Because it is cheaper to store them on site than to dispose of them, while Japan awaits the opening of a reprocessing plant.

因为日本在等待后处理厂开工时，就地储存比处理它们成本低。

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However, there are two more things that you need: you need an enrichment plant and a reprocessing plant.

FORBES: Stuxnet and the Iranian Nuclear Program

The answer lies in its plant for reprocessing spent fuel, built partly to take used fuel from Germany and Japan (which largely paid for its construction) and partly to produce the plutonium that was once expected to be needed as fuel for fast-breeder reactors, in Britain and elsewhere.

ECONOMIST: British Nuclear Fuels: Horrid stuff | The

The Sellafield nuclear reprocessing plant has been shut down because of severe weather conditions.

BBC: Sellafield shutdown: Severe weather closes nuclear site