ties for spent nuclear fuel (SNF), high-level radioactive waste (HLW), and transuranic (TRU) waste.

6.1 DISPOSAL OF WASTE FORMS

Waste forms containing radioactive waste are intended for disposal in engineered facilities constructed in stable geologic formations. Geologic repositories are designed for the disposal of higher-hazard wastes3 such as SNF, HLW, and TRU waste. These facilities are constructed in geologic formations located hundreds of meters below Earth’s surface. Shallow-land disposal facilities are designed for the disposal of lower-hazard wastes such as low-level radioactive waste (LLW). These facilities are typically excavated into sediments located within 10 meters or so of the Earth’s surface (Figure 6.1).

Given the focus of this report on HLW, the discussion in this chapter focuses on geologic repositories. However, many of the environmental processes that govern waste form performance in geologic repositories would also apply to shallow-land facilities.

Geologic repositories are designed with multiple barriers to isolate waste from the environment (NRC, 2003; OECD-NEA, 2003). They contain both engineered barriers, which include the waste form, disposal canisters, and backfills, if present, and natural barriers such as the host rock. These barriers are intended to work in concert, passively providing different safety functions at different levels of effectiveness and reliability and at different times into the future for long-term isolation of radioactive waste (Figure 6.1). Barrier safety functions can overlap, providing so-called latent safety functions, with each barrier contributing to waste isolation at varying levels and times. Taken collectively, the barriers and their safety functions define the safety concept for the disposal system.

There are two basic isolation strategies to achieve long-term (103-106 years) safety of multiple-barrier disposal facilities (Apted and Ahn, 2010):

  • Containment of radionuclides within the engineered and natural barriers of the disposal facility. Containment allows time for radioactive decay, which reduces the hazard of the radioactive component of the waste.
  • Attenuation of the concentrations of radionuclides that are released from the disposal facility. This strategy may, for example, rely on

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3 Hazard depends on several factors, including the types and concentrations of radioelements and their mobility in the environment. The classification of radioactive wastes in the United States is based on waste origin rather than hazard. Nevertheless, wastes destined for geologic disposal generally have a higher hazard than wastes disposed of in shallow facilities.



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