Post-Fukushima: The Weekly Scientist on Nuclear Accidents

An entire month has passed since Japan raised its nuclear terror alert level from 4 to 7, but today the country is still very much in the throes of its nuclear crisis. Just like Chernobyl, the radioactive fallout from Fukushima Daiichi is exceeding scientist’s expectations. Radioactive caesium and iodine is being found in soils lying outside the 30 km evacuation zone, and an enormous 3,355 times the legal limit of radioactive iodine has now made it into coastal waters at the Fukushima region. But away from these very real episodes of cataclysm, this humanitarian disaster has morphed into a wholly political issue, with both pro- and anti-nuclear power camps banking on the guilt and fears of our greenhouse-gas-spewing generation to make their arguments truly sink in. Amid all this articulate mud-slinging, Joe Public is finding it very hard to grasp the scale of the issue.

So, let us get the facts about nuclear accidents well and truly straight. There have been approximately 60 nuclear accidents since the first-ever atomic bomb was tested in the desert of New Mexico in 1945. These 60 accidents have led to 21 deaths, seven of which having occurred in the US, ten in the Soviet Union. The important thing to note here is that with “deaths”, I am solely referring to the immediate victims of a nuclear accident, and not the potentially hundreds of leukaemia-stricken children that might be dying decades after a radiation leak.

When speaking of nuclear accidents, experts tend to use words such as “criticality” and/or “meltdown”. Both scenarios are exceedingly catastrophic, but there are some very important differences to consider. Criticality, or criticality excursion, refers to a nuclear chain reaction that has reared so out of control that it releases enormous amounts of heat and lethal blasts of ionising radiation from its uranium or plutonium core. This chain reaction is normally rigorously controlled in a nuclear reactor, but uncontrolled (and hence left to cause as much devastation as possible) in an atomic bomb.

A highly publicised example of such a criticality accident was the death of Harry Daghlian Jr. in 1945, an American physicist who had been working on the Manhattan Project that created the Hiroshima and Nagasaki bombs. Simply by dropping a block of tungsten onto a plutonium core, Daghlian made the plutonium core go critical. It released a lethal dosage of neutron radiation into the air, killing him 25 days later.

To understand how such a small mishap can have such devastating effects, we have to go back to the actual science behind a nuclear reaction. Nuclear energy, as the name implies, is directly derived from the neutron nucleus of an atom. Elements such as uranium (U-235 to be precise) are bombarded with neutrons to destabilise them. This causes the uranium nucleus to split into two other nuclei of smaller elements in attempt to regain stability. While doing so, the uranium nucleus produces excesses of heat and ionising radiation, the former of which is used to generate our electricity.

More neutrons are expelled after uranium has split, and these go on to initiate several other fission reactions that eventually escalate into a chain reaction. When Daghlian dropped the tungsten on the plutonium core, neutrons that were being released from the sphere were reflected back into it, causing a fission reaction that promptly magnified into criticality. Such are the nature of most criticality accidents, with one particularly gruesome example at the Mayak nuclear facility in Russia involving three workers acting as “human neutron reflectors” before they were killed by the subsequent burst of radiation.

So, what is a “meltdown” and why is that word constantly being used in association to Fukushima Daiichi? In layman’s terms, a meltdown is the actual melting of a plutonium or uranium core, releasing hot radioactive fuel that can physically burn through its steel container and leak into the environment. This can be triggered by a criticality excursion. In the cautiously controlled environment of a nuclear reactor, however, a more likely meltdown scenario involves the coolants. To stop it from becoming too hot, nuclear reactor cores need to be constantly immersed in water. Engineers at a nuclear facility are in a continuous race to replace the water that is being boiled off by the hot core. In modern reactors, this is largely controlled by machines, so nuclear plants normally depend on an external power source to maintain their cooling systems at all times.

The earthquake and tsunami that hit Japan on March 11 triggered a blackout that shut down this very cooling system at the Fukushima Daiichi plant. Backup generators that would have allayed the risk of a meltdown also failed and thus the reactors were becoming very hot very quickly. The subsequent explosions probably resulted from a hydrogen gas build-up, created by coolant water reacting with hot zirconium in the fuel rods. Parallels can be drawn with the Chernobyl disaster (the only other Level 7 incident ever to have occurred), where a power surge led to overheating of the reactor core, sparking the explosion that killed 31 people and churned out a staggering 4000 times more radioactive material than at the bombing of Hiroshima.

In a desperate move to restore the coolant supply and avoid a potential meltdown, operators at Fukushima Daiichi resorted to pumping seawater into the hot reactor cores. Experts say that this had never been tried before and would effectively destroy the reactors, a testament to how grave the situation had gotten. Yet, things are still looking dire, with some reports claiming that some fuel rods had already begun to melt. Then, there is the leak of radioactive material that could go on to contaminate food supplies, as well as the environment.

Engineers are now to fighting to keep any more fuel rods from melting. Still, with no lives having been claimed yet, Fukushima is set make much less of an impact than Chernobyl did. 6,000 people who were under age of 18 at the time of the Chernobyl incident went on to suffer from thyroid cancer by 2006. Furthermore, the radioactive fallout that followed the explosion at Chernobyl’s nuclear power plant was detected all over Europe. But only time will tell whether Fukushima will have as devastating an effect on Japan’s public health and the environment. For now, with Japan deciding not to build any more nuclear reactors post-Fukushima, one of the first victims of this disaster appears to be nuclear power itself.

Eric John

(Image courtesy of Thierry Ehrmann)

2 Comments on this post.
  • Stuart Neyton
    13 May 2011 at 10:14
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    This is a fantastic article.

    It really isn’t surprising that the Fukushima disaster has reopened the nuclear debate and it’s certainly welcome. There has seemed to be a status-quo argument that the dangers of nuclear are tiny and when compared to the risks posed by the greenhouse gas emissions of all other non-renewable energy sources, risks are insignificant.

    As far as reducing emissions goes, any benefits from new nuclear plants will come far too late for this to have a good enough role. Emissions need to be reduced now, not in 20 years time when the first new nuclear plant will open. The only way to do this is by vast reductions in energy use (mostly by industry, but this is where free insulation, changing our light bulbs, not leaving our TVs on standby and turning lights off when leaving rooms all come into play) and by immediately switching to decentralised renewable sources. Most of Europe’s winds pass over the British Isles, the fact that the amount of it we’re harnessing as green energy compared to the rest of Europe is pretty miserable.

    As far as Fukushima is, of course, an exceptional circumstance. I’m not going to claim there will be an apocalypse here but remember with nuclear the waste takes thousands of years to decompose and is currently held on site as there’s no safe way to dispose of it, the source is expensive and relies on supplies from some heinous countries, it takes many decades to safely decommission a nuclear plant, the technology is closely linked to nuclear weapon proliferation, and its cost means that vast subsidies are needed by the tax payer to make it viable.

    Quite frankly, i’d rather have a windmill in my back garden than a nuclear power plant.

  • Phil
    16 May 2011 at 20:34
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    I’d far rather live near a nuclear power plant than a noisy wind turbine, and considering that the equivalent amount of power generation to one medium nuclear reactor would be thousands of wind turbines, well, I think the choice is pretty obvious.

    As for supply, well, Australia is a major exporter of uranium, so (sporting rivalry aside) we needn’t buy it from evil regimes. And while nuclear is more expensive than fossil fuels (and gas in particular), it is still far, far cheaper than wind power per kWh of energy generated. Wind relies on even massive-er state subsidy, dwarfed only by the green pipe-dream of solar generation, and is the reason for our soaring electricity bills. Wind is also simply too unreliable for the UK baseload of electricity. In reality we have to say that a responsible energy policy for the UK has to include wind only as a small proportion and focus on more reliable sources for baseload generation. In order to be economical that is probably going to be largely gas. However, long-term we really need to move to a higher proportion of nuclear for reasons of energy security rather than importing gas from Russia and the Gulf.

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