Magnesium potassium phosphate ceramic for 99Tc immobilization

Technetium-99, present in the US Department of Energy’s (DOE) high-level waste (HLW) as a by-product of fission reactions, poses a serious environmental threat because it has a long half-life, is highly mobile in its soluble Tc7+ oxidation state and is volatile at high temperatures. Magnesium potassium phosphate (MKP) ceramics have been developed to treat 99Tc that has been partitioned and eluted from simulated high-level tank wastes by means of sorption processes. Waste forms were fabricated by adding MKP binder and a reducing agent (SnCl2) to the 99Tc-containing aqueous waste. In addition, waste forms were fabricated by first precipitating 99Tc from the waste and subsequently solidifying it in MKP. 99Tc loadings in the waste forms were as high as 900 ppm by weight. Waste form performance was established through various strength, leaching, and durability tests. Long-term leaching studies, as per the ANS 16.1 procedure, showed leachability indices between 11 and 14 for 99Tc under ambient conditions. The normalized leach rate for 99Tc, according to the product consistency test, was as low as 1.1 × 10−3 g/m2 d. The waste forms exhibited a compressive strength of ≈30 MPa and were durable in an aqueous environment. Containment of 99Tc in MKP ceramics is believed to be due to a combination of appropriate reducing environment (determined from Eh–pH measurements) and microencapsulation in a dense matrix.