Conditioning of spent electrolyte surrogate LiCl-KCl-CSCl using magnesium potassium phosphate compound

Abstract

The current work was aimed at developing a new conditioning method of spent electrolyte-radioactive waste (RW) generated during the pyrochemical reprocessing of mixed nitride uranium-plutonium spent nuclear fuel. Magnesium potassium phosphate (MPP) compound samples were synthesized under solidification of the electrolyte surrogate solution in a LiCl-KCl-CsCl system. The phase composition and structure of obtained compounds were studied by XRD and SEM-EDS methods. It was found that the compounds possessed a high compressive strength of 17-26 MPa. Hydrolytic stability of the compounds was evaluated in accordance with the long semi-dynamic test GOST R 52126-2003 and with the static PCT test. The 137Cs content in the leached solutions was determined by gamma-ray spectrometry, and other compound components were determined by ICP-AES and ICP-MS methods. The differential leaching rate of Cs at 25 ◦C from monolithic samples on the 91st day of samples contact with water was 5-11 × 10−5 g/(cm2·day) (GOST R 52126-2003), and was 4-29 × 10−7 g/(cm2·day) on the 7th day at 90 ◦C from crushed samples (PCT). The thermal stability of the compound at 180 ◦C and 450 ◦C was shown. The characteristics of the obtained MPP compound correspond to the current regulatory requirements for materials for RW conditioning.