The shape of things to come
June 1st 2006
From The Economist print edition
How tomorrow's nuclear power stations will differ from today's
AP
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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