separation of valuable components after its controlled storage, because the 239Pu, 235U, and 238U contents do not change in SNF while its radioactivity decreases. The fuel removed from reactors should be called irradiated and considered a potential energy resource, rather than waste.
In Russia some SNF types (from VVER-440, BN-600, and BN-350 reactors, some naval-propulsion reactors, and research reactors) are recycled at the RT-1 plant (Mayak in the Chelyabinsk region). Some other SNF types (from VVER-1000 reactors) are planned to be recycled at the RT-2 plant under construction (Zheleznogorsk near Krasnoyarsk). The SNF from RBMK, AMB, EGP-6, some transport facilities and research reactors, and faulty fuel is in storage, and its further treatment has not yet been considered. The primary portion of radioactivity (greater than 3 billion Ci) of SNF is accumulated in the RBMK reactors. This SNF has relatively low contents of fissionable species, and it will probably be recycled only in the far future. The problem of long-term safe storage and disposal of SNF in underground repositories is considered in this paper.
General geological recommendations for the long-term safe storage of SNF are deposition in the low-permeable rocks in seismically stable blocks with low velocities of vertical displacements and free of active volcanism and mineral deposits (Krauskopf, 1988a; Laverov et al., 2001). If these requirements have been met, the probability of the repository destruction, its exposure to erosion, transportation of radionuclides by magmas, and penetration of the repository by mining workings is minimized. The environmental pollution in this case can be related to radionuclide removal from the repository by groundwater in dissolved or colloidal forms. This mechanism is always taken into account in the assessment of the security level of HLW repositories.
SNF radioactivity decreases progressively during its storage (see Figure 1). For example, the radioactivity of SNF of a pressurized water reactor (PWR) (counted upon one ton of uranium) 10, 100, and 1000 years after its removal from the reactor will be approximately 400,000, 40,000, and 1700 Ci, respectively (Roddy et al., 1985). The strongest environmental impact would be expected if radionuclides escaped to groundwater at the earlier stages of SNF disposal, when the SNF contains short- and medium-lived radioisotopes with very high radioactivity. These isotopes decay in 500–1000 years. Current technology of long-term storage and disposal of HLW envisages HLW isolation from groundwater by engineered barriers for this or longer periods. The barriers comprise concrete tanks, corrosion-resistant containers, envelopes for nuclear fuel, and bentonite backfill. The SNF can interact with groundwater only after the engineered barriers lose their insulative properties. From this point on, actinides will present the main hazard for the biosphere. The geological medium will become the only barrier retaining the actinide migration. Below we analyze the conditions of safe SNF disposal provided by the insulative properties of the geological medium.