(published in The New Citizen, February 2002, reprinted in April 2006 edition.)
This diagram, a cutaway of Fig. 1, illustrates the breathtakingly
simple function of a Pebble
Bed Modular Reactor (PBMR),
as designed by South Africa’s Eskom
company. The steel pressure
reactor vessel of the PBMR is six
metres in diameter and 20 metres
high, inside a building that is 21
metres below ground. The walls of
the reactor vessel are lined with
one- metre thick graphite bricks.
Inside the reactor vessel are
310,000 fuel “pebbles” which are
the size of tennis balls, plus
130,000 graphite balls, which
moderate the reaction. The fuel
pebbles contain uranium, which
releases the neutrons that cause fissioning
in other uranium, thereby
releasing even more neutrons that
expand the process in what is
known as a chain reaction, while
the moderator pebbles slow the
neutrons down enough to ensure a
controlled chain reaction.
The fuel pebbles consist of
about 15,000 tiny particles of uranium
oxide, each coated with layers
of ceramics and silicon carbide,
forming an impenetrable barrier
which contains the fuel. These particles
are then mixed with graphite
and moulded into pebbles.
These pebbles can operate even at
very high temperatures of nearly
900 degrees Celsius, and in fact can withstand
temperatures at which normal fuel
rods in conventional reactors
would fail.
A further safety aspect
of these pebbles is that the radioactive
fission products of the spent
fuel are locked inside the fuel particles,
thanks to their silicon carbide
coating. Therefore, even in
the worst conceivable emergency,
the radiation is safely
contained, and after use,
the pebbles can be safely
stored, very cheaply.
The interior of the fuel pebbles
that contain the nuclear reaction and by-products, and make PBMRs “meltdown
proof”.
To produce electricity
in a PBMR, helium gas at
500 degrees Celsius is inserted at the
top of the reactor, and
passes among the fissioning
fuel pebbles, leaving
the reactor core at 900 degrees Celsius.
From there it passes
through three turbines,
the first two driving compressors,
and the third the
generator. There the natural
thermal expansion of
the helium is transformed
into the rotational motion
to generate electricity.
The expanded helium is
then recycled into the reactor
core by two turbo-compressors.
The helium
leaves the recuperator at
about 140 degrees Celsius, and its temperature is lowered further to about 30
degrees Celsius in a water-cooled
pre-cooler. The helium
is then repressurised,
and moves back to the
heat exchanger to pick up
heat before going back to
the reactor core. This direct
cycle helium turbine
simplifies the normal reactor
operations, and
makes many standard aspects
of conventional reactors
unnecessary. The
outlet temperature of
900 degrees Celsius is also far higher than the
280–330 degrees of conventional reactors,
and gives this type of reactor
its name: high temperature reactor.
The inherent and passive safety systems of the PBMR make it
“meltdown proof”. In any imaginable
accident scenario, the reactor
shuts itself down, without any
additional safety systems. Further,
there is a self-stabilising temperature
effect in the reactor core: if
the temperature rises, it slows
down the neutron production that
is central to the chain reaction fissioning
process, and fission decreases,
because of the large
amount of unfissionable uranium-
238 in the fuel particles which capture
the neutrons.
Continue onto Thorium: The Preferred Nuclear Fuel of the Future
Return to Nuclear and Thorium Power
