Stability of Modified Technetium-Containing Magnesium Potassium Phosphate Matrix under Repository Conditions with Regard to Biogeochemical Effects

In this work, the biological stability of magnesium potassium phosphate (MPP) matrix was assessed for the first time under conditions modelling near-surface and deep repositories of radioactive waste (RW) containing technetium. It was found that MPP-matrix is potentially susceptible to microbial degradation, so it is necessary to introduce modifying additives. The efficiency of introduction of modifying additives -tetraphenylphosphonium chloride (TPP), thiourea (THIO), polyhexamethyleneguanidine hydrochloride (PHMG), polyethyleneimine (PEI), shungite (SHU), chalcopyrite (CP), orpiment (ORP) at immobilisation of pertechnetate ion in MPP-matrix has been estimated. The mechanical strength and water resistance of the obtained compounds were investigated. The maximum leaching rate of technetium was found to be 0.34 g cm-2 day-1 from the blank sample without additives. The addition of ORP and PHMG reduces the leaching rate of technetium by 90% and 62%, respectively, relative to the pure sample. The addition of other modifying additives reduced the leaching rate of pertechnetate ions by 15-25% on average. At the same time, the addition of PEI and THIO led to a decrease in compressive strength below the standard requirement of 4.9 MPa.a. Samples containing PHMG and ORP up to 0.5 wt% showed higher safety margin compared to the blank sample. ORP, PHMG and TPP can be proposed as modifying additives to reduce microbial activity during immobilisation of pertechnetate ion using MPP matrix.