Abstract
The key task in the solidification of high-level waste (HLW) into a magnesium potassium phosphate (MPP) compound is the immobilization of mobile cesium isotopes, the activity of which provides the main contribution to the total HLW activity. In addition, the obtained compound containing heat-generating radionuclides can be significantly heated, which increases the necessity of its thermal stability. The current work is aimed at assessing the impact of various methodological approaches to HLW solidification on the thermal stability of the MPP compound, which is evaluated by the mechanical strength of the compound and its resistance to cesium leaching. High-salt surrogate HLW solution (S-HLW) used in the investigation was prepared for solidification by adding sorbents of various types binding at least 93% of 137Cs: ferrocyanide K-Ni (FKN), natural zeolite (NZ), synthetic zeolite Na-mordenite (MOR), and silicotungstic acid (STA). Prepared S-HLW was solidified into the MPP compound. Wollastonite (W) and NZ as fillers were added to the compound composition in the case of using FKN and STA, respectively. It was found that heat treatment up to 450 °C of the compound containing FKN and W (MPP-FKN-W) almost did not affect its compressive strength (about 12–19 МPa), and it led to a decrease of high compressive strength (40–50 MPa) of the compounds containing NZ, MOR, and STA (MPP-NZ, MPP-MOR, and MPP-STA-NZ, respectively) by an average of 2–3 times. It was shown that the differential leaching rate of 137Cs on the 28th day from MPP-FKN-W after heating to 250 °C was 5.3 × 10−6 g/(cm2∙day), however, at a higher temperature, it increased by 20 and more times. The differential leaching rate of 137Cs from MPP-NZ, MPP-MOR, and MPP-STA-NZ had values of (2.9–11) × 10−5 g/(cm2∙day), while the dependence on the heat treatment temperature of the compound was negligible.
Highlights
Liquid radioactive waste (LRW) of various activity levels are generated during spent nuclear fuel (SNF) reprocessing
This work is aimed at studying the influence of the natural zeolite (NZ), MOR, ferrocyanide K-Ni (FKN), and silicotungstic acid (STA) sorbents on the efficiency of preliminary binding of Cs+ cations in the high-salt high-level waste (HLW) solution after SNF reprocessing, as well as determination of thermal stability of the obtained magnesium potassium phosphate (MPP) compounds, which is estimated from its compressive strength and stability to 137 Cs leaching
A combination method for cesium isotopes binding was used, which included the precipitation of cesium in S-HLW according to the reaction (2) using STA (“JSC Reahim” LLC, Moscow, Russia) followed by sorption of cesium remaining in the solution by NZ, which was a reinforcing additive to increase the mechanical strength of the compound, as we showed earlier [36]
Summary
Liquid radioactive waste (LRW) of various activity levels are generated during spent nuclear fuel (SNF) reprocessing. For temporary controlled storage and following final disposal, HLW must be immobilized into a solid compound that will contribute to radioecological safety for the environment. This compound should possess physical and chemical stability, including thermal stability due to its possible significant heating based on the intense heat-generating of HLW radionuclides, which is largely due to the radioactive decay of fission products, cesium isotopes (134 Cs, 137 Cs), the activity of which provides the main contribution to the Energies 2020, 13, 3789; doi:10.3390/en13153789 www.mdpi.com/journal/energies. It is known that various sorbents are used to remove and concentrate cesium from natural and technogenic solutions
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have