New data released from Japan may help researchers to determine exactly how radioactive materials were released into the atmosphere from the Unit 1 reactor during the early days of the Fukushima Daiichi nuclear disaster and determine the effectiveness of venting operations.
After the earthquake and tsunami struck Japan on March 11th, 2011, 14 monitoring posts around the Fukushima Daiichi nuclear power plant recorded radiation levels every 20 seconds.
The data recorded by the monitoring posts show that the radiation levels dramatically spiked an hour before the hydrogen explosion ripped apart the Unit 1 reactor building.
One post located 3.5 miles northwest of the plant recorded surging radiation levels after 14:10 on March 12th. The levels peaked at 4.6 millisieverts per hour around 14:40:40.
The annual exposure limit for members of the public is 1 millisievert, which the data shows could have been reached in around 20 minutes.
A senior researcher at the Japan Atomic Energy Agency, Masamichi Chino, says that the spike in radiation levels may have been caused by emergency venting operations meant to reduce pressure in the Unit 1 containment vessels, which were carried out around 14:00 by TEPCO workers.
The nuclear disaster at Fukushima Daiichi was the first time that Japan’s venting procedures were tested, but they failed to prevent the hydrogen explosions which released radioactive materials into the atmosphere.
Within 12 hours of the earthquake, pressure in the Unit 1 reactor was twice the design levels, which also made pumping coolant water into the reactor difficult.
By 06:00 AM on March 12th, the Unit 1 reactor had already experienced a meltdown and fuel was migrating into the lower part of the Reactor Pressure Vessel and leaking into the Containment Vessel. This allowed radioactive materials like Iodine-131, Cesium-134, Cesium-137, and Tellurium-132 to leak hours before the venting operations were performed.
It has been well-documented since the 1960s that the GE Mark 1 containment design is extremely small, which can lead to terrific pressure buildups which can destroy the reactor containments. The hydrogen which builds up also produces a risk of the hydrogen igniting after combining with oxygen from the water or in the atmosphere.
Venting operations are meant to reduce the pressure building up in the containment vessels and hydrogen concentrations in the containment, but also release radioactive materials. It is thought that lesser amounts of radiation would be released through venting operations than if hydrogen explosions were to damage the reactor containment.
When venting operations are carried out, all noble gases, Iodine, Cesium, and other radionuclides are released from the reactor to the atmosphere.
When workers vented the air, they scrubbed through water in order to reduce the amount of cesium which would be released. Scrubbing, or pushing the air through water in the Suppression Chamber binds a fraction of aerosols in the water. TEPCO officials thought that the scrubbing process would reduce cesium levels to 1/1000th of the original levels, but the data shows the measure may also not have been effective.