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.

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

“…It was shown that lithium and cesium are present in the compound as components of low-soluble phosphates, for example, cesium-MgCsPO 4 •6H 2 O (Phase #4, main reflexes at 5.40 and 3.44 Å), indicating the replacement of potassium ions with cesium in the matrix structure, while lithium-Li 3 PO 4 (Phase #5, lithiophosphate, main reflexes at 3.99 and 3.82 Å). This is confirmed by the presence of the KCl phase (Phase #6, sylvite, main reflexes at 3.15 and 2.23 Å) in the compound, which was also noted earlier in the immobilization of CsCl in the MPP compound [18].…”

“…Earlier [21], it was shown that heat treatment at a temperature of ≥300 • C resulted in thermal decomposition of the formed FKCN. Thus, the thermal stability of the MPP compound was studied at 270 • C for 4 h. The samples of the MPP compound were preliminarily kept at 180 • C for 10 h for removal of bound water from MgKPO 4 •6H 2 O-based compounds in a muffle furnace SNOL 30/1300 (AB UMEGA GROUP, Utena, Lithuania), as previously shown in [18].…”

“…At present, a magnesium potassium phosphate (MPP) matrix, obtained at room temperature and being a synthetic analogue of the natural mineral K-struvite, is considered as an alternative to glass for HLW immobilization. The MPP matrix is an effective mineral-like material with high hydrolytic, mechanical and radiation resistance, as shown earlier in [18][19][20]. Therefore, these materials were chosen for the immobilization of the spent electrolyte.…”

Chemical Technologies for Closing the Nuclear Fuel Cycle

“…This indicates that the use of synthetic analogs of minerals as matrices for immobilization of radioactive waste is promising. Breakthrough results in recent years [29][30][31] were obtained in the study of the possibility of using crystalline double orthophosphate of magnesium and potassium (MPP) as synthetic matrices for long-term and environmentally safe storage (or disposal) of liquid radioactive waste, including actinide-containing waste. The synthesis of the MPP matrix occurs in an aqueous solution according to the reaction The nonthermal process (room temperature, atmospheric pressure) of obtaining MPP matrices is similar to cementing and is characterized by low energy consumption, ease of implementation, and mobility of the curing process, and the simplicity of the hardware design of the method leads to minimization of "secondary" radioactive waste.…”

Optimization of the Solidification Method of High-Level Waste for Increasing the Thermal Stability of the Magnesium Potassium Phosphate Compound

“…Previously, in a number of literary sources [2][3][4][5][6][7][8][9][10][11][12][13], it was shown that the compound based on the magnesium potassium phosphate (MPP) matrix MgKPO 4 × 6H 2 O, which is formed at room temperature and is an analogue of the natural mineral K-struvite [14], is a promising material for immobilization of various LRW types, including HLW. A key task during HLW immobilization is to ensure binding of mobile cesium isotopes in the MPP compound.…”