Nuclear-Fueled Power Plants - Energy and the Environment - page 132

facilities a country possesses, on economic incentives of a particular government, and on the wish
of a country to produce weapons grade plutonium, it may be more advantageous to operate a
natural uranium/heavy water reactor than to operate an enriched uranium/light water reactor. In the
United States, because of the availability of enriched uranium, no heavy water power plants are
operating.

6.4.3 Gas-Cooled Reactor (GCR)

A reactor type that was developed in particular in Great Britain is the gas-cooled reactor .In
fact, the first commercial power plant put into operation, in 1956 at Calder Hall, was a GCR.
Generally, these reactors are fueled by natural or enriched uranium, either metallic or ceramic
uranium oxide. The moderator is graphite; and as the name implies, the coolant is a gas, nor-
mally CO 2 , but helium can also be used. Because of the lower heat transfer capacity of gases as
compared to liquids, the contact surfaces and flow passages in the reactor must be larger than
those in liquid-cooled reactors. In order to obtain a reasonable thermal efficiency, GCRs are run at
higher temperatures than PWRs or BWRs. This necessitates cladding and piping materials that can
withstand the higher temperatures. Some GCRs are using enriched uranium to boost the thermal
efficiency.
An example of a high-temperature gas (CO 2 )-cooled reactor is operating at Hinkley Point in
the United Kingdom. Its net output is 1250 MW. The fuel is UO 2 with 2.6% 235 U. The CO 2 leaves
the reactor at 655 ◦ C and 4.3 MPa. The CO 2 is pumped to a heat exchanger where steam is generated
at 540 ◦ C and 17 MPa. The plant achieves a thermal efficiency of close to 42%.
In the United States a 40-MW high-temperature gas-cooled power plant was constructed and
operated near Philadelphia, Pennsylvania. It is now decommissioned. Another 330-MW plant was
operated near Platteville, Colorado. While providing interesting experience, the plant had many
engineering problems and is by now also decommissioned. Research and development on GCRs
is continuing, and their revival may occur in the future.

6.4.4 Breeder Reactor (BR)

In a breeder reactor, fissile nuclei are produced from fertile nuclei. The principal breeder mechanism
is the conversion of 238 Uto 239 Pu as shown in equation (6.2). The intermediary 239 U has a half-life
of 23 minutes, then converts to neptunium 239 Np with a half-life 2.4 days, which in turn decays to
239 Pu, with a half-life of 24,000 years. The 239 Pu formed, while a fissile nucleus, does not participate
to a significant extent in the chain reaction, but accumulates in the spent fuel from which it is later
extracted and reused.
Unlike 235 U, which efficiently undergoes fission with slow thermal electrons whose energy is in
the tenths of eV range, 238 U captures efficiently fast neutrons in the MeV range. To obtain this wide
spectrum of neutron energies, a coolant/moderator other than light or heavy water is required. The
preferred coolant is liquid sodium, and such a reactor is called the liquid metal fast breeder reactor
(LMFBR). The sodium nucleus has a larger mass than does hydrogen or deuterium; therefore a
neutron colliding with a sodium nucleus bounces off with nearly its original momentum, whereas
a neutron colliding with a hydrogen nucleus (e.g., a proton) imparts nearly half of its momentum
to hydrogen.