Fig. 2.6. Typical wastes from reactor operations (image courtesy of ANDRA).

2.4.1.4. Reprocessing

A SF assembly essentially consists of a large number of long rods (containing the spent fuel pellets), which are held in place by grids and relatively large metallic end and bottom pieces. The objective of reprocessing is to separate the fission and transuranic elements (4 per cent) from the uranium (95 per cent) and plutonium (1 per cent) contained within the spent fuel pellets (Fig. 2.7). In relation to the 25 tonnes of fuel mentioned above, reprocessing would result in 1 tonne of fission products and transuranics, 23.75 tonnes of uranium and 0.25 tonnes of plutonium.

Fig. 2.7. A simplified overview of the reprocessing process (image courtesy of AREVA-NC).

The commercial process basically consists of chopping the fuel rods into small pieces and dissolving the fuel pellets in concentrated nitric acid. The liquid containing the waste fission products and transuranics is then separated from the uranium and plutonium. Following this, the uranium and plutonium are then separated from each other, purified and re-used. The high-level waste liquid containing the fission products and actinide (both transuranics and residual uranium and plutonium) isotopes is dried by a calcination process to produce a solid which is then mixed with borosilicate glass and vitrified. The resulting glass is then cast into stainless steel containers and stored. For geological disposal, it is envisaged that each of these containers will be separately placed in a massive metal container (overpack) (e.g., steel in the Swiss and Japanese concepts (Nagra, 1985; JNC, 2000), copper in the Swedish (Werme, 1998) and Finnish (Raiko and Salo, 1999) concepts – see also Chapter 5 for further details).

After dissolution of the spent fuel, the remaining fuel structural materials (Fig. 2.8) and heavily contaminated materials (such as pumps, clothing, etc.) and process residues (effluent treatment) are conditioned as L/ILW-LL. Other wastes, such as lightly contaminated materials, are conditioned and disposed of as L/ILW-SL.

2.4.1.5. Reactor decommissioning

On closure of nuclear facilities, the buildings are generally decontaminated and dismantled. In the case of NPPs, the first stage of decommissioning consists of the removal of all of the SF, accounting for 99 per cent of the radioactivity, from the facility. This then allows the facility to move from an operating to a decommissioning licence. Following this, the reactor (Fig. 2.9) is then completely dismantled to a greenfield site (with no conditions on its subsequent use), with the number of stages and the amount of time required to reach this final stage being dependent on the approved strategy.