Rise in radiation an hour before the explosion

March 11, 2014

Radiation surge detailed in 2011 accident

http://www3.nhk.or.jp/nhkworld/english/news/20140311_33.html 

Data recorded by radiation monitoring posts near the Fukushima Daiichi nuclear power plant show the environmental radiation level rose sharply 1 hour before a hydrogen explosion took place at the plant.

14 monitoring posts around the plant recorded the radiation level every 20 seconds after the plant was damaged by the earthquake and tsunami on March 11th of 2011.

Data recorded by one of the monitoring posts, located 5.6 kilometers northwest of the plant, show that the radiation level began surging after 2:10 PM on March 12th.

At 2:40 and 40 seconds, the post measured 4.6 millisieverts per hour, the highest level of the day. That was about 1 hour before a hydrogen explosion occurred at the No.1 reactor of the plant.

The data suggest the accumulated doses of radiation would have reached 1 millisievert in about 20 minutes. 1 millisievert is the annual exposure limit for ordinary people.

Masamichi Chino, senior researcher at the Japan Atomic Energy Agency, says the rise in the environmental radiation level may have been caused by an emergency operation to protect the No.1 reactor by reducing pressure within the containment vessel. Tokyo Electric Power officials began the so-called vent work at around 2:00 PM.

The vented air was released after going through water to reduce the amount of radioactive cesium. The step is intended to reduce the substance to 1 thousandth of its original level. But the measure may not have been effective.

Chino says the data can help researchers investigate how radioactive substances were released into the atmosphere and study the effectiveness of the venting process.

Mar. 11, 2014 - Updated 12:09 UTC