Chain reactions reignited at Fukushima nuclear plant after tsunami: study

By KentuckyFC 
9 May 2011 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. … Cesium-137, with a half life of 30 years, takes much longer than iodine-131 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. Today, Tetsuo Matsui at the University of Tokyo, says the limited data from Fukushima indicates that nuclear chain reactions must have reignited at Fuksuhima up to 12 days after the accident [pdf]. Matsui says the evidence comes from measurements of the ratio of cesium-137 and iodine-131 at several points around the facility and in the seawater nearby. He has calculated what the starting ratio must have been by assuming the reactors had been operating for between 7 and 12 months. … “The data of the water samples from the unit-4 cooling pool and from the sub-drain near the unit-2 reactor show anomaly which may indicate, if they are correct, that some of these fission products were produced by chain nuclear reactions reignited after the earthquake,” he says. These chain reactions must have occurred a significant time after the accident. “It would be difficult to understand the observed anomaly near the unit-2 reactor without assuming that a significant amount of fission products were produced at least 10 – 15 days after X-day,” says Matsui. So things in reactor 2 must have been extremely dangerous right up to the end of March. …

Chain Reactions Reignited At Fukushima After Tsunami, Says New Study via Ex-SKF