Explaining the Fukushima crisis

The still-unfolding tragedy of Japan may be told simply through the statistics of death and destruction. However, the scientific complexity underlying the crisis at the Fukushima Daiichi nuclear power plant is much harder to report.

There are many kinds of radiation and more than one view as to the short- and long-term effects of exposure. Trying to explain in a clear, accurate fashion what is happening to the plant’s damaged reactors and the workers struggling to bring the plant under control is not helped by occasionally contradictory pronouncements from the management of Tepco, which operates Fukushima (at one point Tepco officials said radioactivity levels in one part of the plant were “10 million times higher than normal”, then retracted this statement within 24 hours).

One of the key issues confusing some readers is the way levels of radiation were compared. Early on in the crisis we compared a rate of dose as being “equivalent” to an amount of dose, which one reader said was not a like-for-like comparison and therefore meaningless. It was the subject of a long clarification on 31 March.

Another problem area for the use of the word “equivalent” is in saying such-and-such a dose of radiation in an hour is “equivalent” to, for instance, twice the natural background radiation in a year. It is accurate to talk about equivalent amounts of radiation, but journalists are on dangerous ground if we make out they have equivalent health risks. The Guardian’s science correspondent puts the point this way: “You can think of it like exposure to the sun’s rays. I could move somewhere and get twice as much sunshine in a year, but if I received all that sun in one hour, I would be toast. Because one hour in very intense sunlight will do me more harm than the same amount of sunlight evened out over the year.”

A useful briefing note for journalists has been produced by the Science Media Centre of Canada. The biological effect of radiation in the human body, called the dose equivalent, is measured in a unit called a sievert (Sv): “The basic idea of this unit is to approximate the degree of damage to the cells of our bodies, proportional to the number of ionising particles that pass through them. The nuclear incidents in Japan are reporting dosages using sieverts as the unit. Most doses that humans are exposed to are smaller than one sievert – they are in the range of millisieverts (thousandths (0.001) of a sievert, or mSv), or even microsieverts (thousandths of a millisievert, or 0.000001 Sv).

“Often, exposures are described by millisieverts per year, as that’s the scale of background exposures … According to the UN Scientific Committee on the Effects of Atomic Radiation, the average person receives about 2.4 mSv (0.0024 Sv) of radiation a year through background sources such as cosmic rays, soils, and food. In addition, the average person receives about 1 mSv/year from medical X-rays. This can vary widely depending on where you live.”

One reader wrote to point out that one article published on 29 March states that “One of the plant’s three reactors contains plutonium in its fuel mix”, while in fact, due to the nature of fission reactors, all of the reactors contain some plutonium. Of the three damaged reactors at Fukushima, only reactor three has plutonium in its fresh fuel, but the reader is right in the general sense. According to our science correspondent: Plutonium-239 cannot be made from the fissioning, or splitting, of uranium-235. In the Fukushima reactors, plutonium-239 is a by-product of nuclear fission, and occurs when uranium-238 in the fuel absorbs neutrons. This means that all three reactors then contain plutonium at small levels. All of the spent fuel rods kept in the storage pools at each reactor will also contain some plutonium[see footnote].

Another reader pointed out – correctly – that a reference in a story published on 27 March to iodine-134, which has a half life of 53 minutes, must be wrong: “A fission product with a half-life of less than an hour couldn’t still be around over two weeks after the reactor was shut down. That’s 400 half-lives, meaning that every single atom of it should have decayed by now.”

The Fukushima crisis means that for months to come the Guardian will be reporting on the nuclear debate in even more detail than hitherto. The challenge will be one of explanation as much as disclosure.

• This article was amended on 4 April 2011. The original said that “some of the uranium in the fuel rods will fission into plutonium”. This has been corrected.

This column was originally published in The Guardian on April 1, 2011.

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