In the ‘once-through’ fuel cycle concept, the 3-5% 235U enriched fuel is burned once in the power reactor. It is then removed from the reactor, stored temporarily, and ultimately packaged for disposal in a deep geologi­cal repository. Because the once-through fuel cycle has been the official policy for managing irradiated commercial nuclear fuel in the US, the geo­logical repository program in the US was highly developed. It had advanced to the point that the US Department of Energy (DOE) submitted the license application for the Yucca Mountain repository to the US Nuclear Regulatory Commission in 2008 (OCRWM, 2008). Although the DOE recently withdrew the license application for the Yucca Mountain reposi­tory, the development of this repository represents a good example appropriate for the once-through fuel cycle. For this reason, the discussion in this section will focus on the Yucca Mountain repository concept.

The site of the proposed Yucca Mountain repository is located in an arid region, about 145 km northwest of Las Vegas, Nevada. The geology and hydrology of this site have been extensively studied, as have the volcanic and seismic characteristics of the area (OCRWM, 2002). The Yucca Mountain repository is designed to accommodate three primary waste types: (1) commercial irradiated nuclear fuel from power generating plants, (2) DOE-owned irradiated fuel, including naval reactor fuel, and (3) vitri­fied high level waste (HLW). The latter category consists mostly of HLW borosilicate glass canisters generated from vitrifying radioactive tank wastes at the Hanford and Savannah River sites in the US (see Chapter 18). Under current US law, a maximum of 70,000 metric tonnes heavy metal (MTHM) can be disposed of at the Yucca Mountain repository. Of this, 63,000 MTHM is allocated to the disposition of commercial irradiated fuel, representing approximately 221,000 fuel assemblies (OCRWM, 2008). The DOE-owned irradiated fuel would constitute 2,333 MTHM, and the remaining 4,667 MTHM would be allocated for disposal of vitrified HLW. It is interesting to note that under the current statute, the Yucca Mountain repository (if built) would already be oversubscribed because the 4,667 MTHM allocated to vitrified HLW represents only approximately 9,334 of the 22,000 canisters expected to be produced and also 292,000 commercial irradiated fuel assemblies are expected to be produced by 2040, far in excess of the space allotted for 221,000 assemblies (OCRWM, 2008).

There is a single design for the Yucca Mountain waste package, but it has six configurations providing flexibility to accommodate the different types of waste to be received. Figure 5.3 illustrates the six waste package configu­rations (OCRWM, 2008). All six configurations consist of two concentric cylindrical containers. The primary (inner) waste container is made of 316 stainless steel and has walls 50.8 mm thick (Skinner et al., 2005). The outer secondary containment has 20.3 mm thick walls and is made of alloy C-22. The secondary container is designed to provide corrosion resistance, so it is referred to as the outer corrosion barrier. Each waste package has three welded lids, one on the primary container and two on the corrosion barrier. After welding of the inner lid, the primary waste vessel is evacuated and back-filled with helium. The helium serves three primary purposes:

1. It inhibits internal corrosion.

2. It improves heat transfer between the waste and the waste package.

3. It provides a tag for leak testing of the inner vessel closure welds.

Because of the intense radiation involved, all the waste package closure operations must be conducted robotically (Skinner et al., 2005 ).

5-DHLW/DOE SNF 2-MCO/2-DHLW Short

5.3 Approved waste package configurations for the Yucca Mountain repository.

In the design concept for Yucca Mountain, the loaded waste packages were to be transported to emplacement drifts carved within the repository. The emplacement drifts are nominally 5.5 m in excavated diameter with an average length of approximately 600 m. The length of the emplacement drifts is constrained to no more than 800 m to allow for efficient ventilation. The drifts have been excavated in parallel, spaced 81 m apart. This spacing is designed to prevent thermal interaction between adjacent drifts and allows infiltrating water from the surface to percolate past the drifts.

Encroachment of water onto the waste packages was to be further miti­gated by installation of a titanium drip shield. Water percolating into the emplacement drift from above the waste package is directed to a point below the waste package by the drip shield. Figure 5.4 provides a cross­sectional illustration of an emplacement drift, as designed for the Yucca Mountain Repository (OCRWM, 2008).

Sweden also practices a once-through nuclear fuel cycle and substantial progress has been made in that country on the establishment of a geological repository (see Chapter 13). These efforts are led by the Swedish Nuclear Fuel and Waste Management Company (SKB). Three decades of research and development and a 20-year site development process has resulted in the selection of Forsmark in the municipality of Osthammar as the site for the Swedish geological nuclear repository. In contrast to the arid environ­ment of Yucca Mountain, the Forsmark site is located along a coastal area. However, there are relatively few water-conducting fractures in the bed­rock at the depth of the fuel emplacement (500 m) at the Forsmark site (SKB, 2009). For emplacement into the Swedish repository, it is planned that the irradiated assemblies will be packed into cast iron baskets, which

in turn will be placed within thick copper canisters. Once placed within the repository, the loaded copper canisters will be packed in bentonite clay.

Finland is taking a similar approach to Sweden, having chosen the Olkiluoto site for the Finnish HLW repository (Okko and Rautjarvi, 2004). In this case, the waste packages will be placed in excavated tunnels hun­dreds of meters below the surface. The tunnels will be separated by a dis­tance of 25 m. As for the Swedish repository, the irradiated fuel will be packaged into nodular cast iron containers, which will then be enclosed in a 5 cm thick copper shell. The waste packages will be surrounded with ben­tonite clay to absorb water and to protect the waste from minor movements in the surrounding bedrock.