The first estimate of the amount of radiation that leaked from the damaged Fukushima nuclear power plant in the wake of the devastating tsunami have been released by atmospheric chemists from the University of California, San Diego.
Their estimates, which were published in the early online edition of the journal Proceedings of the National Academy of Sciences, suggest that 400 billion neutrons were released per square meter surface of the Fukushima cooling ponds between March 13 — when engineers started pumping seawater into the cooling ponds — and March 20.
This equates to levels approximately 365 times higher than natural levels.
“In any disaster, there’s always a lot to be learned by analysis of what happened,” said senior author Mark Thiemens, Dean of the Division of Physical Sciences at UC San Diego. “We were able to say how many neutrons were leaking out of that core when it was exposed.”
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The measurements were taken by a sensitive instrument at the end of the pier at UC San Diego’s Scripps Institution for Oceanography, where Thiemens and his group continuously monitor atmospheric sulphur levels. On March 28, 2011, 15 days after engineers at the Fukushima power plant had begun pumping seawater into the damaged reactors and pools holding the spent fuel rods, Thiemens’ group observed an unprecedented spike in the amount of radioactive sulphur in the air in La Jolla, California.
When the fuel rods at the Fukushima power plant started melting, neutrons and other products started to leak. The seawater absorbed these neutrons, which collided with chloride ions in the salt water which, in turn, knocked a proton out of the nucleus of a chloride atom, transforming the atom into a radioactive form of sulphur.
When the water hit the reactors, the vast majority of the water was vaporized into steam and vented into the atmosphere: along with the radioactive sulphur.
These radioactive sulphur then travelled across the Pacific Ocean on prevailing westerly winds to the Scripps instrument which recorded the levels. The team were then able to use a model based on NOAA’s observations of the atmospheric conditions at the time to determine that the radioactive sulphur had come from Fukushima.
They then calculated how much radiation must have been released in order to leave such a signature over California.
“You know how much seawater they used, how far neutrons will penetrate into the seawater and the size of the chloride ion. From that you can calculate how many neutrons must have reacted with chlorine to make radioactive sulfur,” said Antra Priyadarshi, a post-doctoral researcher in Thiemens’ lab and first author of the paper
“Although the spike that we measured was very high compared to background levels of radioactive sulfur, the absolute amount of radiation that reached California was small. The levels we recorded aren’t a concern for human health. In fact, it took sensitive instruments, measuring radioactive decay for hours after lengthy collection of the particles, to precisely measure the amount of radiation,” Thiemens said.
Source: University of California, San Diego
Image Source: Gerardo Dominguez