Nuclear reactors produce radioactive by-products that decay at different rates. One common by-product is iodine-131 which has a half life of about 8 days while another is cesium-137 with a half life of about 30 years.
When a reactor switches off, the iodine decays more quickly so the ratio between these two isotopes changes rapidly over a period of days. That's why measuring this ratio is a good way to work out when the nuclear reactions terminated.
There are some complicating factors, however. The most important of these is that the ratio of iodine-131 and cesium-137 to start with depends on how long the reactor has been operating and so is not constant.
That's because, after a reactor has been switched on, the levels of iodine-131 reach an equilibrium on a timescale similar to its half life of about 8 days.
But cesium-137, with a half life of 30 years, takes much longer to reach equilibrium. To all intents and purposes, the levels of cesium-137 in a reactor continue to grow steadily during the timescales over which reactors are usually operated.
The Fukushima reactor was struck by a magnitude 9 earthquake at 14:46 local time on 11 March. The three operating reactors there were immediately shut down.
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