Centrifuges are typically used to separate undissolved fission product particles and fuel cladding debris from the dissolved nuclear fuel prior to reprocessing. A typical unit, used in the Thermal Oxide Reprocessing Plant (THORP) at Sellafield in the UK, and similar to the units used at the La Hague nuclear site in France, is shown in Fig. 3.4. This has a rotating inner bowl that spins at several thousand rpm contained within an outer container. As was described in Section 3.2 on the Type 3 PSC, the outer

container is permanently welded into the in-cell pipework, while the inner bowl is suspended on a shaft from the cell top and can be withdrawn through a cell roof plug into a flask if this is ever needed. The motor and gearbox are located outside the cell and so can readily be maintained without breaking the cell containment.

This unit is operated “batch continuously” with the dissolved nuclear fuel solution (“dissolver liquor”) being fed into the inner bowl for an extended period, with the clarified product overflowing from the top of the bowl, over a weir inside the outer container, so that it then flows from the bottom of the outer container to a receipt tank.

The removed solids progressively build up on the inner bowl wall and, when this layer reaches a set thickness, the feed of dissolved fuel is stopped and the centrifuge changed to solids recovery mode. In this mode, high pressure water jets from a spray lance inside the inner bowl remove the solids “cake” and slurry it from the bottom of the bowl into a separate receiving tank. Feed of the dissolved fuel is then restarted and the cycle is repeated. Two such centrifuges are usually operated together to provide continuous solids removal capability.