A Microscopic and Tof-SIMS Study on the Ra Uptatke by barite

Long-term safety assessment studies of nuclear waste repositories for spent nuclear fuel commonly consider a scenario, in which radium released from the fuel comes into contact with an aqueous solution saturated with barium [1]. An open question is whether a Ra containing solution will equilibrate with solid BaSO4 under repository relevant conditions due to complete recrystallization. Previous studies have indicated that uptake of Ra is not limited by pure adsorption at close to equilibrium conditions but involves a significant fraction of the bulk solid [2,3]. So far, only macroscopic data indicate that barite may fully recrystallize to radiobarite. For this microscopic (SEM) and ToF-SIMS study, samples of two different barites were prepared in batch sorption experiments to investigate the recrystallization of barite in the presence of Ra. Based on gamma spectrometry, it was observed that one barite incorporates Ra faster than the other barite. A broad grain size distribution was determined via SEM observation for the fine grained barite. During recrystallization of the crystals, an additional effect on the grain size evolution due to the presence of Ra was detected compared to the coarsening of a blank sample due to Ostwald ripening. In contrast, the narrow grain size distribution of the coarse grained barite didn’t show significant changes with time and no influence of the presence of Ra. Here, morpholgical changes were observed instead. The results of ToF-SIMS indicate a homogeneous uptake of Ra into the crystals. After 443 days the integrated Ra concentration corresponds with the size of the barite particles. The Ba/Ra ratio was calculated from several ToF-SIMS measurements. The Ra/Ba intensity distribution has its maximum between 2 and 4 . 10-3, corresponding well with the macroscopic results from bulk solution chemistry. [1] Grandia, F. et al.: SKB Technical Report TR-08-07 2008 [2] Bosbach, D. et al.: SKB Technical Report TR-10-43, 2010. [3] Curti, E. et al.: Geochim. et Cosmochim. Acta, 2010, 74, 3553-3570