Today, an insight into the conditions in the region surrounding the Fukushima Nuclear Power plant soon after the magnitude 9 earthquake and resultant tsunami which caused the reactors to explode.
Fukushima Medical University sits some 60 kilometres northwest of the power station. In the run up to the accident, a physicist at the university, Tsuneo Konayashi, had been measuring the background levels of gamma radiation, the numbers of secondary particles from cosmic ray impacts and the amount of radon in the atmosphere.
So when the accident struck, Konayashi and his colleagues were in a good position to measure exactly how things changed.
First, the background levels of gamma radiation changed little immediately after the earthquake but then spiked, reaching 9.3 times the usual levels on 16 March, five days after the quake and just hours after a hydrogen explosion occurred at the plant That’s a level of 11.9 micro Seiverts per hour.
By August, Konayshi says the levels had dropped to 1.5 times the usual levels.
Indoor levels were significantly lower. The Japanese Self-Defense Force screened people entering the university and anyone with more than 10,000 counts per minute had to be decontaminated at separate decontamination tent before entering the campus.
“The maximum value found was 100,000 cpm,” says Konayashi. That’s a significant level of contamination but it’s not possible to say exactly how high because the number of counts for a given level of radiation depends on the measuring apparatus. Other news reports on the web mention a worker at the plant who was sprayed with highly radioactive water while moving a hose leading to readings of 100,000 cpm.
Konayashi says his data can best be explained by fitting it to a model in which there are two types of radiation with short and long half lives. The best fit is for short-lived isotopes, such as iodine-131, that decay with an average half life of about 3.6 days, and with the longer-lived ones, such as cesium-134 and strontium-90, having a half life of 181 days.
The radon levels were unaffected in most places. However, the levels rose appreciably in some indoor locations because the university had reduced ventilation to prevent radioisotopes from entering the building and this also prevented the radon from escaping.
In some places the levels rose to 250 becquerels per cubic metre, almost double the maximum allowable levels in the US. These places had to be sealed off from normal use.
Curiously, the cosmic ray measurements decreased slightly soon after the accident, a phenomenon that Konayashi puts down to an increase in atmospheric pressure which would have shielded the ground from these particles.
During the first few months, Konayashi says he was distributing his data round the campus on a daily basis, to let people know whether there had been further leaks at the plant.
Konayashi says efforts to monitor radiation are ongoing and look likely to continue for the foreseeable future.
We wish them well.
Ref: arxiv.org/abs/1111.2395: Radiation Measurements At The Campus Of Fukushima Medical University Through 2011 Off The Pacific Coast of Tohoku Earthquake And Subsequent Nuclear Power Plant Crisis