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 (90)Sr investigation. For decomposition, a microwave method was applied, where 1g of dry-ashed slurry was partially dissolved (HNO(3) (65%)/H(2)O(2) (30%); v:v=8:2). In this slurry we obtained an (90)Sr 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 1g of dry-ashed "Si-free" slurry at 1100 degrees C in a muffle furnace. We achieved an (90)Sr activity of (7.8+/-0.3)Bq/g, yet observing BaSO(4) precipitation during the chromatographical separation of Sr. An alkali fusion using Na(2)CO(3) was done using the Bunsen burner and the muffle furnace for 20 min at 1000 degrees C, in combination. During formation of the hot glass, the surplus of Na(2)CO(3), produced Na(2)SO(4) and BaCO(3) in solid form. The hot glass was dissolved in deionised water, removing thus the SO(4)(2-) ions. Dissolving the residue directly in HNO(3), solves Ba as Ba(NO(3))(2) and thus we achieved over 80% of the (133)Ba activity in the solution, as measured by gamma-spectrometry. (85)Sr tracer of 88.0%+/-3.3% was recovered, yielding on average in (7.4+/-0.3)Bq/g of (90)Sr activity. The increase of 2.1-2.5 Bq/g of (90)Sr 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 SrSO(4) in the slurry.