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.