Long-term controlled storage or disposal is one of the major stages of liquid radioactive waste (LRW) management. The preparation of LRW for this stage includes the conversion of waste into a stable solidified form. In recent years, researches devoted to the development of new methods of LRW solidification are carried out using a magnesium-potassium-phosphate (MPP) compound based on the MgKPO4∙6H2O matrix. The matrix is formed in an aqueous solution at room temperature as a result of the acid-base reaction between magnesium oxide and potassium dihydrogen phosphate and it is an analog of the natural mineral K-struvite. It was shown that the LRW immobilization method in the MPP compound combines versatility, simplicity of implementation and relative cheapness similar to cementing, as well as high physicochemical stability of the compound, which is not inferior to glass.
The samples of the MPP compound were synthesized during the immobilization of nitric acid solutions of uranium and lanthanum in this research work. It has been established that uranium in the MPP compound is in the form of potassium uranyl orthophosphate K(UO2)PO4∙3H2O (meta-ankoleite), and lanthanum - in the form of phosphates LaPO4∙0.5H2O (rhabdophane-La) and Mg0.6K0.68La0.36PO4·6H2O. The differential rate of leaching of uranium and lanthanum at the 28th day of contact of the compound with water under 23 °C is, g / (cm2∙day): 1.7∙10-6 and 6.4∙10-6, respectively. The degree of leaching of uranium and lanthanum is 0.02 and 0.05%, respectively.
The mechanism of uranium and lanthanum leaching of MPP compound was studied according to the de Groot and van der Sloot model. It was shown that the mechanism of uranium and lanthanum leaching varies during the contact of the compound with water. During the first 7 days uranium leaching occurs due to dissolution of the surface layer of the compound, where single particles of hydrated uranyl nitrate were probably localized, however then the uranium leaching is uniquely determined by the diffusion mechanism from the inner layers of the compound. The behavior of lanthanum in leaching was almost the same. During the first 14 days leaching of lanthanum occurs due to dissolution of the surface layer of the compound, then during the subsequent 14 days - due to its depletion, however then leaching of lanthanum similar to leaching of uranium is uniquely determined by the diffusion mechanism from the inner layers of the compound.