Effect of Natural Organic Matter on Radionuclide Partitioning in Nuclear Fuel Storage Ponds - 17136

One of the nuclear fuel storage ponds at Sellafield is open to the air, and has contained a significant inventory of corroded Magnox fuel and sludge for several decades, with some fission products having been released into solution. The sludge is predominantly composed of brucite (Mg(OH)₂), a corrosion product from the magnesium-based fuel cladding, amongst other corrosion products resulting from the uranium metal fuel. Sr-90 is known to constitute a small mass of the radionuclides present in the pond, but due to its solubility and activity, it is a radionuclide that is at risk of challenging effluent discharge limits. Organic molecules are known to be present also due to the ponds being open to the air, with occasional algal blooms restricting visibility. Understanding the chemical interactions of these components is important to inform ongoing sludge retrievals and effluent management. Batch sorption-desorption experiments have been performed with brucite, Sr-90 and natural organic matter (NOM) (namely humic acid (HA) and Pseudanabaena catenata cyano-bacterial growth supernatant) in both binary and ternary systems at high pH. Ionic strength, pH and order of addition of components were varied. Sr-90 was shown not to interact strongly with the bulk brucite surface in binary or ternary systems under pH conditions relevant to the pond. Humic acid in both binary and ternary systems demonstrated a strong affinity for the brucite surface. In ternary systems HA appeared to enhance sorption of Sr-90, and vice versa, regardless of the order of addition of components. There was no distinct effect of varying ionic strength or pH. (authors)