1. 6  Predicting Safety of Radioactive Waste Disposal for a Million Years
- Diffusion of Radionuclides in Rock
 

Fig. 1-12 Granite appears to be very solid and not to have pores. A micrograph, however, shows micropores are present (upper left). Adsorbed dye showing the extent of micropores in granite is displayed in the other micrograph (upper right). Owing to this absorption mechanism, radionuclides transported with groundwater along rock fractures diffuse into pores in the rock and are adsorbed on mineral surfaces (lower example of mass transport in fractured rock mass).

Fig. 1-13 Effective diffusivity of solutes in granite The green line represents the correlation between effective diffusivity and diffusivity in water when solutes diffuse through pore water in granite. A higher diffusivity was obtained for Sr2+ and Ba2+, which is due to diffusion in the adsorbed state on mineral surfaces (surface diffusion).

---

After placing radioactive waste in a deep underground repository, long-lived radionuclides may be leached from the waste and subsequently transported by groundwater through fractures in the rock mass surrounding the repository. Radionuclides transported by groundwater will diffuse into the rock matrix and be adsorbed on minerals (Fig. 1-12). Although the radionuclides have been released from the repository, their migration is greatly impeded. Further, the concentration of these radionuclides in groundwater is diluted significantly before the nuclides return to our biosphere. These mechanisms greatly mitigate the risks of this method of radioactive waste disposal.  Transport of radionuclides in an intact rock sample was investigated and Fick's diffusion law was confirmed to be applicable to predicting transport behavior. The distance plutonium diffuses in the rock mass in a million-year period, for example, is predicted to be 5 m using the diffusivity value obtained in this study. It became clear that radionuclides diffuse through pore water in rock (pore diffusion), and some radionuclides diffuse in the adsorbed state on mineral surfaces (surface diffusion). This knowledge is used in the long-term performance assessment of radioactive waste disposal.

---

Reference T. Yamaguchi et al., Diffusivity of U, Pu and Am Carbonate Complexes in a Granite from Inada, Ibaraki, Japan Studied by through Diffusion, J. Contaminant Hydrology, 35, 55 (1998).

Select a topic in left column

---

Persistent Quest - Research Activities 2001 Copyright(c) Japan Atomic Energy Research Institute