It was believed that around 23:00 JST the 4m long fuel rods in the reactor were fully exposed for the second time.[104][106] At 00:30 JST of 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at unit 3 may have caused a glitch in the cooling system of unit 2: Four out of five water pumps being used to cool unit 2 reactor had failed after the explosion at unit 3. In addition, the last pump had briefly stopped working.[107] To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. The unit's air flow gauge was accidentally turned off and, with the gauge turned off, flow of water into the reactor was blocked leading to full exposure of the rods.[104][108]

As of 04:11 JST, 15 March, water was being pumped into the reactor of unit 2 again.[109] At 10:38 JST, 15 March, water level was reported to be at 1.20 meters and rising.[110]

Explosion in reactor 2 building

An explosion was heard after 06:14 JST[111] on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel.[112][113] The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate all non-essential workers from the plant.[114] Only a skeleton crew of 50 men, also referred to as the Fukushima 50, was left at the site.[115] Soon after, radiation equivalent dose rates had risen to 8.2mSv/h[116] around two hours after the explosion and again down to 2.4mSv/h, shortly after.[117] Three hours after the explosion, the rates had risen to 11.9mSv/h.[118]

While admitting that the suppression pool at the bottom of the containment vessel had been damaged in the explosion, causing a drop of pressure there, Japanese nuclear authorities emphasized that the containment had not been breached as a result of the explosion and contained no obvious holes.[119]

In a news conference on 15 March the director general of the IAEA, Yukiya Amano, said that there was a "possibility of core damage" at the No. 2 unit of the damaged Fukushima power plant. He went on to add that the damage was estimated as being "less than five percent".[120] The Nuclear and Industrial Safety Agency stated 33% of the fuel rods were damaged, in news reports the morning of 16 March.[95]

Reactor unit 3

Unlike the other five reactor units, reactor 3 runs on mixed uranium and plutonium oxide, or MOX fuel, making it potentially more dangerous in an incident due to the neutronic effects of plutonium on the reactor and the carcinogenic effects in the event of release to the environment.[68][121][122] Units 3 and 4 have a shared control room. [123]

Cooling problems at unit 3

Early on 13 March 2011, an official of the Japan Nuclear and Industrial Safety Agency told a news conference that the emergency cooling system of Unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel in order to prevent a meltdown of its reactor core.[124] At 05:38 there was no means of adding coolant to the reactor due to loss of power. Work to restore power and vent pressure continued.[125] At one point, the top three meters of mixed oxide (MOX) fuel rods were not covered by coolant.[126]

At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health"[127] and manual venting took place at 08:41 and 09:20.[96] At 09:25 JST on 13 March 2011, operators began injecting water containing boric acid into the primary containment vessel (PCV) via a fire pump.[128][129] When water levels continued to fall and pressure to rise, the injected water was switched to sea water at 13:12.[125] By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased.[130] A rise was eventually recorded but the level stuck at 2m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.[96]

Injection of sea water into the PCV was discontinued at 01:10 on 14 March because all the water in the reserve pool had been used up. Supplies were restored by 03:20 and injection of water resumed.[129]

Explosion of reactor 3 building

At 12:33 JST on 13 March 2011, the chief spokesman of the Japanese government, Yukio Edano said that hydrogen was building up inside the outer building of unit 3 just as it had in unit 1, threatening the same kind of explosion.[131] At 11:15 JST on 14 March 2011, the envisaged explosion of the building surrounding Reactor 3 of Fukushima 1 occurred, due to the ignition of built up hydrogen gas.[132][133] It was reported that as with unit 1, the top section of the reactor building was blown apart, but the inner containment vessel was not breached. The explosion was larger than that in unit 1 and felt 40 kilometers away. Pressure readings within the reactor remained steady at around 380kPa at 11:13 and 360kPa at 11:55 compared to nominal levels of 400kPa and a maximum recorded of 840kPa. Water injection continued. Radiation rates of 0.05mSv//h were recorded in the service hall and of 0.02mSv/h at the plant entrance.[134] Eleven people were reported injured in the blast.[135][136]

On the morning of 15 March 2011 (JST), Secretary Edano announced that according to the TEPCO, at one location near reactor Units 3 and 4, radiation at an equivalent dose rate of 400mSv/h was detected,[26][27][28] but this might have been due to debris from the explosion in unit 4.[137]

Around 10:00 JST, 16 March, NHK helicopters flying 30km away videotaped white fumes rising from the Fukushima I facility. Officials suggested that the reactor 3 building was the most likely source, and said that its containment systems may have been breached.[138] The control room for reactors 3 and 4 was evacuated at 10:45 JST but staff were cleared to return and resume water injection into the reactor at 11:30 JST.[139] At 16:12 JST Self Defence Force (SDF) Chinook helicopters were preparing to spray water on unit 3, where white fumes rising from the building was believed to be water boiling away from the fuel rod cooling pond on the top floor of the reactor building, and on unit 4 where the cooling pool was also short of water. The mission was cancelled when helicopter measurements reported radiation levels of 50mSv.[140][141] At 21:06pm JST government reported that major damage to reactor 3 was unlikely but that it nonetheless remained their highest priority.[142] Early on 17 March, TEPCO requested another attempt by the military to put water on the reactor using a helicopter[143] and water dumps were underway by 9:45 JST.[144]

Reactor unit 4

At the time of the earthquake unit 4 had been shut down for maintenance and refueling since 30 November 2010.[145] All fuel rods had been transferred in December 2010 from the reactor to the spent fuel pool on the top floor of the reactor building[28] where they were held in racks containing boron to damp down any nuclear reaction.[137] These recently active fuel rods were hotter and required more cooling than the spent fuel in units 5 and 6.[146] At 04:00 JST on Monday 14 March water in the pool had reached a temperature of 84°C compared to a normal value of 40-50°C.[137]

/ Wikinews has related news: Fukushima reactor suffers multiple fires, radiation leak confirmed

At approximately 06:00 JST on 15 March, a loud explosion was heard within the power station, and later it was confirmed that the 4th floor rooftop area of the Unit 4 reactor building had sustained damage.[147] At 09:40 JST on 15 March 2011, the Unit 4 spent fuel pool caught fire, likely releasing radioactive contamination from the fuel stored there.[148][149] TEPCO said workers extinguished the fire by 12:00.[150][151] As radiation levels rose, some of the employees still at the plant were evacuated.[152] The reason for the fire seems to have been a hydrogen explosion.[153]

On the morning of 15 March 2011 (JST), Secretary Edano announced that according to the Tokyo Electric Power Company, radiation dose equivalent rates measured from the reactor unit 4 reached 100 mSv per hour.[26][27] Government speaker Edano has stated that there was no continued release of radiation.[154] The dose after which the symptoms of acute radiation poisoning typically appear is approximately 1000mSv, or 1Sv, received over one day. An exposed worker would be expected to begin experiencing radiation sickness soon after receiving a 100mSv/h dose rate for 10 hours of a day, or a 400mSv/h dose rate for 2.5 hours of a day.

Japan's nuclear safety agency reported two holes, each 8 meters square (64 m2 or 689 sq. feet -- not 8 sq. meters each) in a wall of the outer building of the number 4 reactor after an explosion there.[155] Further, at 17:48 JST it was reported that water in the spent fuel pool might be boiling.[156][157]

As of 15 March 2011 21:13 JST, radiation inside unit 4 had increased so much inside the control room that employees could not stay there permanently any more.[158] Seventy staff remained on site but 800 had been evacuated.[159] By 22:30 JST, TEPCO was reported to be unable to pour water into No. 4 reactor's storage pool for spent fuel.[137] At around 22:50 JST, it was reported that TEPCO was considering using helicopters to drop water on the spent fuel storage pool.[159][160][161] However, TEPCO soon dismissed the option of helicopters because of concerns over safety and effectiveness. TEPCO went on to consider the use of high-pressure fire hoses instead.[162]

A fire was discovered at 05:45 JST on 16 March in the north west corner of the reactor building by a worker taking batteries to the central control room of unit 4.[163][164] This was reported to the authorities, but on further inspection at 06:15 no fire was found. Other reports stated that the fire was under control.[165] At 11:57 JST, TEPCO released a photograph of No.4 reactor showing that "a large portion of the building's outer wall has collapsed."[166] Technicians were reported to be considering spraying boric acid on the building from a helicopter.[167][168]

Possibility of criticality in the spent fuel bay

This new fire cast into doubt the earlier hope that the Tuesday blaze in the Unit 4 housing was caused by lubricating oil pumps; instead at approximately 14:30, TEPCO announced its belief that the storage pool may have begun boiling, raising the possibility that exposed rods would reach criticality.[169][170][29] BBC commented that criticality would not mean a nuclear bomb-like explosion; however, a sustained release of radioactive materials would be a possible scenario.[169]

Around 20:00 JST on 16 March it was planned to use a police water cannon to spray water on unit 4.[171]

Julius Andreev, former director of the Soviet Spetsatom clean-up agency involved in the Chernobyl clean-up, as well as Laurence Williams, professor of nuclear safety at the University of Central Lancashire, speculate that the Fukushima management could have been engaged in an unsafe industry practice of re-racking spent rods in the pool well beyond its rated capacity, in effect heightening danger of melting and pool boil-off. [172][173]

On 16 March the chairman of United States Nuclear Regulatory Commission, Gregory B. Jaczko, said in Congressional testimony that the NRC believes all of the water in the spent fuel pool has boiled dry.[174] Although Japanese nuclear authorities and TEPCO downplayed the concern, with a TEPCO spokesman saying "[w]e can’t get inside to check, but we’ve been carefully watching the building’s environs, and there has not been any particular problem," later in the day Jaczko stood by his claim saying it had been confirmed by sources in Japan.[175]

Reactor units 5 and 6

Both reactors were off line at the time the earthquake struck (reactor 5 had been shut down on 3 January 2011 and reactor 6 on 14 August 2010). Although an IAEA report indicated that the fuel rods are still in the reactor vessels of both units and not in the spent fuel pools as in Unit 4,[176] Kyodo News said that there were rods in the pools, but only one-third as many in the pools as compared to Unit 4.[177]

Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well.[154][178] At 21:00 on 15 March water levels in unit 5 were reported to be 2m above fuel rods, but were falling at a rate of 10cm per hour. Unit 6 was reported to have operational diesel generated power and this was to be used to power pumps in unit 5 to supply more water.[28]

The removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape.[176] The BBC later reported that units 5 and 6 were believed to be heating up.[179] At 18:31 on 16 March, TEPCO was reported to be pouring water into both reactors.[180]

Reactor status summary

The Japan Atomic Industrial Forum (JAIF) has developed a status summary table for the Fukushima nuclear power plants and is publishing semi-regular updates.[181][182]

Status of FukushimaI at 9:00 JST(UTC+9) on 17 March[182] / Unit 1 / Unit 2 / Unit 3 / Unit 4 / Unit 5 / Unit 6
Power output (MWe) / 460 / 784 / 784 / 784 / 784 / 1100
Type of reactor / BWR-3 / BWR-4 / BWR-4 / BWR-4 / BWR-4 / BWR-5
Status at earthquake / In service / In service / In service / Outage (scheduled) / Outage (scheduled) / Outage (scheduled)
Fuel integrity / 70% damaged[95] / 33% damaged[95] / Damaged, dangerous MOX fuel includes plutonium / Spent Fuel Damaged / Not damaged / Not damaged
Containment integrity / Not damaged / Damage suspected / Damage suspected / Not damaged (defueled) / Not damaged / Not damaged
Core cooling system 1 (ECCS/RHR) / Not functional / Not functional / Not functional / Not necessary (defueled) / Not necessary / Not necessary
Core cooling system 2 (RCIC/MUWC) / Not functional / Not functional / Not functional / Not necessary (defueled) / Not necessary / Not necessary
Building integrity / Severely damaged / Slightly damaged / Severely damaged / Severely damaged / Not damaged / Not damaged
Pressure vessel, water level / Around half of the fuel / Recovering after dried-up / Around half of the fuel / Safe (defueled) / Safe but dropping / Safe
Pressure vessel, pressure / Stable / unknown; battery dead / Stable / Safe (defueled) / Safe / Safe
Containment pressure / Stable / D/W: Unknown, S/P: Atmosphere / Stable / Safe (defueled) / Safe / Safe
Seawater injection into core / Continuing / Continuing / Continuing / Not necessary (defueled) / Not necessary / Not necessary
Seawater injection into containment building / Continuing / To be decided / Continuing / Not necessary / Not necessary / Not necessary
Containment venting / Continuing / Preparing / Continuing / Not necessary / Not necessary / Not necessary
Integrity of fuel in Spent Fuel Pool (SFP) / (no data) / (no data) / SFP level low,
Preparing water injection / SFP level low,
Preparing water injection,
Damage to fuel rods suspected (700 rods total)[citation needed] / SFP temperature increasing / SFP temperature increasing
Environmental effect (NPS border) / 1472 μSv/hour (1.472 mSv/hr) at 16:20 on 16 March
Evacuation radius / 20km from Nuclear Power Station (NPS). People who live between 20km to 30km from the FukushimaI Nuclear Power Station are to stay indoors.
INES / Level 4 (estimated by Japanese NISA and accepted by the international IAEA); Level 6 (estimated by the French nuclear authority and the Finnish nuclear authorities)[2][183][184]

Radioactive contamination

Radiation readings from the TEPCO website for 3 months before the earthquake[185]

Radiation readings from TEPCO during the accident

Radiation levels at the stricken Fukushima I power plant have varied up to 1,000mSv/h(millisievert per hour),[6] which is a level that can cause radiation sickness.[186] The level of radiation within the 20 km exclusion zone surrounding the power plant is such that people have been advised to evacuate, and people within the 20-30km zone are being advised to stay indoors.[187]

Chief Cabinet Secretary, Yukio Edano, said that on 15 March 2011 radiation rates had been measured as high as 30mSv/h between the Units 2 and 3, as high as 400mSv/h[28][26] near Unit 3 between it and Unit 4, and 100mSv/h near Unit 4. He indicated that "There is no doubt that unlike in the past, the figures are the level at which human health can be affected,"[188] Prime Minister Naoto Kan urged people living between 20 and 30 kilometers of the plant to stay indoors, "The danger of further radiation leaks (from the plant) is increasing," Kan warned the public at a press conference, while asking people to "act calmly".[189][190]

A spokesman for Japan's nuclear safety agency said TEPCO had told it that radiation levels in Ibaraki, between Fukushima and Tokyo, had risen. "The level does not pose health risks," the spokesman said. The Tokyo metropolitan government said it has detected radioactive material, such as iodine and cesium, up to 40 times normal levels in Saitama, near Tokyo.[189][188] Radiation levels in Tokyo were at one point measured at 0.8μSv/hour although they were later measured at "about twice the normal level".[191] Later, on 15 March 2011, Edano reported that radiation levels were lower. A changed wind direction dispersed radiation away from the land and back over the Pacific Ocean.[192] Thousands of Tokyo residents are reported to have left for cities further south, although Edano insisted that levels in Greater Tokyo were not hazardous.[193]

On 16 March power plant staff were briefly evacuated after smoke rose above the plant and radiation levels surged to 1,000mSv/h before coming down to 800–600mSv/h, and staff returned.[6] Japan's defence ministry criticized the nuclear safety agency and TEPCO after some of its troops were possibly exposed to radiation when working on the site.[194] The Japan's ministry of science measured radiation levels of up to 0.33 millisieverts per hour 20 kilometers northwest of the power plant.[195]

International commentators were divided in their analysis of the scale of the danger, with French Foreign Minister, Alain Juppe, saying that the threat was "extremely high" while others said it was too early to make comparisons to the 1986 Chernobyl disaster.[193]