Nuclear power - How tomorrow's nuclear power stations will differ from today's

Nuclear Power The shape of things to come Jun 1st 2006 From The Economist print edition How tomorrow's nuclear power stations will differ from today's THE agency in charge of promoting nuclear power in America describes a new generation of reactors that will be “highly economical” with “enhanced safety”, that “minimise wastes” and will prove “proliferation resistant”. No doubt they will bake a mean apple pie, too. Unfortunately, in the world of nuclear energy, fine words are not enough. America got away lightly with its nuclear accident. When the Three Mile Island plant in Pennsylvania overheated in 1979 very little radiation leaked, and there were no injuries. Europe was not so lucky. The accident at Chernobyl in Ukraine in 1986 killed dozens immediately and has affected (sometimes fatally) the health of tens of thousands at the least. Even discounting the association of nuclear power with nuclear weaponry, people have good reason to be suspicious of claims that reactors are safe. Yet political interest in nuclear power is reviving across the world, thanks in part to concerns about global warming and energy security. Already, some 441 commercial reactors operate in 31 countries and provide 17% of the planet's electricity, according to America's Department of Energy. Until recently, the talk was of how to retire these reactors gracefully. Now it is of how to extend their lives. In addition, another 32 reactors are being built, mostly in India, China and their neighbours. These new power stations belong to what has been called the third generation of reactors, designs that have been informed by experience and that are considered by their creators to be advanced. But will these new stations really be safer than their predecessors? Clearly, modern designs need to be less accident prone. The most important feature of a safe design is that it “fails safe”. For a reactor, this means that if its control systems stop working it shuts down automatically, safely dissipates the heat produced by the reactions in its core, and stops both the fuel and the radioactive waste produced by nuclear reactions from escaping by keeping them within some sort of containment vessel. Reactors that follow such rules are called “passive”. Most modern designs are passive to some extent and some newer ones are truly so. However, some of the genuinely passive reactors are also likely to be more expensive to run. Safety chain? Nuclear energy is produced by atomic fission. A large atom (usually uranium or plutonium) breaks into two smaller ones, releasing energy and neutrons. The neutrons then trigger further break-ups. And so on. If this “chain reaction” can be controlled, the energy released can be used to boil water, produce steam and drive a turbine that generates electricity. If it runs away, the result is a meltdown and an accident (or, in extreme circumstances, a nuclear explosion—though circumstances are never that extreme in a reactor because the fuel is less fissile than the material in a bomb). In many new designs the neutrons, and thus the chain reaction, are kept under control by passing them through water to slow them down. (Slow neutrons trigger more break ups than fast ones.) This water is exposed to a pressure of about 150 atmospheres—a pressure that means it remains liquid even at high temperatures. When nuclear reactions warm the water, its density drops, and the neutrons passing through it are no longer slowed enough to trigger further reactions. That negative feedback stabilises the reaction rate. Most American nuclear reactors are pressurised-water reactors of this type. So is the reactor being built at Olkiluoto in Finland—the largest planned to date. This reactor will produce 1,600 megawatts when it starts generating electricity in 2009, enough by itself to supply the needs of 1.8m households. The Olkiluoto reactor has several protective measures against accidents in addition to its innate design. These include four independent emergency-cooling systems, each capable of taking heat out of the reactor after a shutdown, and a concrete wall designed to withstand the impact, accidental or otherwise, of an aeroplane. A second plant of similar design may be built at Flamanville in France. If this proposed power station withstands the scrutiny of a public inquiry and gets planning permission, it could be producing electricity by 2012. There are also plans to build four such nuclear plants in China. Canada, a country that has spent its entire history trying to distinguish itself from its southern neighbour, has its own nuclear design, too. Its pressurised heavy-water reactors, known as CANDU, are similar to ordinary pressurised-water reactors (or light-water reactors, as they are sometimes known) but they contain water in which the hydrogen atoms have been replaced by their heavier cousins, deuterium. Heavy water is expensive. However, the fuel used by CANDU is cheap. Light-water reactors rely on enriched uranium. The thing enriched is a rare but highly fissile isotope of the element. Enrichment is an expensive process. CANDU, by contrast, uses natural uranium. The cheapness of this fuel balances the cost of the heavy water. Moreover, instead of using a single large containment vessel, the fuel is held in hundreds of pressure-resistant tubes. CANDU reactors can thus be refuelled while operating, making them more efficient than light-water reactors. India has nuclear power plants based on the CANDU design, as does China. CANDU is passive in that the neutron-absorbing rods needed to stop the reactor rely only on gravity to drop into the reactor core. A South African design, called the “pebble-bed”, is, however, truly passive. Instead of water, it uses graphite to regulate the flow of neutrons, and instead of making steam, the reactor's output heats an inert or semi-inert gas such as helium, nitrogen or carbon dioxide, which is then used to drive the turbines. The name of the design comes from the fact that the graphite is used to coat pebble-like spheres of nuclear fuel. Like the CANDU design, pebble-bed reactors can be refuelled while running. China is also developing pebble-bed reactors. Further into the future, engineers are developing designs for so-called fourth-generation plants that could be built between 2030 and 2040. Work on these designs is the job of a ten-nation research programme whose members include America, Britain, China, France, Japan, South Africa and South Korea. Three of these designs are for fast reactors (which work without any need for the neutrons to be slowed down). These would have the nifty trick of generating their own fuel, since fast neutrons can convert non-fissile isotopes of uranium into highly fissile plutonium. But fast reactors have complicated designs that could prove expensive to build. They also operate at very high temperatures, so in two cases the cooling fluids pumped through their cores are liquid metals (sodium and lead). Whether such reactors would be apple-pie safe is a different question. But 2030 is still a long way away. Plenty of time for the sloganeers to sharpen their pencils.

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