Nuclear Energy

Pressurised Heavy Water Reactor Heavy water is a more
efficient moderator than
ordinary water as it
absorbs less neutrons
than the latter. Further,
the design of the PHWR
is such that most of the
moderator which is in the
Calandria is at lower
temperature than the
moderator in PWRs:
therefore, the neutrons in
PHWRs are at optimum
speeds to cause fission,
implying that the PHWR
is more efficient in fissioning U-235 nuclei. Both these advantages mean that the PHWR can sustain
a chain reaction with lesser number of U-235 nuclei in uranium as compared to PWRs, which is
why it uses unenriched uranium as nuclear fuel.
Thus, in PHWRs, enrichment costs are saved, but the disadvantage is that heavy water is
also very costly, costing hundreds of dollars per kilogram.
(iii) Fast Breeder Reactors
In the uranium fuelled reactors discussed above, U-235 was fissioned by slow moving
neutrons, and the energy released in the fission reaction is used to generate steam to run a turbine
and generate electricity. Neutrons produced by fission have high energies and move extremely
quickly. These so-called fast neutrons do not cause fission as efficiently as slower-moving ones, so
they are slowed down using a moderator.
In contrast, a fast breeder reactor uses a mix of oxides of plutonium-239 (or Pu-239) and
uranium-238 as the fuel (also called MOX fuel). [Plutonium is usually obtained by reprocessing
waste from the uranium fuelled reactor.] Pu-239 is the fissile material. It is even better at fissioning
than U-235. The energy released in the fission reaction is transferred via the coolant to produce the
steam used to power the electricity generating turbines. All current fast reactor designs use liquid
metal (generally sodium) as the primary coolant.
The second difference with U-235 fuelled reactors is that the coolant used in FBRs is not a
moderator. So its neutrons are fast moving. While these neutrons are not good at causing fission,
they are readily captured by U-238 to transform it into U-239 which then beta decays (that is, emits
electrons from its nucleus – see Chapter 3 for more on this) to form Pu-239, which then can be
reprocessed and used as more fuel for the reactor.
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