Effect of Detonation Nanodiamonds on Physicochemical Properties and Hydrolytic Stability of Magnesium Potassium Phosphate Composite

This study focuses on improving the operational properties of a magnesium potassium phosphate (MPP) matrix MgKPO4 × 6H2O for the immobilization of radioactive waste (RW) by introducing detonation nanodiamonds (NDs). The study evaluates the impact of NDs on the phase composition of the resulting composite based on the MPP matrix (further referred to as MPP-ND composite), as well as its compressive and flexural strength, porosity, thermal conductivity, and leaching resistance to actinides (239Pu, 238U) and europium (as a lanthanide simulator). It was found that the optimal content of NDs in the composite is 1 wt%, along with 20 wt% of wollastonite as a reinforcing additive. This MPP-ND composite exhibited high compressive and flexural strengths of 24 and 4 MPa, respectively, a thermal conductivity coefficient of (0.5–1.0) W/(m∙K) in the interval of (47–510) °C, and a minimal open porosity of no more than 5%. An increase in hydrolytic stability to leaching of actinides and europium due to their prior sorption on NDs was observed. The leaching rates of 239Pu, 238U, and Eu from the MPP-ND composite on the 28th day of sample contact with water were 3.5 × 10−6, 1.5 × 10−4, and 4.0 × 10−6 g/(cm2·day), respectively. Thus, for the first time, data on the influence of NDs on the physicochemical properties and hydrolytic stability of MPP-ND composite demonstrating the practical applicability of this composite for RW immobilization have been obtained.