Electrically Accelerated Leaching of Simulated Cementitious Wasteforms

UK disposal plans for intermediate-level nuclear waste (ILW) foresee its encapsulation in blends of Blast Furnace Slag and Portland cement (BFS:PC), held in steel drums and stored in a Geological Disposal Facility. Following groundwater ingress, wasteform degradation through leaching is possible and radionuclides could escape. Due to the long lifetime of hazardous waste, real-time experiments are infeasible, although it is necessary to gain detailed information about leaching performance of the wasteforms for key radionuclides and their transmutation products. This study aims at better understanding the durability of the wasteform by investigating the interactions of caesium and barium with the cement matrix (3:1 BFS:PC) through an accelerated degradation method: Electrically accelerated leaching. 137Cs, a problematic radionuclide in ILW due to its abundance and mobility, decays into 137Ba over time. This study revealed that when intermixed above ~2.8 wt.%, CsNO3 will precipitate without interacting with the 3:1 BFS:PC matrix. When barium is incorporated as Ba(NO3)2 and Ba(OH)2∙8H2O, barite (BaSO4) forms even at 1 wt.%-equivalent of CsNO3, and ettringite formation decreases. The study also revealed that Ca leaching was accelerated by 19.3±1.9 times when 3:1 BFS:PC is electrically leached at a current density of 25 Am-2 for two weeks. This ‘Leaching Acceleration Factor’ was different for every element studied. Leaching of cations into the anode tank, and vice versa for anions, is actually suppressed compared with static leaching. Electric migration of Ba into pre-cured 3:1 BFS:PC containing Cs resulted in formation of barite in the near-surface region of the cement. This suggests that formation of barite likely takes place following the nuclear decay of 137Cs, which is a desirable result as barite is highly insoluble, becoming strongly immobilised in the cementitious wasteform