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IN 1984, Lawrence Lidsky, a professor of nuclear engineering at the
Massachusetts Institute of Technology, found himself in a quandary. For
20 years he had worked on nuclear fusion, most recently on the problems
of building a commercial power plant that would harness the energy generated
if nuclei could be forced to unite. But Lidsky was coming to the conclusion
that it was a hopeless task. ‘Each year,’ he recalls, ‘I would present a
design for a power plant to my class. And each year, we would conclude that
the design was large, expensive, complex and ugly.’ According to Lidsky,
‘the light slowly dawned’ that magnetically confined fusion of deuterium
and tritium, two isotopes of hydrogen, would never be a practical source
of energy.

Lidsky decided to check his discouraging results by comparing them with
the designs of a few novel fission reactors. It was a fateful step, for
he stumbled onto a design that impressed him so much that he dropped his
research on fusion entirely. The reactor he came across is known as the
high-temperature, gas-cooled reactor and Lidsky has switched his research
at MIT to the task of perfecting it.

The HTGR is the most prominent of a new generation of nuclear power
reactors that are expected to be much safer and more reliable than existing
light-water reactors, such as the PWR, the pressurised-water reactor. Only
a few HTGRs have been built, and none has operated profitably. In August
last year, the US Department of Energy (DOE) gave a powerful impetus to
the HTGR’s prospects. It proposed to buy one of the latest types of HTGR
from General Atomics, a firm based in San Diego, as the second of two new
reactors to produce tritium for nuclear weapons. The other one will be a
heavy-water reactor, similar to those at the DOE’s Savannah River Plant,
South Carolina, that are shut down for safety reasons. The DOE is willing
to sink $2 billion into building an HTGR, mainly because of its commercial
potential rather than the reactor’s value as a producer of tritium. The
HTGR will produce only half as much tritium as the heavy-water reactor,
and its untested technology carries a higher risk of developing unexpected
problems. But officials at the DOE hope the HTGR will operate so smoothly
that it will persuade the public that the new reactor is safe and efficient.
It is not clear if Congress will approve funds for both reactors, though
General Atomics is lobbying hard. One key selling point is the HTGR’s ability
to generate electricity while it produces tritium. By comparison, the heavy-water
reactor generates little heat to produce steam to drive a turbine. ‘Let’s
say peace breaks out,’ says Harold Agnew, a retired senior executive at
General Atomics, ‘the HTGR won’t be a white elephant.’

The high-temperature, gas-cooled reactor gets its name from the method
by which it is cooled. Helium gas circulates through the core of the reactor,
reaching temperatures of up to 700 Degree C; the fuel itself heats up to
1100 Degree C during normal operation. In a light-water reactor, by contrast,
there is a much greater difference between the operating temperatures of
coolant and fuel; the water that cools the reactor stays below 350 Degree
C, while the temperatures inside the reactor’s fuel rods exceed 2000 Degree
C. As a result, an interruption in the supply of water in a light-water
reactor leads rapidly to catastrophe; fuel will start to disintegrate within
minutes if water drains from the reactor’s core while it is running. The
HTGR’s great attraction comes from its apparent immunity to this type of
disaster, known as a loss-of-coolant accident. General Atomics says that
the cooling system of its HTGR can be out of action for days and the reactor
will still operate at full power without running out of control.

Unfortunately, this self-cooling works only in small reactors, which
the nuclear industry has generally seen as uneconomic for producing electricity.
But General Atomics proposes to build a ‘modular’ HTGR where four separate
reactors are linked to drive a common set of steam turbines and generators.
This is the type of HTGR that the US Department of Energy wants to buy.
Each reactor will produce 135 megawatts of electricity. Together, the four
reactors will produce only half the capacity of a water-cooled nuclear reactor
but, according to General Atomics, this is enough to make the modular quartet
more economic than a modern coal-fired plant.

Gas-cooled reactors have a long history. More than 35 years ago, British
engineers developed the Magnox reactors, which are cooled with carbon dioxide;
two Magnox plants, at Calder Hall and at Chapelcross, produce plutonium
for nuclear weapons. The Magnox reactors served as prototypes for the second
generation of nuclear power units in Britain, the advanced gas-cooled reactors
(AGRs). Seven pairs of AGR power stations operate in Britain; each pair
has a generating capacity of more than 1200 megawatts. But, outside of Scotland,
the AGRs have a poor record, which has given gas-cooled reactors a bad name.
The reason, some experts say, is that the development of AGRs was too big
a technological step from Magnox reactors; there has been more time to perfect
the HTGR.

Carbon coat a success

Some of the earliest work on the HTGR was done during the 1950s in Britain.
One crucial innovation involved enclosing the reactor fuel in ceramic materials
that could endure very high temperatures without disintegrating. Engineers
at the Atomic Energy Research Establishment at Harwell, now known as AEA
Technology, stumbled onto the idea of coating reactor fuel with hard, impermeable
layers of carbon instead of wrapping it in a metal, which other researchers
were investigating. ‘I heard that the Royal Air Force, at Farnborough, were
developing pyrocarbon for use on rocket nozzles,’ recalls Peter Fortescue,
who was a deputy chief scientific officer at the Harwell laboratory. Fortescue
decided to use the pyrocarbon to cover columns of fuel in a small gas-cooled
reactor that an international team of researchers was developing. This 20-megawatt
reactor, called the Dragon, was the first HTGR. The hard carbon layer kept
moisture from coming into contact with the reactor’s uranium carbide fuel;
this was particularly important as water and uranium carbide can react chemically
to release acetylene, an extremely combustible gas. Engineers discovered
that the coating also trapped most of the radioactive by-products of fission,
although these tended to make the coating crack into pieces. Additional
years of research, notably in Britain, West Germany and the US, produced
the fuel particles used in modern HTGRs, which have a coating that does
not crack up. This is because the particles have an inner layer of porous
carbon, which acts as a buffer, absorbing some of the pressure of the radioactive
by-products.

Proponents of the HTGR believe that the reactor’s ‘inherent safety’
will calm the fears that have helped to stall the use of nuclear power in
many countries. Critics recall the exaggerated claims of the past, such
as the imminent arrival of electricity that would be ‘too cheap to meter’.
According to Robert Pollard of the Union of Concerned ¿ìè¶ÌÊÓƵs in the
US, ‘ ‘inherent safety’ is the functional equivalent of ‘too cheap to meter’.
This enchantment with future panaceas, ignoring the problems that are out
there (in current nuclear power plants); I guess I will never understand
¾±³Ù.’

Even if safety concerns could be allayed, the HTGR has financial hurdles
to cross. According to General Atomics, the first commercial version of
its modular HTGR will cost $1.8 billion to build. The company estimates
that it will have to cut the reactor’s price to $1.1 billion if it is going
to sell one. The $2 billion price tag on the HTGR for the US Department
of Energy arises from the need for additional technology to produce tritium.
If Congress endorses the DOE’s plans, government funding for an HTGR will
cut the price of future commercial gas-cooled reactors by as much as $500
million, according to General Atomics. This is because the DOE’s HTGR will
cover the cost of assembling the first set of machine tools that will be
required to build the reactors. But this leaves the cost of a commercial
reactor, at around $1.3 billion, still too high to attract the electricity
companies. General Atomics is looking to the government for more money.
Pollard says that the reactor doesn’t deserve any help: ‘The nuclear industry
should join the free enterprise system.’

Although the DOE’s nuclear weapons programme is now the HTGR’s best
hope for survival, it was the American armed forces that sent gas-cooled
reactors into near-oblivion 20 years ago. ‘Originally, the civil LWRs rode
on the back of the navy reactor test programme. That’s where they got the
money to get started,’ recalls Agnew, who joined General Atomics after retiring
as director of Los Alamos National Laboratory. Naval reactors needed to
be compact; they had to generate large amounts of power within a small space.
The high ‘power density’ resulted in extremely high fuel temperatures, so
that the fuel elements required constant cooling. Thus was born the light-water
reactor, now the dominant form of nuclear power plant. Engineers also proposed
alternative designs for nuclear reactors, including gas-cooled versions,
but they could not overcome the industrial momentum propelling commercial
light-water reactors. The US’s Atoms for Peace programme, established during
the 1950s, promoted civilian nuclear power based on established designs
of light-water reactors, and helped the LWR to take over the market for
nuclear power in most of Western Europe and Japan.

Electricity utilities tried to build bigger and bigger reactors, believing
that larger power sources would produce cheaper electricity. Light-water
reactors, providing huge quantities of concentrated power, soon dominated
the market. HTGRs were regarded as a mere curiosity of technology, symbolised
by the Dragon – a reactor that was not even linked to a turbine to produce
electricity. One year after the dragon began operating in Winfrith, Dorset
in 1966, the US and West Germany introduced their own small HTGRs. In 1976,
General Atomics finished a medium-sized HTGR, with a generating capacity
of 350 megawatts, at Fort St Vrain in Colorado. Although the fuel and reactor
core functioned perfectly, the plant quickly became an embarrassment: water-lubricated
bearings, used in fans that circulated the cooling helium, sprang frequent
leaks. There was no drain to remove the water, so operators had to shut
down the reactor frequently to dry it out. ‘It’s an albatross around our
neck,’ admits Agnew.

Only West Germany continued to pursue the development of HTGRs vigorously.
Several firms, including Siemens, the West German manufacturer, and ASEA-Brown
Boveri, the Swedish/Swiss group, hoped to use hot helium from an HTGR to
turn coal into gas, rather than to produce steam to drive turbines and generate
electricity. In 1985, a prototype HTGR, called the THTR-300, producing 300
megawatts of electricity, went into operation near Hamm, Nordrhein-Westfalen.
The reactor has been a financial disappointment, however, losing about $29
million per year. Last December, the operator of the THTR-300, the local
electricity authority, VEW, asked federal and state governments, which provided
most of the $2.3 billion for the project, for more money towards the plant’s
running costs. The request was turned down and the plant is now being decommissioned.

Spherical fuel

West German and American HTGRs both use fuel particles encased in layers
of carbon and silicon carbide. West German reactors use a ‘pebble bed’ core
in which the fuel particles are contained within small spheres of graphite,
6 centimetres across; earlier types of gas-cooled reactors use fuel rods
surrounded by blocks of graphite stacked on each other. American HTGRs use
tiny round particles of uranium, not much bigger than grains of sand, embedded
in large blocks of graphite. The pebble-bed reactor can be refuelled while
it is running: operators drop fresh spheres into the core and withdraw spent
ones from the bottom. General Atomics’ version of the HTGR, which is the
only one being developed in the US, is refuelled by replacing the entire
core.

Faced with shrinking markets and tight government funding, the world’s
builders of HTGRs have begun to join forces. Since 1987, General Atomics
and Siemens have together been trying to persuade an electricity utility,
probably in the US, to buy an HTGR. The partnership, which is trying to
sell reactors simultaneously to the Soviet Union and to the US Department
of Energy, has been a tricky one to manage. According to Richard Dean, vice-president
of General Atomics, the US administration has advised the firm, whose reactor
may become a mainstay of the American nuclear weapons programme, not to
participate in West Germany’s contract to build an HTGR in the Soviet Union.
Soviet officials have told the West German consortium to keep away from
any of General Atomics’ work on nuclear weapons.

Current versions of the HTGR use the hot helium that circulates through
the reactor core to produce steam, which then drives a turbine. Lidsky is
investigating whether gas turbines could be used to harness the energy of
the helium directly, rather than transferring it first to steam. If this
could be done, less of the heat would be lost, making the reactor a more
efficient producer of electricity. In Lidsky’s workshop at MIT, researchers
from West Germany and Japan are cooperating in an effort to show that the
idea is practical.

Changing times, not new technology, is the main reason why renewed enthusiasm
surrounds the HTGR. The light-water reactor has led nuclear power to a standstill
in the US, following the accident at Three Mile Island, Pennsylvania, in
1979. Since then, as a result of public opposition and rising costs, no
nuclear power stations have been ordered in the US; many that were already
ordered have been cancelled. Even those that were being built, and are now
ready to use, are not guaranteed an easy life. At Shoreham, near New York
City, public protests have blocked the operation of a plant that has just
been finished, after more than a decade of construction.

Proponents of the HTGR say that the companies have blocked the way for
safer alternative reactors by refusing to admit that their reactors have
serious shortcomings. The HTGR now waits in the wings. It is unlikely to
emerge onto the centre stage of energy production until demand for power
once again outstrips supply. Even then, it may languish, unless government
sources invest additional billions of dollars to demonstrate the practical
usefulness of new generations of gas-cooled reactors.

Lidsky, who believes in the HTGR with the zeal of a new convert, is
convinced that it represents the only way forward for nuclear power. ‘The
LWR is no more a surrogate for the potential of nuclear power than the Hindenburg
was for the potential of air transportation,’ he says. ‘The HTGR is absolutely
essential if we are to have a renewal of nuclear power.’

Dan Charles is a science and technology writer based in Boston, Massachusetts.