The Influence of Zeolite (Sokyrnytsya Deposit) on the Physical and Chemical Resistance of a Magnesium Potassium Phosphate Compound for the Immobilization of High-Level Waste

Abstract

The manuscript presents the results of the development of new material for high-level waste (HLW) management: the magnesium potassium phosphate (MKP) compound. The possibility of using zeolite (Sokyrnytsya deposit) to increase the mechanical, thermal, and hydrolytic resistance of this compound with immobilized HLW was studied. The main component of the used natural zeolite is a mineral of the clinoptilolite–heulandite series, and quartz, microcline, and clay minerals (illite, sepiolite, and smectite) are present as impurities. The compressive strength of the compound, containing at least 4.2 wt % zeolite, is about 25 MPa. Compound containing 28.6 wt % zeolite retains high compressive strength (at least 9.0 MPa), even after heat treatment at 450 °C. The adding of zeolite to the composition of the compound increases its hydrolytic stability, while the leaching rate of the mobile nuclides 137Cs and 90Sr decreases up to one order of values. Differential leaching rate of radionuclides from the compound containing 28.6 wt % zeolite is 2.6 × 10−7 for 137Cs, 2.9 × 10−6 for 90Sr, 1.7 × 10−9 for 239Pu, and 2.9 × 10−9 g/(cm2∙day) for 241Am. Thus, the properties of the resulting compound correspond to the requirements for solidified HLW in Russia.

Conditioning of Spent Electrolyte Surrogate LiCl-KCl-CsCl Using Magnesium Potassium Phosphate Compound

“…Thermal stability of the MPP compounds, obtained after solidification of LiCl-KCl-CsCl aqueous solutions, was investigated in accordance with the current requirements [27] for solidified HLW. For this purpose, the samples were kept at 180 • C, 10 h and at 450 • C, 4 h (hereinafter named MPPZ_180 and MPPZ_450, respectively) in a muffle furnace (SNOL 30/1300, AB UMEGA GROUP, Utena, Lithuania), as in previous work [25].…”

“…The samples of the MPP compound obtained by solidification of LiCl-KCl-CsCl aqueous solutions and containing 28.6 wt% zeolite (hereinafter the samples are named MPPZ) were also synthesized. The natural zeolite of the Sokyrnytsya deposit, Transcarpathian region (ZEO-MAX LLC, Ramenskoye, Moscow region, Russia) with a particle size of 0.07-0.16 mm and a specific surface area of 17.5 m 2 /g was used for preliminary cesium binding and for increasing the mechanical strength of the MPP compound just as in the previous work [25], where its mineral composition was also determined.…”

“…Cubic samples of the MPP compound with dimensions of 2 cm × 2 cm × 2 cm were prepared as required by [26]. It was decided to keep the samples for at least 15 days at ambient atmospheric conditions in accordance with previous studies [17,25].…”

“…At the same time, it was noted that under heating to 450 • C of the compound, the samples practically lost their compressive strength. Earlier in [25] it was established, that the addition of about 29 wt% zeolite to the composition of the MPP compound led to a significant increase in both mechanical strength and thermal stability of samples. To study the effect of zeolite on the efficiency of immobilization of spent electrolyte, compound samples obtained during solidification of LiCl-KCl-CsCl solution and contained 28.6 wt% zeolite were used.…”

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

“…Earlier, in some articles, the effectiveness of using NZ as a reinforcing mineral filler of the MPP compound [35,36], as well as using FKN for sorption of cesium isotopes [5,37,38] during solidification of certain LRW types, was demonstrated.…”