Elements of Modern Physics
354
have a large thermal capacity). This agent or the coolant gives this energy to
water (see Fig. 9.9) transforming it into steam which operates the steam turbines.
Breeder Reactors
So far mainly moderator-operated reactor bases on thermal neutrons has been
discussed. It is possible to run a reactor on fast neutrons by using a fuel with
enriched
235
U or
239
Pu. The fast neutrons released in the fission are used to
transform
238
U into
239
Pu (or
232
Th into
233
U) which can undergo fission by
capturing thermal neutrons. The core of such a reactor will contain two materials,
say, fissionable fuel
239
Pu and the potential fuel
238
U. If the conditions are such
as to produce more fuel than the amount burnt, the reactor is known as breeder
reactor (it breeds fissionable fuel).
Uncontrolled Chain Reactions
If two or more pieces of almost pure
235
U (or
239
Pu), each of sub-critical mass,
are brought together, the total mass may be over critical. In such a case, the
neutrons rapidly multiply and the energy release will be explosive, as in the
case of an atomic bomb.
Steam to
steam turbine
Water
in
Core
Coolant
Control rods
Pump
Fig. 9.9 Schematic diagram of a power reactor.
In an atomic bomb, the chain reaction can be triggered by secondary cosmic
ray neutrons. Here, the pieces must be kept together under pressure so that the
chain reaction can build up to explosive proportions. This is done by using
ordinary explosives to shoot one piece into another. Nevertheless, only a part of
the fuel has time to react before the final explosion.