Japan nuclear crisis: What's in the smoke emerging from Fukushima I?

Smoke plumes continue to rise from parts of Japan's devastated Fukushima I nuclear plant. On Wednesday, black smoke suddenly billowed up from reactor No. 3, causing workers to evacuate the area and stopping work at the plant for a few hours. Crews later returned, and officials said radiation levels did not spike during the incident. But smoke has been a continuing problem at Fukushima I (also called Fukushima Daiichi) – white and grey plumes have erupted at the plant a number of times this week.

What’s causing this smoke? Is it a matter of concern?

In general, workers are making progress stabilizing Fukushima I, said the International Atomic Energy Agency on Wednesday. Electricity has returned – in some measure – to most of the plant’s six reactors. Control room instruments are now powered at all except No. 3, for instance.

But seeing light at the end of the tunnel is not the same thing as reaching open air. The bursts of smoke are among pieces of evidence indicating that Fukushima I is not yet under complete control.

“The overall situation remains of serious concern,” said Graham Andrew, special advisor to the IAEA Director General for Scientific and Technical Affairs.

At this point, officials do not know how serious the smoke bursts are because they are not certain what their cause is.

As Japanese workers have powered up reactors in recent days, shorts or other electrical problems could have ignited debris from last week’s containment building explosions, said David Lochbaum, director of the nuclear safety project at the Union of Concerned Scientists, in a Wednesday phone briefing for reporters.

Smoke from such a cause is not necessarily something to worry about. On the other hand, it is also possible that the spent-fuel pool at No. 3 has overheated to the point where water has boiled away and exposed fuel assemblies, causing them to overheat and release particulate matter into the atmosphere. That could cause plumes of radioactive smoke like the black smoke seen Wednesday.

“That might be an additional cause for concern,” said Lochbaum.

Lochbaum added that Japanese authorities continue to inject seawater laced with boron into reactor Nos. 1, 2, and 3. Boron absorbs neutrons and thus is used to moderate nuclear fission. Its presence in the water indicates that Japanese officials remain worried that fuel in the reactors may have melted and slumped inside the containment vessels, changing shape in a way that might restart a nuclear chain reaction.

“That’s pretty clear evidence of significant damage,” he said.

Meanwhile, on Tuesday the US Department of Energy for the first time released detailed radiation readings taken by US ground and airborne detectors deployed in Japan.

Unlike the periodic measurements taken at the Fukushima I plant gates, the new US readings give a sense of how radiation has settled over the area surrounding the Fukushima I complex.

The good news is that the readings are relatively low – all are less than 0.3 mSv (300 µSv) per hour – according to the Energy Department. The worrisome news is that the data shows a plume of somewhat elevated radiation levels, higher than 0.125 mSv (125 µSv) per hour, extending up to 25 miles northwest into Japan’s interior, instead of east or southeast towards the ocean. [Editor's note: The original version of this paragraph incorrectly reported 300 mSv (milliSieverts) released from the Fukushima I plant each hour. The correct leakage rate was 300 µSv (microSieverts). The radiation leakage was thus overstated by a factor of 1000.]