Radiation Soars at Fukushima — to 10 Sieverts Per Hour


Image from NHK.

NHK English, and Al Jazeera (but, weirdly, not yet The Japan Times) are reporting that the damaged Fukushima nuclear power plant is showing levels of localized radiation potentially lethal to humans in just minutes –high enough to actually go off the scale of the plant operator’s Geiger counters. Now, the reports are of localized radiation of at least 10 sieverts per hour — according to the plant operator TEPCO, that’s enough to cause severe radiation sickness in humans after just a few seconds of direct exposure.

I say “at least” because that’s as high as the Geiger counters go, so there’s no way to tell. The speculation is that the radiation comes from material left over from emergency venting in the first few days of the crisis, rather than some new breakdown. But it’s worth pointing out that such an answer would mean the radiation had been there the whole time.

Interestingly, while NHK refers to the hotspot as an “exhaust pipe” — ulp — Al Jazeera prefers to call it a “ventilation stack” — slightly less scary, eh? The radiation has shown up, from what I can tell, in two spots in the same pipe/stack. Here’s what NHK says:

The operator of the crippled Fukushima nuclear complex is searching for radioactive hotspots after finding record high radiation near an exhaust pipe at the plant.

Tokyo Electric Power said on Monday that over 10,000 millisieverts per hour had been detected at the bottom of the exhaust pipe in between reactor buildings No.1 and No.2. That’s the highest level detected since March when the quake and tsunami disabled the plant.

A photo released on Tuesday shows workers taking measurements with a detector attached to the tip of a 3-meter-long arm. The level of radiation where the workers stood reportedly reached 40 millisieverts per hour.

TEPCO says the exhaust pipe was used when radioactive air was vented from the No.1 reactor’s containment vessel one day after the March 11th disaster.

The company subsequently revealed that the reactor had suffered a nuclear fuel meltdown. The utility believes highly radioactive substances that leaked from the container flowed into the pipe and accumulated inside.



What appears to be a second hotspot is mentioned by Al Jazeera:

“Authorities are working on the theory though that it has come from those initial hydrogen explosions that we’ve saw at the plant in the days after the earthquake and tsunami,” [the correspondent said]. …”It is now looking more likely that this area has been this radioactive since the earthquake and tsunami but no one realised until now.”

On Tuesday, TEPCO said it found another spot on the ventilation stack itself where radiation exceeded 10 sieverts per hour, a level that could lead to incapacitation or death after just several seconds of exposure.

The company used equipment to measure radiation from a distance and was unable to ascertain the exact level because the device’s maximum reading is 10 sieverts.


A level of 10 Sieverts (Sv) per hour, incidentally, is kind of a lot of radiation. I was slightly dubious of the correspondent’s claim that it can cause radiation sickness after a few seconds, but TEPCO’s math is actually not that far off the mark, especially since the level could be far higher than that.

The safety limits for workers at Fukushima is 250 millisieverts (mSv) per year. Acute exposure to 1,000 mSv, or one Sievert (Sv) total is generally considered enough to reliably cause mild radiation sickness (often only half that, but it varies widely). Keep in mind that the measured localized radiation is ten times that — but only because that’s where the equipment stops measuring it.

Image by Randall Munroe at XKCD.com.

According to XKCD’s handy radiation exposure chart (reposted at right), acute human exposure to 8 Sv total is thought to result in death even with treatment; 4 Sv is usually fatal even with prompt treatment; 2 Sv causes severe and possibly fatal radiation sickness. A total exposure of 10 Sv would absolutely be fatal, even with treatment, so the fact that 10 Sv/hour is the limit of the equipment is pretty significant.

At 10 Sv/hour, my sketchy math skills give me something like two minutes and fifteen seconds to possible acute radiation sickness, so…”a few seconds” might be pushing it, but it’s sure as hell not a cakewalk. Radiation exposure over a short period of time also causes more acute effects.

Los Alamos physicist Harry Daghlian, by comparison, received an estimated dose of 5.1 Sv in the August, 1945 criticality experiment with the “demon core” that also killed self-described “bomb putter-togetherer” Louis Slotin the following year. Daghlian died of acute radiation syndrome 25 days after the first accident. Slotin (who, incidentally, consoled Daghlian as he died) probably received 21 Sv in the second accident, and died in 9 days. Both exposures were for just a few seconds.

The local Fukushima levels measured — that is to say, the limits of the equipment — therefore represent about twice Daghlian’s exposure, or half Slotin’s, per hour, compared to the few seconds that resulted in the exposures in the two Los Alamos accidents.

Harry Daghlian, Creative Commons from Wikipedia.

It’s slightly disingenuous to compare these events, because they have so little to do with each other. Daghlian and Slotin were not wearing protective gear. They both received direct bursts of radiation from weapons-grade plutonium, which is the same grade of plutonium used in Fukushima’s mixed-oxide or MOX fuel — but in MOX it’s not in pure form. Many things affect the level of ionizing radiation absorbed by human tissues, including the exact kind of radiation, and what tissue is absorbing it; its not just apples-to-apples, here. The number of case studies of acute human exposure to high levels of ionizing radiation from fissionable material is, thankfully, pretty damn limited. We don’t know all that much about what radiation this intense would do to a person, because luckily not that many people through history have been exposed.

And it’s also worth observing that Slotin’s and Daghlian’s exposures were only for a few seconds because the accidents only lasted a few seconds. The measured radiation at Fukishima has, the theory goes, endured since the inception of the crisis (or shortly thereafter).

There’s no evidence that this specific 10+ Sv/hour source of radiation has any access to the environment — the ocean, groundwater, etc. — now, or at any point in the last few months. But the radiation, by definition, must be coming from some material.

Is that material properly isolated from the environment? Clearly not by intention, since plant workers just found out about it. Probably not by design, since it’s somewhere it’s not expected, and the protective structures at Fukushima were unquestionably compromised. Assuming it’s been there for months, might it have been kept isolated from the local environment by pure chance and dumb luck?

Louis Slotin's Los Alamos badge photo. Public Domain, from Wikipedia.


That said, the currently available descriptions represent no specific threat whatsoever from this specific radiation to the immediate environment, let alone the global environment (overall at Fukushima? That’s another story entirely…). However, I mention the examples above to describe just how astronomical the difference is between the levels reported thus far at Fukushima and the levels being discussed now. No humans have been exposed to that level of radiation — and again, keep in mind that no one involved in the Fukushima accident has died from radiation, despite the widespread (though short-lived) belief in the West that the “Fukushima 50” had “sacrificed themselves.”

But in immediately local terms (ie, at the bottom of the ventillator stack) these levels are orders of magnitude worse than what was seen during the original acute phase of the crisis after the earthquake and tsunami. We’re not talking about anything even remotely close to the levels that sent plant workers to the hospital after they stepped in dirty water. We’re talking about many times that.

If these reports are accurate, then they represent the worst radiation danger to plant workers (and, by extension, the local environment) that has ever been seen at Fukushima — including the initial crisis when things were exploding and cores were melting down. It makes the local radiation levels described in the heart of the crisis seem like child’s play. That’s partially true because, obviously, the cleanup requires plant workers to go (or send their equipment) into places that no sane person would have gone immediately following the explosions. Cleanup requires plant workers to dig deep…so they’re discovering just how serious the release of radiation was, and is.

Before North Americans reading this start packing up their Volvo station wagons with shotgun ammo and cat-carriers, you should know that the levels are more than just localized to the power plant site — they’re localized to the bottom of a single shaft (so far). But again, these levels are coming from somewhere, and it’s reasonable to think that the material putting out that kind of radiation must be going somewhere — like out to sea, for instance. Al Jazeera’s silent on that matter.


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