Separation of 90Sr from radioactive waste matrices—Microwave versus fusion decomposition

Radioactive waste (slurry) from a detention pond deriving from two research reactors and several inactive and active drain outlets at the Paul Scherrer Institute are the basis for the current 90Sr investigation. For decomposition, a microwave method was applied, where 1 g of dry-ashed slurry was partially dissolved (HNO3 (65%)/H2O2 (30%); v:v=8:2). In this slurry we obtained an 90Sr activity of 5.3±0.2 Bq/g in solution. In a second run, we applied a borate-fusion (Li metaborate/Li tetraborate (80:20 w/w%) dissolving 1 g of dry-ashed “Si-free” slurry at 1100 °C in a muffle furnace. We achieved an 90Sr activity of (7.8±0.3) Bq/g, yet observing BaSO4 precipitation during the chromatographical separation of Sr. An alkali fusion using Na2CO3 was done using the Bunsen burner and the muffle furnace for 20 min at 1000 °C, in combination. During formation of the hot glass, the surplus of Na2CO3, produced Na2SO4 and BaCO3 in solid form. The hot glass was dissolved in deionised water, removing thus the SO42− ions. Dissolving the residue directly in HNO3, solves Ba as Ba(NO3)2 and thus we achieved over 80% of the 133Ba activity in the solution, as measured by γ-spectrometry. 85Sr tracer of 88.0%±3.3% was recovered, yielding on average in (7.4±0.3) Bq/g of 90Sr activity. The increase of 2.1–2.5 Bq/g of 90Sr activity achieved with the alkali fusion, and the Li metaborate/Li tetraborate 80:20 w/w% fusion, respectively, clearly shows that some Sr must have been present as SrSO4 in the slurry.