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 SO₄^2- 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.