Fukushima Reactor 2 released two large plumes of radiation spike on March 15

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The belt of high-level radioactive contamination that extended to the northwest of the stricken Fukushima No. 1 nuclear power plant was the result of weather conditions on the afternoon of March 15, analysis by the Japan Atomic Energy Agency has shown.

Rainfall subsequently deposited large quantities of radioactive substances, discharged from the crippled No. 2 reactor, onto the ground.

The belt extends about 40 kilometers to the northwest of the nuclear plant and runs through the town of Namie and Iitate village.

The radiation dose survey results, released Sept. 1 by the government, also showed that zones of high doses were concentrated in the vicinity of the nuclear plant and to the northwest of the plant.

In the no-entry zone, the area within a 20-km radius of the nuclear plant, the maximum dose at a 1-meter height above ground was 139 microsieverts per hour in the Ottozawa district of Okuma town, about 1 km southwest of the nuclear plant.

In the planned evacuation zone, designated along the belt of high contamination beyond that radius, the maximum dose was 41.3 microsieverts per hour in the Hirusone district of Namie town, about 22 km to the northwest.

Haruyasu Nagai, leader of the Research Group for Environmental Science at the JAEA, estimated that large quantities of radioactive substances were discharged from the crippled No. 2 reactor over two periods: between 7 am and 11 am and between 1 pm and 3 pm, both on March 15.

During the second spell of discharge in the afternoon, a cloud of radioactive substances accumulated in gas form drifted first to the west and gradually changed direction to the northwest.

“Weak winds and slow plume movements contributed to the high concentrations of radioactive substances, but the largest contribution came from the rain,” said Hiromi Yamazawa, a professor of radiation-related environmental safety at Nagoya University.

Source: www.asahi.com, via Twitter search for Fukushima
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1 Comment

  1. Lot of discussion is going on why maximaum radioactive material was released from unit 2 of Fukushima. Pirma facie it looks surprising even unbelivable as unit 2 building is least damaged, no hydrogen explosion took place in unit 2, its cooling system (RCIC) lasted for 70 hrs after the earthquake, core damage started later as compared to other units and no venting was done from unit 2 containment. Then why more radioactive material got released from unit 2 than oter units. Answer of this is very simple, that is, because no venting was done or could not be done from unit 2 containment.
    As no venting could be done from unit 2, its containment got pressurised and it failed due to overpressure. Venting could not be done as rupture disc in hardened vent header did not rupture. Containment failure took place at weakest links such as penetrations, seals and gaskets. Activity released from drywell through these leaky points and came into reactor building. From reactor building it came into atmosphere through blowout panel of reactor building which had got opened during unit 1 hydrogen explosion on 12th March. Since release was from drywell, it was unfiltered release and that too it was ground level release. It is a case of ‘suppression pool bypass’.Because of failure of containment penetrations, water injected for core cooling came into turbine building through pipe trenches/cable trenches and it contributed in release through water route. In contrast, containment venting could be done in unit 1 and unit 3. This venting was done from suppression air space. Radioactive material coming to suppression air space has to pass through suppression pool water. During this passage much of the radioactivity (around 99%) is filtered out. Water acts as filter in this case, since most of the volatile fission products like cesium, iodine, telerrium get dissolved in it and particulates get suspended in it. Remaing 1% contains mostly noble gases which get dispersed in atmosphere, do not dissolve in water/rain and thus do not fall out on the ground.
    One line answer to the question why maximum radioactivity was released in atmosphere from unit 2 is that “Activity released from unit 2 was mostly unfiltered while from unit 1 &3 it was mostly filtered.”

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