328 ENVIRONMENTAL, ENGINEERING
Reactor core (Uranium fuel)Figure 16-5. 'Qpical pressurized water nuclear reactor.the core. One such reaction is
9~l.J~~~ + on1 = 42Mog5 + 57LaI3' + 2 on1 + 204 MeV.
Several features of this reaction merit discussion.0 A large amount of energy is released in the reaction: 204 MeV per uranium atom,
or 80 million Btu per gram of uranium. Uranium fission released about 100,000 times as
much heat per gram of fuel as natural gas combustion. The development of commercial
nuclear power is based on this phenomenon.
0 One neutron is required for the fission reaction, but the reaction itself produces
two neutrons, each of which can initiate another fission reaction that will produce
two neutrons, and so on, resulting in afision chin reaction. However, the con-
centration of fissile material (U-235 in commercial power reactors) must be high
enough so that neutrons produced are likely to collide with U-235 nuclei. The mass
of fissile material needed to sustain a fission chain reaction is called the critical
0 Neutron flow in a reactor may be interrupted, and the fission reaction stopped, by
inserting control rods that absorb neutrons (Fig. 16-5). Although inserting control rods
stops the fission reaction, the insertion does not stop heat generation in the core, since
the many radioactive fission products continue to emit energy. Therefore, continued
coolant flow is critical.
0 In this particular reaction, Mo-95 and La-139 are the products of the fission.
However, a fissile nucleus can split apart in about 40 different ways, yielding approxi-
mately 80 different fission fragments. Althoughmany of these have very short half-lives
and decay very quickly, some have long half-lives. These long half-life fission productsmass.