Solidification of high level waste using magnesium potassium phosphate compound

Abstract

Compound samples based on the mineral-like magnesium potassium phosphate matrix MgKPO4 × 6H2O were synthesized by solidification of high level waste surrogate. Phase composition and structure of synthesized samples were studied by XRD and SEM methods. Compressive strength of the compounds is 12 ± 3 MPa. Coefficient of thermal expansion of the samples in the range 250–550 °C is (11.6 ± 0.3) × 10−6 1/°C, and coefficient of thermal conductivity in the range 20–500°С is 0.5 W/(m × K). Differential leaching rate of elements from the compound, g/(cm2 × day): Mg - 6.7 × 10−6, K - 3.0 × 10−4, P - 1.2 × 10−4, 137Cs - 4.6 × 10- 7; 90Sr - 9.6 × 10−7; 239Pu - 3.7 × 10−9, 241Am - 9.6 × 10−10. Leaching mechanism of radionuclides from the samples at the first 1–2 weeks of the leaching test is determined by dissolution (137Cs), wash off (90Sr) or diffusion (239Pu and 241Am) from the compound surface, and when the tests continue to 90–91 days - by surface layer depletion of compound. Since the composition and physico-chemical properties of the compound after irradiation with an electron beam (absorbed dose of 1 MGy) are constant the radiation resistance of compound was established. Keywords: Magnesium potassium phosphate compound, High level waste, Immobilization, Leaching rate, Leaching mechanism, Radiation resistance