Researchers from the University of Tokyo, the Japan Atomic Energy Agency and other experts have examined the crisis response log from the Fukushima Daiichi nuclear disaster. They also checked radiation levels in surrounding areas, namely the radiation data from the Fukushima Daini plant 10 kilometers south of the Daiichi plant.
They found that workers attempts to prevent explosions at the Unit 2 reactor may have in fact caused a substantially large radiation leak. They proved that hypothesis by pairing data of the recorded radiation levels which rose sharply 3 times at Fukushima Daini monitoring posts over a period of 5 hours during the night of March 14th.
The report reveals that by that time, the amount of nuclear fuel which had already melted down in the core was already leaking into the containment vessel, filling it with radioactive materials, prior to the start of seawater injection operations.
The researchers report shows that workers were unable to get coolant water into the Unit 2 reactor, due to the high pressures inside of the containment from steam leaking out of the reactor pressure vessel due to core damage and fuel meltdown.
When the water level in the reactor pressure vessel increased after the seawater injection, the pressure in the containment rose as well until it reached over two times the designed limits, which combined with the extreme heat would threaten to crack or damage the concrete containment vessel.
These rises came 1 hour after each time workers at Unit 2 released radioactive steam from the core in a critical effort to lower its pressure, and prevent an explosion the likes of which had devastated Unit 1 and Unit 3 reactor buildings.
The experts propose that the radioactive materials escaped from cracks in the concrete containment vessel, and were carried south towards the Fukushima Daini nuclear power plant.
This is also corroborated by what data has been revealed regarding the radiological release, which also implies a substantial amount of radioactivity was released prior to the explosion at 06:00 JST on March 15th.
The largest spike of radiation measured at Fukushima Daiichi in the first week, were measured around or shortly after 00:00 on March 15th, which correlates with TEPCO’s documented ‘dry vent’ of Unit 2. A ‘dry vent’ operation refers to releasing gas directly from the drywell pressure release valves into the outside air, without passing it through the pipes and the water in the suppression chamber, which may filter part of the isotopes.
As the fission products were being directly released into the environment, inside of the reactor their absence would have been quickly replaced by newly-produced steam from the melted nuclear fuel.
Tokyo Electric Power Company, the operator of the Fukushima Daiichi nuclear power plant, says opening the valve was the only solution at the time.
The Japan Atomic Energy Agency’s Masamichi Chino says the leaks were serious and need to be further studied even though they occurred as a result of work to protect the reactor. Researchers say the radiation from Fukushima Daiichi Unit 2 was 10 to 20 times higher than the other Units onsite which were devastated by catastrophic hydrogen explosions.
After the explosion was heard inside of Unit 2, the data also suggests that even larger amounts of radioactivity were released, possibly in combination with other units.
TEPCO would later hypothesize due to elevated temperatures, among other factors, that a breach roughly 10 cm wide was created in the reactor’s containment vessel 21 hours after the quake. Tepco also said it believes that parts used to ensure air tightness may have broken from overheating.
Earlier this year the details emerged that the core of Unit 2 had completely liquefied, and has yet to be located inside of the reactor building.
Co–editors: Larry B; Eric G; Vince Guarisco