The diffusion behavior of neptunium (Np) in compacted bentonite with bicarbonate was investigated at various NaHCO3 concentrations. Typical Japanese purified Na-bentonite, Kunipia-F, which contains approximately 95 wt % montmorillonite, was used. The Na-bentonite was compacted with a jig into cylindrical pellets 10 mm in diameter and 10 mm high with dry densities of 0.7 to 1.4 Mg/m3. Each pellet was inserted into an acrylic resin column and saturated with carbonated water containing 0.25 to 1.0 M NaHCO3 for more than 1 month. The bottom interface of the compacted bentonite saturated with carbonated water was spiked with 5 µL of tracer solution containing 10 ppb of Np in 0.1 M HNO3, and the diffusion periods were in the range of 2.7 years. After the diffusion period, the bentonite specimen was pushed out of the column and cut into 0.3- to 1.0-mm-thick slices. Each slice was submerged in 1 N HNO3 to extract the tracer ion and the liquid phase was separated by centrifugation. The supernatant was analyzed for Np by inductively coupled plasma-tandem mass spectrometry. The Np concentration profile increased over time and reached the final profile in 2 years. The final profile showed logarithmic decay with a decay or filtration coefficient of 3.2 ± 0.1 mm−1. This final profile indicated that Np could diffuse but would be filtered by the bentonite medium. Np may exist as carbonate complexes, which could be larger than the pores constituting the diffusion path.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering