Environmental Engineering Reference
In-Depth Information
Technical Note
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reactor to what the design can stand without damage. It also
transfers the heat to the power unit where the electricity is
generated. In an LWR the coolant is ordinary water.
Another kind of reactor is moderated and cooled with what
is called heavy water. Ordinary water, called light water in the
reactor business, is composed of two atoms of hydrogen and
one of oxygen. Heavy water has two atoms of deuterium,
instead of hydrogen. The nucleus of deuterium has a neutron
combined with the proton of ordinary hydrogen giving it twice
the mass and hence the name heavy hydrogen. In the moder-
ating process ordinary hydrogen can capture some of the
neutrons while deuterium has a much smaller capture prob-
ability since it already has a neutron bound to the proton of the
nucleus. LWRs need enriched uranium to make sure that a
neutron sees a uranium atombefore it is captured. Heavy water
reactors, because of the low capture probability in the moder-
ator, can use natural uranium as fuel. They are not widely used
because of their expense even after taking into account the
savings from the use of natural uranium fuel. A relative of the
heavy water moderated reactor is one moderated by carbon
like North Korea
s. It too can run on natural uranium.
Economies of scale have driven up the size of the present
generation LWRs, most of which are in the gigawatt range.
There is increasing interest in smaller reactors for places that
are not connected to a high-capacity power grid or where
the economy is not developed enough to be able to use the
full output of a big reactor. Several groups have begun
development of smaller reactors. Toshiba, the Japanese
reactor builder, has developed its
'
S reactor with an output
ranging from
MWe. This
design has a sealed core that only needs refueling every
MW electricity (MWe) to
years, making it much easer to operate and monitor for
potential proliferation problems. Two other companies,
Hyperion Power Generating Systems in New Mexico and
