UC Researchers Produce First Quantitative Estimate of
Fukushima Radiation Leak
Tiffany Kaiser - August 16, 2011
UC San Diego researchers concluded that 400 billion neutrons
were released per square meter surface of the cooling pools
at Fukushima Daiichi
The 9.0-magnitude earthquake that shook Japan and crippled
the reactor at Fukushima Daiichi nuclear plant back in March
caused quite a bit of havoc with the release of radioactive
water, contamination of crops and of course, the thousands
of lives lost. At the same time, news networks like CNN and
MSNBC sensationalized reports, causing unnecessary
nuclear-related fear. U.S. senators even demanded the
Nuclear Regulatory Commission to repeat an expensive
inspection of U.S. nuclear plants.
In an attempt to clear some confusion and understand exactly
how much radiation actually leaked from the damaged nuclear
reactor at the Fukushima Daiichi nuclear plant in Japan on
March 11, atmospheric chemists at the University of
California, San Diego, have produced the first quantitative
estimate of how much radiation actually leaked from the
Mark Thiemens, study leader and Dean of the Division of
Physical Sciences at UC San Diego, along with post-doctoral
researcher Antra Priyadarshi, and a team of researchers,
observed the amount of radioactive sulfur in the air soon
after the earthquake in Japan and was able to report a
quantitative measurement of the amount of radiation leaked.
When fuel rods melt, products like neutrons leak from the
fuel rods. Seawater is used to cool the hot reactors, and
absorbs the leaked neutrons. These neutrons "collide" with
chloride ions in the seawater, which results in the loss of
a proton out of the nucleus of a chloride atom and turns the
atom into a radioactive form of sulfur. Most of this
vaporizes into steam when the saltwater comes into contact
with the hot reactors, and to avoid explosions due to the
collection of hydrogen, operators vent the steam into the
atmosphere. Once in the air, the sulfur reacts with oxygen
to create sulfur dioxide gas and eventually sulfate
On the other side of the Pacific Ocean in La Jolla,
California on March 28, 2011, Thiemens and his team noticed
an "unprecedented spike" in radioactive sulfur in the air.
They used a model, which was based on the NOAA's
observations of atmospheric conditions, to determine the
path the air took to get to California over the previous 10
days, and found that it had come from Fukushima Daiichi.
The next step was to calculate how much radiation had leaked
from the reactor based on the path over the Pacific Ocean.
They took into account that some sulfate particles had
fallen into the ocean or decayed along the way, and
concluded that 400 billion neutrons were released per square
meter surface of the cooling pools. They predicted that this
occurred between March 13, 2011 and March 20, 2011. March 13
was when operators began flooding the reactor with seawater.
"You know how much seawater they used, how far neutrons will
penetrate into the seawater and the size of the chloride
ion," said Priyadarshi. "From that, you can calculate how
many neutrons must have reacted with chlorine to make
To achieve the levels observed in California, the team said
the concentrations a kilometer above the ocean close to
Fukushima must have been 365 times above normal levels. Over
the four days that the team took measurements, which ended
March 28, Thiemens measured 1501 atoms of radioactive sulfur
in sulfate particles per cubic meter of air. They mentioned
that this was the highest they had seen in two years of
observations and recordings.
According to the researchers, the radioactive sulfur
observed was produced by partially melted nuclear fuel in
the storage ponds or reactors. While cosmic rays can produce
radioactive sulfur, the team noted that these rays rarely
mix into the layer of air right above the ocean.
Despite the high levels of radioactive sulfur recorded in
California, Thiemens and his team said these levels were not
dangerous to human health.
"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," said
Thiemens. "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."