Underground Disposal of HLWs and Bentonite

There are plans to dispose of HLW s produced from nuclear power plants in deep underground repositories sited several hundred meters deep (Fig. 1.4). The HLW s 

The radioactivity of HLW s lasts for several thousands years before its radioactivity reduces to the level of natural uranium ore, because it includes long-lived fission products (FP s) and transuranium nuclides (TRU s). The radionuclides released from the stored waste can be transported via ground water flow from the repository. Scientifically, it is difficult to prove the safety of a HLW disposal facility because the long time scale involved is beyond human experience. 

Bentonite is the key material of the EBS, which retards the transport of radionuclides. This is because (1) it is an extremely low permeable material, which prevents movement of water and eventually radionuclides, (2) it seals cracks in the rock mass because of its swelling properties, and (3) it shows a high ability for cation adsorption, therefore it can retard the migration of radionuclides. 

At the microscale level bentonite is a micro-inhomogeneous material, which consists of smectic clay minerals and macro-grains, mainly quartz, water and air (Fig. 1.5). The composition of Kunigel VI , which is a candidate buffer material for the Japanese proposals for HLW disposal, and its purified Kunipia F , is shown in Table 1.1. 

The main component of bentonite is smectic clay, such as montmorillonite and beidellite, and the unique properties mentioned above are due to the properties of these smectic clays (Pusch et al. 1990). 

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