Fatal Radiation Level Found at Japanese Plant

By MARTIN FACKLER
Published: August 1, 2011

TOKYO — The operator of the Fukushima Daiichi nuclear plant said Monday that it measured the highest radiation levels within the plant since it was crippled by a devastating earthquake. However, it said the discovery would not slow continuing efforts to bring the plant’s damaged reactors under control.

The operator, Tokyo Electric Power, said that workers on Monday afternoon had found an area near Reactors No. 1 and 2, where radiation levels exceeded their measuring device’s maximum reading of 10 sieverts per hour — a fatal dose for humans.

The company said the reading was taken near a ventilation tower, suggesting that the contamination happened in the days immediately after the March 11 earthquake and tsunami, when workers desperately tried to release flammable hydrogen gas that was then building up inside the reactor buildings. The release, known as venting, failed to prevent crippling explosions that destroyed the reactor buildings.

The company said the workers who found the reading were safely protected by antiradiation clothing. Tokyo Electric said it has closed off an area of several yards around where the lethal radiation level was found. The company said this would not hamper efforts to build a new cooling system and remove contaminated water.

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http://www.nytimes.com/2011/08/02/world/asia/02japan.html?_r=1&hp

State that the 10 Severt/Hour dose rate was the upper limit of the equipment (a Teletector 6112M ). The actual dose rate may be a lot higher.

http://ex-skf.blogspot.com/

There is a lot of Japanese sourced reporting at the site. Among other things a Kyoto University Prof. opines that the hotspots are possibly pieces of spent fuel rods blown out by the explosion.

The reporting isn't getting any better... the NYT piece is misleading in so many ways, beginning with a headline confusing does rates with dose. (A 10 Sv/hr source isn't "fatal," a 10 Sv dose is, which is why exposure time is important). Then they pass on an essentially meaningless TEPCO statement that "...the workers who found the reading were safely protected by antiradiation clothing." What does that even mean? What it really means, I think, is that the protective gear kept radioactive particles off their bodies, but it's pretty obvious that those garments provide essentially zero protection from acute exposure to an external source (as the analyst pointed out in the link kristopher provided).

Previous reporting made it sound as if some fixed radiation monitor suddenly hit 10 SV/hr, which was hard to reconcile with the overall course of attempts to manage this mess. But if there are chunks of "spent" fuel lying about thanks to those explosions (and it's pretty plausible from the videos of those blasts that debris from the pools could be anywhere on or near the plant site) finding super-hot point sources would be expected. The photos on the site kristopher linked help a lot.

Incidentally, I doubt the claim that the worker who received a reported a 4 mSv dose taking that reading only spent about a second or two near the hot spot. If you look at the picture, the instrument is at the end of what looks like the proverbial "10-foot pole." If our source is a small chunk of spent fuel (and the gamma picture does seem to show a localized source) then the dose rate drops with the square of distance. If the sensor is a foot away and the worker is 10 feet away, the does rate at the worker's location is 1/100 the rate where the sensor is. So something more like 100 mSv/hour would be a more accurate guess of exposure rate, which could correspond to more like a minute. (Of course, the reading was at or above the top of the scale, so it could be few times higher...)

But it does point out the perils of working in this environment - to work at all safely there you do need a good survey of "hot spots," and it seems likely there are plenty of unsecured chunks of very radioactive materials that could move around substantially.

And when it comes to this particular task, if it were me I'd pretty much stop once the meter "pegged" - there's no more information to glean by hanging around! I'm guessing maybe 10-20 seconds max in that location.

I am certain that the hot spots are fuel fragments, or pieces of very high dose rate reactor internals. To get dose rates that high just about requires it.