Anne Miller, Facebook asked:
How can water become radioactive? Or is the water just carrying radioactive particles which could technically be filtered out?
We put this question to Dr Ian Farnan from the Department of Earth Sciences at Cambridge University:
That's the way that the radioactivity is transported away from the reactor. Small particles and elements like caesium will actually dissolve in water very readily, so will the iodine of course. People are probably familiar with iodine solutions. Those can be separated out using ion exchange resins. In fact, when people operate power stations, they regularly filter the water which is used in the cooling cycles or the heat transfer cycles with these resins and then those resins form part of our nuclear waste inventory. But the problem was that we had this large leak, a crack, and it was going out unregulated. One of the operations that's going on now is that the plant operators are asking the Japanese government for permission to release water from a holding pool and then let some of the more radioactive water into the waste treatment plants. They want to release this water from the waste treatment plant and let this more radioactive water into that plant so it can go through all this filter system.