Perspective Compounds for Immobilization of Spent Electrolyte from Pyrochemical Processing of Spent Nuclear Fuel

Abstract

Immobilization of spent electrolyte–radioactive waste (RW) generated during the pyrochemical processing of mixed nitride uranium–plutonium spent nuclear fuel is an acute task for further development of the closed nuclear fuel cycle with fast neutron reactors. The electrolyte is a mixture of chloride salts that cannot be immobilized directly in conventional cement or glass matrix. In this work, a low-temperature magnesium potassium phosphate (MPP) matrix and two types of high-temperature matrices (sodium aluminoironphosphate (NAFP) glass and ceramics based on bentonite clay) were synthesized. Two systems (Li0.4K0.28La0.08Cs0.016Sr0.016Ba0.016Cl and Li0.56K0.40Cs0.02Sr0.02Cl) were used as spent electrolyte imitators. The phase composition and structure of obtained materials were studied by XRD and SEM-EDS methods. The differential leaching rate of Cs from MPP compound and ceramic based on bentonite clay was about 10−5 g/(cm2·day), and the rate of Na from NAFP glass was about 10−6 g/(cm2·day). The rate of 239Pu from MPP compound (leaching at 25 °C) and NAFP glass (leaching at 90 °C) was about 10−6 and 10−7 g/(cm2·day), respectively. All the synthesized materials demonstrated high hydrolytic, mechanical compression strength (40–50 MPa) even after thermal (up to 450 °C) and irradiation (up to 109 Gy) tests. The characteristics of the studied matrices correspond to the current requirements to immobilized high-level RW, that allow us to suggest these materials for industrial processing of the spent electrolyte.

Medium-Temperature Glass-Composite Phosphate Materials for the Immobilization of Chloride Radioactive Waste

“…We considered iron phosphate glass [28], the synthesis temperature of which is lower than that of aluminophosphate and borosilicate glasses, as well as a magnesium-potassium phosphate matrix [29,30] obtained at room temperature as promising for chloride wastes. The technology for obtaining glass composite materials of various synthesis temperatures in the range of 450-750 • C was also tested based on known glass compositions [31].…”

The Use of Dolomite to Produce a Magnesium Potassium Phosphate Matrix for Radioactive Waste Conditioning

“…Glass is the only industrial matrix for conditioning high-level waste (HLW) used in Russia, UK, the USA, France, Germany, Belgium, China, Japan, and other countries [2,4].…”

“…One possible alternative to these matrices is chemically bonded phosphate ceramics [11], which include a mineral-like magnesium potassium phosphate (MPP) matrix. Earlier, we Energies 2023, 16, 5513 2 of 13 showed in [4,12,13] that MPP matrix of the composition MgKPO 4 × 6H 2 O is an effective and multipurpose material for conditioning of RW, including ILW and HLW. The MPP matrix is produced at room temperature by reaction (1) and is an analogue of the natural mineral struvite-K [14].…”

“…Chemical immobilization of waste components is provided due to ion-exchange properties of K-struvite in the structure of which substitution of potassium and magnesium for other univalent cations (NH 4+ , K + , Rb + , Cs + , Tl + ) and divalent cations (Mg 2+ , Ni 2+ , Zn 2+ , Co 2+ , Cd 2+ , Cr 3+ , Mn 2+ , VO 2+ ) respectively is possible [6,[18][19][20]. At the same time the MPP matrix has high radiation resistance to gamma and alpha irradiation [21], including electron irradiation up to 10 8 Gy [4].…”