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ACCORDING to many of its proponents within the US Department of Energy,
the modular HTGR that General Atomics is trying to sell is the safest possible
generator of electricity from nuclear power. Almost nothing, from a really
dedicated saboteur to the most poorly trained operator, could turn the helium-cooled
reactor into a public menace, they say.

The first key to the reactor’s safety is its small size. According to
Harold Agnew, a former executive at General Atomics, now retired, the firm’s
engineers figured out how large a reactor could be and still survive a loss-of-coolant
accident without any human intervention. ‘That calculation set the size
of the reactor,’ says Agnew. Small reactors, producing only a few hundred
megawatts of thermal energy, heat up less quickly than large ones if cooling
systems fail, and their heat is more easily dissipated. In addition the
steel reactor vessel will be placed underground, but air will be allowed
to flow around it. If the flow of cooling helium through the core is cut
off, the air around the reactor will help cool it through natural convection.
If the air flow were cut off as well, the reactor’s heat would simply dissipate
into the surrounding earth. Even in this event, the fuel would stay below
1500 Degree C, according to calculations done by General Atomics and by
engineers at the US Department of Energy (DOE). The fuel particles are designed
to withstand temperatures of at least 1600 Degree C, and almost all would
survive heat up to 2200 Degree C.

Critics doubt that the safety of any nuclear power plant can be that
simple. They point out that graphite is used for the core of the HTGR, and
that two of the worst nuclear accidents, Windscale in England and Chernobyl
in the Soviet Union, involved burning graphite.

A mild controversy surrounds the question of whether graphite burns.
Engineers at General Atomics maintain that it is prac

tically impossible to burn graphite. They point out that modern graphite
has fewer impurities than the material used at Windscale, and pure graphite
is much less likely to burn than graphite mixed with small amounts of other
substances. According to the DOE investigations, little of the heat produced
at Chernobyl came from the burning graphite.

Visitors to General Atomics who ask about the combustibility of graphite
are shown a video of someone trying to ignite a block of graphite with a
blowtorch, without success. The tape shows the torch heating the graphite
red-hot, while pure oxygen flows over it. Even after several hours, there
is no obvious change in the graphite. At the spot where the torch was focused,
however, a slight depression is created where a small amount of graphite
has oxidised.

That kind of experiment does not convince everyone. Since graphite is
an excellent conductor of heat, the heat from the torch rapidly flows away
from the hot spot. Graphite will burn only when an entire block is extremely
hot, with no cooler areas to siphon off the heat. Critics say the experiment
does not simulate conditions in a reactor core.

Peter Kroeger, a scientist at the DOE’s Brookhaven National Laboratory,
carried out studies of the HTGR for the Nuclear Regulatory Commission, which
oversees nuclear power in the US. He maintains that even if there were a
graphite fire in the HTGR, it would pose little danger. Kroeger, using computer
simulation, analysed an accident in which every nightmare came true. He
assumed that the cooling system failed and the reactor was not shut down.
In addition, holes in both ends of the reactor vessel allowed air to flow
through the hot core, bringing oxygen to supply a fire. He found that the
graphite would burn slowly, but would add only a small amount of heat to
the reactor. If left alone, the graphite would burn away in about two months,
he says.

According to Kroeger’s studies, about 10 per cent of the reactor core
would reach a peak temperature of 1700 or 1800 Degree C after about three
days. This would be enough to cause some damage. A small fraction, less
than a tenth of a per cent, of the casings of the reactor’s fuel particles
could begin to crack, releasing radioactivity through the ruptured reactor
into the rest of the plant. But this scenario is impossible, says Kroeger,
because it would require several things to happen that cannot occur together.

The final key to the HTGR’s safety lies in the novel ceramic coating
of the fuel. The uranium fuel in the American and West German reactors is
surrounded by successive layers of porous carbon, silicon carbide, and pyrocarbon.
When the uranium fissions, it releases a variety of radioactive by-products,
such as isotopes of iodine, strontium and caesium. The porous carbon layer
absorbs gases that could cause the hard outer layer to crack, and layers
of silicon carbide and pyrocarbon prevent the isotopes from escaping into
the core. This means that if the reactor vessel cracked, or walls were blown
apart by saboteurs, there would be less radioactive material free to escape
from the core than there would be from a light-water reactor. General Atomics
has argued that the fuel is so effective at trapping radioactive substances
that the modular HTGR does not need a massive containment structure around
the plant. The Nuclear Regulatory Commission is still studying whether the
reactor should have a containment structure. It is concerned that defective
batches of fuel particles could slip by General Atomics’s computerised inspection
system. If the fuel failed at a lower temperature than expected, large amounts
of radioactivity could enter the environment after the reactor vessel ruptured
during a serious accident.