China’s HLW/SF repository concept

Public concerns over the global ability to manage, and eventually dispose of RAW, especially HLW/SF, remain high. Emplacement in the deep geology is an internationally recognized disposal solution for HLW and SF, and China is planning to use this route. While China’s GDF programme is at an early stage, like all international waste management programmes imple­menting geological disposal, it is considering multi-barrier concepts com­prising engineered and natural barriers between the HLW/SF in the geosphere and the biosphere, while bentonite-based engineered barrier systems (EBS) were considered in China as early as the 1990s [16-17]. The current preliminary geological disposal concept for its HLW/SF is to use a shaft-tunnel model in the saturated zones of granite rock (Fig. 22.4). Over the past 20 years, China has made great strides in its geological repository programme including, as described above, geologically surveying the whole country for its georepository site selection and optimization of backfill/ buffer materials that will be needed for the GDF safety cases [18].

Many other countries are developing similar concepts for permanent disposal of radioactive waste deep underground: solidification of HLW/SF

image256"Bentonite

backfill

A multi-barrier concept

Buffer

22.4 China s preliminary HLW repository concept.

using glass and ceramics, packaging in metal canisters, following temporary storage above ground before permanent geological disposal in natural barrier systems such as a granite rock-body, using a multi-barrier system [16,17]. Chinese researchers have suggested that EBS is a major component in guaranteeing long-term safety, making it necessary to conduct fundamen­tal research on the coupled THMC (thermal-hydrological-mechanical — chemical) behaviour of bentonite under simulated geological disposal conditions, and subsequently to reveal the property changes of the ben­tonite over a long period of time.

The requirements for HLW backfill materials are long-term chemical and physical stability, good mechanical properties, volume expandability in contact with water and very low water penetrability. Other requirements also include the ability to hinder nuclide migration, good thermal conduc­tivity and thermal stability, radiation resistance and stability, natural avail­ability and importantly, low cost.

Many years of research in Europe and China on bentonite backfill mate­rials for the EBS has revealed that bentonite comprising predominantly montmorillonite is considered to give the best performance in terms of low water penetration, high volume expansion, and excellent nuclide absorption and retention, as well as being abundant.

China is rich in mineral reserves and has large bentonite reserves suitable for the EBS backfill/buffer (at one site with a volume of 40 x 40 x 0.7 km) in China’s Inner Mongolia region near Beijing. Bentonite with high content of expandable montmorillonite has been found in an area named Gao — Miao-Zi (GMZ, which in English means Highland Temple). This single reserve, as shown in Fig. 22.5 is over 280 x 106 tonnes.

It is expected that the bentonite at Gao-Miao-Zi will be used in China’s HLW/SF geological repositories. This bentonite is being considered as a part of the EBS due to its ability to retain radionuclides and other hazardous materials. Prior to considering modular designs for canister encapsulation in the GDF, bentonite natural resources, raw mineral analysis, characteriza­tion and processing, need to be investigated, developed and optimized for large-scale cost-effective manufacture. To demonstrate the long-term safety of a GDF in China, the influence of the bentonite composition and the properties of the compacted block/brick must be studied.

Some large-scale mock-up facilities have also been built in China to test the efficacy of backfill/buffer materials such as bentonite with designed canisters. A China mock-up test was recently initiated after a long period of research conducted with international support. It is based on a prelimi­nary concept of the HLW granite rock environment repository in China [19] . It was developed to investigate the THMC properties of compacted GMZ-Na-bentonite as shown in Fig. 22.6 . which reveals the arrangement of compacted bentonite mineral blocks inside the mock-up test steel. The work has been carried out and led by the Beijing Research Institute of Uranium Geology (BRIUG) [20-22] . The device contains a heater, which

image257

22.5 China bentonite GMZ 1,600 million tonnes of Na-based bentonite site in Gao-Miao-Zi, Inner Mongolia, China.

image258

22.6 Compacted engineered bentonite blocks arranged in inside a mock-up test facility within a steel tank (top view). This mock-up THMC test consists of a heater (canister) and bentonite blocks within a cylindrical steel tank.

simulates the heat from a container of HLW/SF, placed inside the com­pacted GMZ Na-bentonite blocks with total dry density 1,600 kg/m3. Water inflow through the barrier from its outer surface simulates the water pen­etration. The device is a large steel tank of 900 mm internal diameter and 2200 mm in height. The experiment is being performed at the BRIUG labo­ratory and the design concept is shown in Fig. 22.6 and Plate VI (between pages 448 and 449). In Fig. 22.6, the compacted engineered bentonite blocks are arranged inside a mock-up test facility within a steel tank (top view). This mock-up THMC test consists of a heater (canister), bentonite blocks within a cylindrical steel tank, as shown in Plate VI as a sketch of the cross section of the China mock-up facility and the arrangement of central heater, steel canister, bentonite blocks/bricks and multiple sensor arrange­ment [22] .

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