physical properties of highly active liquor containing molybdate solids

Available online at www.sciencedirect.com ScienceDirect Procedia Chemistry 21 (2016) 24 – 31 5th International ATALANTE Conference on Nuclear Chemistry for Sustainable Fuel Cycles Physical properties of highly active liquor containing molybdate solids Barbara Dunnetta*, Tracy Warda, Rachel Robertsa, Jonathan Cheesewrightb a National Nuclear Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, U.K b Sellafield Ltd, Sellafield, Seascale, Cumbria, CA20 1PG, U.K Abstract The reprocessing of irradiated nuclear fuel at Sellafield produces a nitric acid based Highly Active Liquor (HAL) waste The liquor, containing fission products and process additives, is concentrated in an evaporator in order to reduce the volume and is then stored in Highly Active Storage Tanks (HASTs) prior to vitrification Caesium phosphomolybdate (CPM) is precipitated during the evaporation process and can convert to zirconium molybdate (ZM) during storage During Post Operational Clean Out (POCO) of the HASTs, it is expected that their highly active content will be reduced by repeated cycles of washing using nitric acid and other reagents Initial washings are likely to have a chemical composition comparable to concentrated HAL, becoming more dilute during the wash-out process It is expected that the wash-out process will also recover significant quantities of molybdate solids (ZM, CPM or a mixture) from the HASTs In order to determine the processing challenges from such washings during POCO, the physical properties of varying concentrations of non-active HAL simulants containing molybdate solids have recently been measured by the UK’s National Nuclear Laboratory The following measurements are presented and discussed: • Particle size distribution • Density • Settling behaviour of solids • Voidage of settled sediment beds • Viscosity • Yield stress • Influence of ZM morphology on physical properties ©©2016 Published by Elsevier B.V.B.V This is an open access article under the CC BY-NC-ND license 2016The TheAuthors Authors Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of ATALANTE 2016 Peer-review under responsibility of the organizing committee of ATALANTE 2016 * Corresponding author Tel.: +44-19497-79325; fax: +44-19467-79007 E-mail address: barbara.f.dunnett@nnl.co.uk 1876-6196 © 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of ATALANTE 2016 doi:10.1016/j.proche.2016.10.004 Barbara Dunnett et al / Procedia Chemistry 21 (2016) 24 – 31 Keywords: Properties; highly active liquor; caesium phosphomolybdate; zirconium molybdate Introduction At Sellafield, in the UK, the Magnox reprocessing plant reprocesses fuel from the Magnox reactors while the more recent thermal oxide reprocessing plant (THORP) is dedicated to reprocessing oxide fuel from the light water reactors (LWR) and advanced gas-cooled reactors (AGR)1 The reprocessing plants chemically separate the uranium and plutonium from the spent fuel for re-use The waste arising from reprocessing includes a nitric acid based Highly Active Liquor (HAL) waste, with that originating from the Magnox reprocessing plant referred to as ‘Magnox’ and that from THORP referred to as ‘Oxide’ in this paper This liquor, containing fission products and process additives, is concentrated in an evaporator in order to reduce the volume prior to storage in Highly Active Storage Tanks (HASTs) before being vitrified which yields a stable and durable waste form suitable for long-term storage and subsequent disposal.2-5 Caesium phosphomolybdate (CPM) is precipitated as a solid phase during the evaporation process and can convert to zirconium molybdate (ZM) during storage An installed agitation system, comprising of air lifts and jet ballasts, is present in each HAST5-8 This is used to re-suspend any solids which have settled to the base of the tank and keep them in suspension The efficiency of the agitation system is dependent on the volume of liquor in the HAST with its efficiency decreasing as the tank content is decreased Hence on final emptying of the tanks, solids are expected to remain, along with a small volume of HAL Post Operation Clean Out (POCO) of the HASTs will gradually reduce the Highly Active (HA) contents remaining in the HAST at the end of their operational life It is currently proposed that this will be carried out by repeated cycles of adding volumes of nitric acid or another reagent, agitation using installed systems, then discharge of part of the HAST contents (the effluent being termed as ‘flushings’) Initial flushings will be slightly diluted HAL, becoming more dilute as the HAST is washed out It is expected that the wash-out process will recover ZM and unconverted CPM The flushings will subsequently be concentrated in an HA evaporator prior to vitrification The presence of significant quantities of solids in the flushings from POCO of the HASTs may lead to processing challenges such as difficulties in mobilization of the solids using the installed agitation systems and subsequent transfer of the flushings to the evaporator and vitrification plant Hence a range of physical properties of CPM and ZM, and flushings containing molybdate solids (i.e ZM, CPM and ZM:CPM mixtures) have been measured, the results of which are presented and discussed Non-active CPM and ZM solids9 were used and the flushings were represented either by nitric acid or nitric acid based HAL simulated using non-active isotopes, omitting those at insignificant molar concentration Fig SEM images of (a) CPM and (b) ZM with cubic morphology 25 26 Barbara Dunnett et al / Procedia Chemistry 21 (2016) 24 – 31 Particle size distribution of CPM and ZM CPM (Cs3PMo12O40.14H2O) crystals nucleate in solution during HA evaporation very rapidly at a very small molecular cluster size (