The new chronology of events at Japan鈥檚 Fukushima Daiichi plant, which we write about here, contains no new bombshells about the disaster. But in dry prose, it gives some additional indications of how bad things were after the quake and tsunami there and will probably be the basis for some lessons learned for the American industry.
    One lesson involves the nature of radiation protection for plant workers in an emergency. Two operators received more than 60 rem (0.6 Sv), far above the level at which changes in blood chemistry can be observed and almost at the level where symptoms like nausea and vomiting begin.
    But most of the dose came not from walking around in places where radiation was present 鈥 it was internal, meaning it was from particles inhaled or swallowed by the workers. Radioactive material that is lodged in the body will deliver a dose for an extended period.
    Tony Pietrangelo, a nuclear safety official at the Nuclear Energy Institute, the American industry鈥檚 trade association, suggested that the reason was inadequate use of respirators and other protective gear at the plant. And the tsunami and power failure knocked out some equipment that measures radiation fields, he said, so the workers may not have realized the magnitude of the dose they faced.
    Radiation doses were high by American standards. The Japanese government set an emergency limit of 10 rem (0.1 Sv). By comparison, nearly all American plant workers get less than two rem (0.02 Sv) a year, and the upper limit in this country is five rem (0.05 Sv).
    But by the end of March, 鈥渁pproximately 100 workers had received doses greater than 10 rem,鈥 the chronology said.
    Natural background exposure for most Americans is about one-third of a rem a year.
    Paradoxically, the new chronology points up another split in radiation thinking between Japan and the United States. The Japanese delayed using emergency vents that they had installed in the reactor containments at Fukushima. If they had used the vents earlier, pressures would have declined and it would have been easier to pump in cooling water. They also would have expelled hydrogen that was being generated in the reactor cores.
    The hydrogen eventually exploded, causing worse problems. But the American chronology says the Japanese waited until surrounding areas had been evacuated, and the government had made an announcement, before turning to the vents. Then they ran into technical problems getting the vents open. Their chances of successful venting might have been higher if they had started earlier, the chronology suggests.
    The Japanese, whose country is the only one where nuclear weapons have ever been used in combat, are ultra-sensitive about radiation exposures to the public.
American reactor operators would vent promptly but tell public safety officials which way the wind was blowing and where it would be advisable for people to take shelter.
    While the industry itself has not drawn final conclusions from the events that began eight months ago at Fukushima, Mr. Pietangelo cited some instances of innovation by workers at the Japanese reactor complex as a lesson for operators elsewhere.
    American-designed reactors, including the ones at Fukushima, were built to withstand something called a 鈥渄esign basis鈥 accident, usually the worst case that engineers thought was plausible. This is generally the instantaneous breaking of the biggest pipe in the plant, shorn away in two locationss so that a segment falls out cleanly. It is known as a 鈥渄ouble-guillotine pipe break,鈥 and it has yet to occur in the United States, Japan or anywhere else.
    The focus now, Mr. Pietangelo said, should go beyond design-basis accidents to include events for which a variety of emergency equipment could be useful, as at Fukushima, rather than just the built-in equipment that is thought to be capable of dealing with the hypothetical double-guillotine break. 鈥淭he way we鈥檝e been weaned in our industry is on very stylized accidents,鈥 he said.

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