Characterisation of plutonium species in alkaline liquors sampled from a UK legacy nuclear fuel storage pond

Abstract

Within the UK there are a number of nuclear legacy fuel storage ponds and silos that contain substantial volumes of corroding spent Magnox fuel pieces. The fuel and wastes within these ponds, including highly radioactive sludges, must be retrieved and processed during decommissioning. Sludges and other intermediate level wastes will then be encapsulated in a wasteform suitable for storage and disposal, whilst residual activity must be removed from pond liquors and process effluents prior to any authorised discharges. Understanding the nature and behaviour of the radionuclides in the ponds, including any potential for activity transfer from solid to solution phases, is critical in the environmental clean up of these nuclear legacy facilities. Plutonium isotopes (with ²⁴¹Am) dominate the α-activity within these ponds. Herein, the Pu species in samples taken from a UK legacy fuel storage pond and downstream Holding Tank on the Sellafield site are shown to be predominantly associated with suspended solid phases. Analyses of the residual soluble Pu concentrations indicate differences in Pu solubility between different areas of the pond, which speciation studies suggest are related to differing Pu oxidation state distributions. The implication is that Pu redox chemistry varies across the pond and this controls Pu solubilities and, by implication, Pu behaviour during waste processing. Simple treatment methods to suppress soluble Pu-α are also suggested.