Nuclear Waste Management

Of particular concern in nuclear waste management are two long‐lived fission products, Tc‐99 (half‐life 220 000 years) and I‐129 (half‐life 15.7 million years), which dominate spent fuel radioactivity after a few thousand years. The most troublesome transuranic elements in spent fuel are Np‐237 (half‐life two million years) and Pu‐239 (half‐life 24 000 years) (Nuclear Decommissioning Authority 2014). Nuclear waste requires sophisticated treatment and management to successfully isolate it from interacting with the biosphere. This usually necessitates treatment, followed by a long‐term management strategy involving storage, disposal, or transformation of the waste into a nontoxic form (Vandenbosch and Vandenbosch 2007). Governments around the world are considering a range of waste management and disposal options, though there has been limited progress toward long‐term waste management solutions (Ojovan and Lee 2014).

In the second half of twentieth century, several methods of disposal of radioactive waste were investigated by nuclear nations (Brown 2004), which are as follows:

1. “Long term above ground storage,” not implemented
2. “Disposal in outer space” (for instance, inside the Sun), not implemented – as it would be currently too expensive
3. “Deep borehole disposal,” not implemented
4. “Rock‐melting,” not implemented
5. “Disposal at subduction zones,” not implemented
6. “Ocean disposal,” done by the USSR, the United Kingdom (World Nuclear Association 2011), Switzerland, the United States, Belgium, France, The Netherlands, Japan, Sweden, Russia, Germany, Italy, and South Korea (1954–1993). This is no longer permitted by international agreements.
7. “Sub seabed disposal,” not implemented, not permitted by international agreements
8. “Disposal in ice sheets,” rejected in Antarctic Treaty
9. “Direct injection,” done by USSR and United States.

In the United States, waste management policy completely broke down with the ending of work on the incomplete Yucca Mountain Repository (The Independent London 1997). At present, there are 70 nuclear power plant sites where spent fuel is stored. A Blue Ribbon Commission was appointed by President Obama to look into future options for this and future waste. A deep geological repository seems to be favored (The Independent London 1997).

Initial Treatment

Vitrification

Long‐term storage of radioactive waste requires the stabilization of the waste into a form which will neither react nor degrade for extended periods. It is theorized that one way to do this might be through vitrification (Blue Ribbon Commission 2012). Currently, at Sellafield the HLW (PUREX first cycle raffinate) is mixed with sugar and then calcined. Calcination involves passing the waste through a heated, rotating tube. The purposes of calcination are to evaporate the water from the waste, and denitrate the fission products to assist the stability of the glass produced.

The “calcine” generated is fed continuously into an induction heated furnace with fragmented glass (National Research Council 1996). The resulting glass is a new substance in which the waste products are bonded into the glass matrix when it solidifies. As a melt, this product is poured into stainless steel cylindrical containers (“cylinders”) in a batch process. When cooled, the fluid solidifies (“vitrifies”) into the glass. After being formed, the glass is highly resistant to water (Laboratory‐scale vitrification and leaching of Hanford high‐level waste for the…model verification 2009).

After filling a cylinder, a seal is welded onto the cylinder head. The cylinder is then washed. After being inspected for external contamination, the steel cylinder is stored, usually in an underground repository. In this form, the waste products are expected to be immobilized for thousands of years (Ojovan et al. 2006).

The glass inside a cylinder is usually a black glossy substance. All this work (in the United Kingdom) is done using hot cell systems. Sugar is added to control the ruthenium chemistry and to stop the formation of the volatile RuO₄ containing radioactive ruthenium isotopes. In the West, the glass is normally a borosilicate glass (similar to Pyrex), while in the former Soviet Bloc it is normal to use a phosphate glass (Nuclear Energy Agency, Paris 1994). The amount of fission products in the glass must be limited because some (palladium, the other Pt group metals, and tellurium) tend to form metallic phases which separate from the glass. Bulk vitrification uses electrodes to melt soil and wastes, which are then buried underground (Ojovan and Lee 2010). In Germany a vitrification plant is in use; this is treating the waste from a small demonstration reprocessing plant which has since been closed down (Pacific Northwest National Laboratory, PNNL‐15198, July 2005).

Ion Exchange

It is common for medium active wastes in the nuclear industry to be treated with ion exchange or other means to concentrate the radioactivity into a small volume. The much less radioactive bulk (after treatment) is often then discharged. For instance, it is possible to use a ferric hydroxide floc to remove radioactive metals from aqueous mixtures (Hensing and Schultz 1995). After the radioisotopes are absorbed onto the ferric hydroxide, the resulting sludge can be placed in a metal drum before being mixed with cement to form a solid waste form (Brünglinghaus, Waste Processing 2013). In order to get better long‐term performance (mechanical stability) from such forms, they may be made from a mixture of fly ash, or blast furnace slag, and Portland cement, instead of normal concrete (made with Portland cement, gravel, and sand).