As usual, The Register has a contrary view.

What has happened is that one of the holding tanks, containing water that had only been through one stage of purification, has sprung a leak and about 300,000 litres of water has got out. Almost all of this was contained by a backup dam which had been built around the tanks when they were set up (this is the nuclear industry, there is always a backup). However, "two shallow puddles" of the water got out of the dam via a rainwater drain valve which has since been sealed off.

After some discussion of the difference between gamma and beta radiation:

So this is a pretty minor industrial-waste spill; thousands of more serious accidents occur every single day.
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Quality is addictive.