Read more: 鈥�Instant Expert 32: Nuclear energy鈥�
Even before the meltdown of the three reactor cores at Fukushima Daiichi, a nuclear renaissance was far from assured. In the US and Europe, any shift to nuclear had stalled more generally amid long-standing questions over safety, disposal of nuclear waste and the possibility that nuclear plants could generate material suitable for weapons
Nuclear disasters
Accidents involving the meltdown of a reactor core are unlikely, but when they do happen the consequences can be far-reaching. Two have directly led to loss of human life: the first in 1986 at the Chernobyl power plant in Ukraine and the second at the Fukushima Daiichi nuclear power plant in Japan in 2011.
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Nuclear fission is not like combustion, where the process is stopped by cutting off the fuel supply. Shut down a reactor in a nuclear power plant, and while the chain reaction ceases, radioactive decay carries on, emitting particles that produce dangerously high heat. Keeping the shut-down reactor cool would boil away 75 tonnes of water the first hour, 960 the first day. If the water supply fails, the resulting meltdown can lead to damage that exposes the reactor core.
Effects on any one nearby would be dire. The International Atomic Energy Agency specifies that for a radiation worker, the maximum yearly dose of radiation without health effects is equivalent to absorbing a mere 0.02 joules of energy per kilogram of body weight. A dose 50 times this warms the tissues by 0.2 millidegrees centigrade and will cause acute radiation syndrome, symptoms of which can include rashes, vomiting and diarrhoea. In severe cases, death ensues in days or weeks.
To block radiation, a reactor core has shielding 2 metres thick. If this is breached in an accident, the release of radioactive isotopes may affect the health of people in the vicinity for years to come. Iodine-131, for example, is taken up by the thyroid gland and this can lead to cancer, although the iodine largely decays within a short time. More persistent and problematic are caesium-137, which is taken up in the body because of its chemical similarity to potassium, and strontium-90, which mimics calcium. A large release of radioactive material may lead to thousands of additional deaths from cancer over many years, although it is never possible to say which cases are linked to radioactivity and which are not.
Chernobyl meltdown
In April 1986, the No 4 reactor at Chernobyl, Ukraine, suffered a catastrophic core meltdown and steam explosion. The reactor, of Soviet design, used uranium held inside thousands of tubes that also contained hot water at high pressure. The meltdown and resulting fire released large amounts of radioactive materials, including uranium fuel particles, in smoke.
The disaster led to 31 deaths in the immediate aftermath, but the long-term effects are uncertain. The 2005 report of the Chernobyl Forum estimates the premature cancer death toll as 4000 among the most highly exposed. However, 90 per cent of the documented radiation exposure was excluded from the report. Had all the data been included, the report would have predicted 40,000 extra deaths.
鈥淭he long-term consequences of the Chernobyl disaster are still uncertain鈥�
The permanent evacuation of 300,000 people has had an unexpected consequence. It created a haven for wildlife, including wild boar, lynx and bison, whose habitats had been destroyed by local industries. Twenty-seven years later, the area is now a wildlife sanctuary.
Reprocessing
A unique feature of nuclear power generation is that the spent fuel from reactor cores contains fissile material. This means it can be treated chemically to remove fission products, leaving behind uranium that can be re-enriched 鈥� and plutonium that can be refabricated 鈥� into fresh fuel. Both can take the place of the uranium-235 that fuels a standard nuclear power reactor. The operation, known as reprocessing, can reduce a light water reactor鈥檚 uranium requirements by about 20 per cent. It also . Commercial reprocessing is performed at plants in France, the UK, Russia, India and Japan.
Reprocessing is politically sensitive because plutonium can be diverted to make nuclear weapons. Six kilograms of plutonium went into the bomb that destroyed Nagasaki at the end of the second world war. That much can be produced in a year in a power plant supplying a town of 25,000 people.
Furthermore, reprocessing plants are not strictly necessary in order to make nuclear weapons from nuclear fuel: the same set of gas centrifuges that would fuel one large power reactor for a year could 鈥� with minor changes in the interconnecting tubing 鈥� provide enough highly enriched uranium for 36 weapons. These concerns are at the heart of sanctions against civilian nuclear programmes in countries such as Iran.
Nuclear waste
One major problem for nuclear power is the waste it produces. A typical nuclear reactor needs to get rid of 20 tonnes of spent fuel per year. The main radioactive components in this waste are caesium-137 and strontium-90; their radioactivity takes 1000 years to diminish to negligible levels. Other dangerous contents include longer-lived fission products and uranium and plutonium, the 239 isotope of which takes more than 150,000 years to decay to negligible amounts.
It is important to keep this material away from people. To that end, the spent fuel initially goes into a pool at the reactor, where it is kept beneath 5 metres of cooling water that blocks its radiation. After some years, the fuel can be removed and put into dry cask storage which, as the phrase suggests, means it is put in big drums at the plant. Two such casks can hold one year鈥檚 spent fuel safely for 100 years. After that, the casks should ideally be put into a mined geological repository 鈥� entombed in deep underground tunnels in a geologically stable area.
There are no such working repositories yet. Why? Past practice and law has been to require each country to have its own repository, but engineering concerns and bureaucracy around siting and planning have stalled the process. In the US, plans for a federal repository at Yucca Mountain in Nevada were recently scrapped.
So what are we to do with the world鈥檚 ever-increasing quantities of nuclear waste? In the US, for example, nearly 65,000 tonnes of spent fuel languishes in interim facilities. A future problem will be disposing of the waste from dismantling defunct reactors: a standard design, once demolished, will produce more than 100,000 tonnes of waste, including concrete, the reactor vessel, control rods and pipes. Because no scrapyards exist for this waste either, so far only 17 of the world鈥檚 135 defunct reactors have been dismantled.
This article appeared in print under the headline 鈥淒anger Zone鈥�
