temperature of stored nuclear waste containers is crucial for health (integrity) monitoring and corro­ sion modelling Currently temperature monitoring is performed with infrared thermometers and spring-loaded thermocouples However, both approaches could have large uncertainties, due to a) unknown surface emissivity and b) background radiation in the former case, c) heat-flux and contact uncertainties in the latter: Unknown surface emissivity: to convert a measured thermal radiance to temperature requires accurate knowledge of the surface emissivity – the emissive power of the surface relative to that of an ideal blackbody (0 < ε < 1) Infrared thermometers are first calibrated against a blackbody standard (with an ε very close to 1) However once used on a practical surface such as a steel storage container, the emissivity will be significantly