(UPDATE 5) Report: Fire Restarted at Fukushima Daiichi Unit 4, 70 Percent of Unit 1 Fuel Damaged

UPDATED 11:18 PM EST --  Video footage from an NHK TV helicopter showed either smoke or steam billowing from the Fukushima Daiichi nuclear power plant. Press conferences for two Japanese government agencies held just before 11 p.m. EST indicate the steam may be coming from unit 3. Radiation levels are too high for workers to approach the reactor building, so it is unknown whether if smoke and/or vapor is coming from the pool storing partially spent MOX fuel.

At 10:45 a.m. local time, radiation levels at the plant gate peaked at 6.4 millisieverts per hour before declining to about 3.4 millisieverts at 11 a.m. Tepco officials guess the increased radiation came from the unit 2 reactor, which is believed to have a leak in its suppression chamber.

Reports indicate the government has more than doubled the legal maximum radiation exposure for plant workers to 250 millisieverts so they can approach more areas of the damaged plant.

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UPDATED 8:04 PM EST -- Japanese media have reported that flames were again sighted above the Fukushima Daiichi unit 4 building, but that they are no longer visible.

Japanese authorities are now considering using a helicopter to drop neutron-absorbing boric acid into the unit's spent fuel tank, as the reactor is too radioactive for workers to approach, according to NHK TV.

Kyodo news also is quoting Tepco as saying 70 percent of the fuel inside reactor unit 1 has been damaged, as has 33 percent of the fuel in unit 2.

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UPDATED 6:35 P.M. EST – The latest news from Japan’s Fukushima Diiachi power plant is mixed, with a reactor building fire extinguished but power-failure-induced cooling problems now affecting parts of all six units.

A fire is no longer burning at unit 4, although reports conflicted Tuesday on the fire's exact location within the plant. At a press conference Director General Yukiya Amano said the fire had occurred in the spent-fuel tank, which likely created the spike in radiation observed outside the unit. Earlier, the Nuclear Energy Institute quoted plant owner Tokyo Electric Power Co. as saying that the fire burned in a corner of the fourth floor of the reactor building and not inside the spent fuel tank.

That fire at unit 4 was believed to have caused an explosion, and afterward two workers in the turbine area were reported missing.

Temperatures in storage tanks at units 5 and 6, which also face a loss of primary power, are now reported at 58.7 C and 57 C, respectively. Units 4, 5 and 6 were down for inspection and not in operation when an earthquake and tsunami caused a station blackout that threatens the integrity of the other three reactors at the site.

If water in spent fuel tanks is not circulated, decay heat can build up, albeit at a much slower pace than fuel inside a recently running reactor. If a tank's water boils off, fuel rods expose to air risk catching fire and releasing substantial amounts of radioactive material into the atmosphere.

The latest reports indicate radiation levels at the gate of the plant have gone down significantly, from 11.93 millisieverts per hour at 9 a.m. local time to 0.5964 millisieverts at 3:30 p.m.

Japanese officials have said radiation levels in Tokyo, while elevated, remain well within healthy limits. Nikkei reported levels of 0.000809 millisieverts, which is less radiation each hour than a standard chest X-ray.

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A fourth unit at the Fukushima Daiichi Nuclear Power Plant now threatens to release radiation into the atmosphere, with officials reporting a fire near a spent fuel tank.

Although unit 4 was down for a routine inspection during Friday's magnitude 9 earthquake, backup power outages at the plant left its spent fuel tank without water circulation to remove decay heat. In a press conference, Prime Minister Naoto Kan said the spent fuel may have heated up enough to release hydrogen and start a fire.

The spent fuel stored at unit 3 is now a concern, as well. NHK-TV reported that the blast yesterday in the building housing that reactor left its spent fuel tank without a cover. The tank's cooling system had not been working. Steam has been visible above the reactor building, although news accounts have conflicted in their description of which units the steam was coming from.

Since September, Unit 3 has used MOX fuel, a blend of spent uranium and plutonium sometimes produced by anti-proliferation programs. While the blending makes the plutonium incapable of achieving the super-criticality required for a nuclear explosion, it also creates fuel with a lower melting point and greater toxicity.

The latest fire follows explosions at all three reactors that were in service at the time of the earthquake. Crews have spent the last three days trying to maintain water and pressure levels within the reactors. Reports indicated crews were even using fire trucks to pump seawater and boric acid into the reactors, while periodically venting irradiated steam into the atmosphere to relieve pressure. Cessium and other elements found outside the plant in small amounts this weekend, as well as Tokyo Electric Power Company reports that fuel rods had been exposed, led several experts to surmise that fuel rod cladding or the fuel itself has been damaged.

Various media reports indicated the explosion at unit 3 disabled four of five fire trucks on the scene Monday, leaving a single truck pumping water into unit 2. When it ran out of diesel, reports indicate water in the core dropped enough to expose at half-to-all of the fuel rods for multiple stretches as crews and their equipment fought to bring the coolant level back up. 

While reports indicate the primary reactor containment of units 1 and 3 remain intact, Japanese officials late Monday said the blast at unit 2 may have damaged the  "pressure suppression room." Immediate translations from Japanese made it difficult to confirm whether or not that damage involved the reactor containment. Analysis on Japanese television indicated the damage occurred in the suppression pool, also known as the torus on General Electric Mark 1 containment designs. The circular chamber beneath the core helps cool the reactor, and damage to it may result in radiation escaping.

While its precise source -- whether from frequent steam venting at three units, a coolant leak or spent fuel exposure -- was impossible to pinpoint, Japanese officials said radiation around the plant has now risen to levels considered damaging to human health. According to NHK, measurements near unit 3 read 400 millisieverts. For context, 1,000 microsieverts make one millisievert; (10 millisieverts equals 1 rem). The Nuclear Regulatory Commission recommends workers in the nuclear field limit radiation exposure to 10 millisieverts per year.

A reading of 100 millisieverts was recorded near unit 4, and a reading of 30 millisieverts was recorded between units 2 and 3.

Shortly after the explosion at unit 3, Tepco evacuated plant workers not immediately involved with pumping water to the reactors, and Japanese authorities have encouraged those living between 20 and 30 kilometers of the plant to stay indoors. About 200,000 people living withing 20 km of the plant were evacuated earlier.

Tepco's second plant in the area, Fukushima Daini, faced similar electrical failure and cooling problems. But as of Monday night, news reports indicated that all three of its reactors that scrammed during the earthquake have achieved cold shutdown.

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  • Anonymous
    Anonymous

    Did you really mean that the fuel was exposed to air? Don't you mean saturated steam inside the reactor? If they are exposed to air that would mean some form of breach or that the core was c0ompletely depressurized allowing air to backflow in through the relief valves.

  • Mike,

    That's a great question. I've seen "exposed to air" used in credible several news sources, but that may be incorrect. Since statements from Tepco and Japanese regulators don't make it entirely clear, I'll update the story and delete "to air" from references to exposed fuel rods within the reactors.

    Thanks,

    Peter

    Nuclear Street News Team

  • Anonymous
    Anonymous

    I was watching an informative and likely explaination of events at theFukushima nuclear plant cenario, on NHK tv, when papers where waved in the presenter's face and he had no choice other than to change track-to some other perhap less sensitive area.  Did anyone else have feelings about this?

  • Andrew - I watched NHK for a few hours last night as they covered the prime minister's press conference. I don't know much about the station, so I haven't had the chance to form an opinion on their reporting one way or another. But last night, they were reporting the same information as other news organizations, and their added analysis from experts was pretty good.

    If want more live updates in addition to what we're running on Nuclear Street, Reuters has a good live stream of earthquake news here: live.reuters.com/.../Japan_earthquake2 .

    Best,

    Peter

    Nuclear Street News Team

  • Ditto about clarity.  Why would they want to risk helicopters and crews when fire fighting equipment could be safer and  more precise?  Does anybody actually know how all of this evolved?

  • Northcoast,

    Here's a set of informative and accurate commentaries regarding the evolution of this problem.

    http://mitnse.com/

  • 70% fuel failure in Reactor 1

    33% fuel failure in Reactor 2

    Dangerous numbers to be sure.  But in my mind, the real monster in this disaster is Reactor 3.  It contains an MOX core load that went critical in September 2010.  Published government and TEPCO assessments indicate that 50% of the core was uncovered for some period of time.  Fuel failure rate is unknown and Containment Vessel integrity is questionable.

    Should Reactor 3 Containment, in fact, be breached and should fuel failure rate be significant, (a distinct possibility due to the inclusion of Plutonium making the fuel melting point lower than the core fuel loads in Reactor's 1 & 2), then Plutonium may well find it's way out of the Reactor and migrate off site.