Monday, 2 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
T. Matsui of the (Fukushima Nuclear Power Plant) Fukushima University of Tokyo Institute of Physics releases a scientific paper analysing the ratio of iodine-131 to caesium-137 taken from water samples, which concludes that a recriticality may have occurred at least 10 - 15 days after the attempted shutdown.
Thursday, 5 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
Workers enter the reactor 1 building. This is the first time since the start of the crisis that a reactor building in the plant is visited by a human being. The workers will connect a ventilation system that should absorb radiation inside the building for the next 4–5 days, allowing them to start installation of the cooling system replacement. Because of protective gear the workers were only exposed to a small amount of radiation (about 2 mSv). TEPCO expects to bring the plant into a cold shutdown within six to nine months. IAEA releases a briefing.
Tuesday, 10 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
In a press release, TEPCO reports that levels of cesium-134, cesium-136, cesium-137, and iodine-131 (half-life of ~ 8 days), had spiked since last sampled on March 2, 2011, when these four nuclides were below detection limits. TEPCO's report gives the newly measured concentration (Bq/cm3) of each nuclide as of sampling date, May 8.
Thursday, 12 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
TEPCO engineers confirm that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor's containment vessel. The utility says fuel rods of the No. 1 reactor are fully exposed with the water level 1 meter (3.3 feet) below the base of the fuel assembly. The government and TEPCO are described as "consistently appeared to be underestimating the severity of the situation.” According to a Japanese press report, there are holes in the base of the pressure vessel, and most of the fuel has likely melted. The nuclear fuel has possibly leaked into the containment vessel, which was damaged by an explosion during the crisis.[188] However, the Nuclear Energy Institute, a nuclear lobbying firm, states that the situation "is in no way alarming. It was anticipated that there was fuel damage in reactors 1, 2 and 3. This is confirmation.
Saturday, 14 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
A third TEPCO (contractor) employee dies, after falling ill at 06:50, being brought to the plant's medical room unconscious. The likely cause of death is a heart attack. TEPCO says he was exposed to 0.17 millisieverts of radiation on Saturday.
Sunday, 15 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
A robot sent to the first floor of unit 1 records a radiation level of 2,000 mSv/h. At this level, workers would only be allowed to stay in the area for 8 minutes. In addition, the reactor's containment vessel is leaking large amounts of water into the basement. A TEPCO worker is able to peer into the basement and determines that the 11 m deep basement is approximately half full of water.
Wednesday, 18 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
Four workers in protective suits and SCBA enter unit 2 for the first time since the March 15 explosion, to check on radiation levels and other conditions inside the building. The workers receive a dose of between 3 and 4 mSv each.
Friday, 20 May 2011.TEPCO president Masataka Shimizu resigns after reporting the largest financial losses in the company's history.
Sunday, 22 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
TEPCO reports that reactor 3 leaked at least 250 tons of radioactive water into the Pacific Ocean over a period of 41 hours beginning on May 10, 2011.
Tuesday, 24 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
On the eve of the arrival in Tokyo of a delegation from the International Atomic Energy Agency, TEPCO admits that the cores of reactor 2 and reactor 3 also melted down in the days immediately following the earthquake in mid-March, 2011. 16 hours after the earthquake and SCRAM, the fuel rods of reactor 1 had "mostly melted and fallen into a lump at the bottom of the pressure vessel — a state that TEPCO officials have described as a 'meltdown'". A TEPCO spokesman Yoshimi Hitosugi stated last night, "The situation inside two and three is almost the same." TEPCO further stated that the fuel in reactor three took about 60 hours to melt and that the reactor melted down 100 hours after the magnitude nine quake struck.
Wednesday, 25 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
TEPCO informs the Nuclear and Industrial Safety Agency and the government of Fukushima Prefecture of the results of soil tests for plutonium (238
Pu, 239Pu and 240 Pu) carried out around the Fukushima Daiichi plant. While the levels were comparable to the fallout in Japan from atmospheric nuclear testing, TEPCO deemed that the plutonium had originated from the accidents.
Saturday, 28 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
TEPCO informs the Nuclear and Industrial Safety Agency and the government of Fukushima Prefecture of the results of soil tests for uranium (234
U, 235U and 238U) carried out around the Fukushima Daiichi plant. The uranium found was considered to be natural, as its isotope ratios were consistent with the natural abundance. At 21:14 a cooling pump at reactor five stops. At 08:12 the next day, work began on a spare pump, and cooling was restored at 12:49. The reactor temperature had risen to 92.2 °C. The cause of the outage is suspected to be motor failure.
Sunday, 29 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
It is reported that 22 out of 23 radiation monitoring systems around the Fukushima plants were disabled by the earthquake and tsunami. Some were directly damaged, but most were disabled due to communication and power lines being cut. Even monitors equipped with backup satellite links failed, probably due to antenna damage. In Miyagi prefecture, 4 out of 7 were disabled by the tsunami, with the remaining three stopping after three hours. In Ibaraki, some 40 monitors stopped working for three hours until power could be restored.
The first of the typhoons of the season is due to strike the area, while Japan states radiation levels at the seabed are several hundreds of times above normal levels off the coast of Fukushima. "The Science Ministry announced late on Friday highly radioactive materials were detected in a 300-km north-south stretch from Kesennuma in Miyagi Prefecture to Choshi in Chiba Prefecture, the Kyodo news agency reported.
Tuesday, 31 May-2011-Timeline of the Fukushima Diichi Neclear Disaster
An oil spill near reactors five and six is detected at 8:00 am, as well as an explosion heard at 2:30 pm near reactor four. TEPCO reports that the explosion was the bursting of an oxygen cylinder damaged by unmanned machinery during debris removal.TEPCO states that there was a temporary oil leak into the sea near the plant, from an oil pipe that may have been damaged in the March disaster. It is stated as being an extremely small leak, possibly caused by recent rainy weather from Typhoon Songda. TEPCO says that the leak has stopped and oil fences have been installed to prevent the liquid from spreading into the Pacific Ocean.
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