The Foundations of Chemistry

with mass numbersgreater than 250 do this because they are too big to be stable. Most

nuclides with mass numbers between 225 and 250 do not undergo fission spontaneously

(except for a few with extremely long half-lives). They can be induced to undergo fission

when bombarded with particles of relatively low kinetic energies. Particles that can supply

the required activation energy include neutrons, protons, alpha particles, and fast elec-

trons. For nuclei lighter than mass 225, the activation energy required to induce fission

rises very rapidly.

In Section 26-2 we discussed the stability of nuclei with even numbers of protons and

even numbers of neutrons. We should not be surprised to learn that both^233 U and^235 U

can be excited to fissionable states by slow neutrons much more easily than^238 U, because

they are less stable. It is so difficult to cause fission in^238 U that this isotope is said to be

“nonfissionable.”

Typically, two or three neutrons are produced per fission reaction. These neutrons can

collide with other fissionable atoms to repeat the process. If sufficient fissionable mate-

rial, the critical mass,is contained in a small enough volume, a sustained chain reaction

can result. If too few fissionable atoms are present, most of the neutrons escape and no

chain reaction occurs. Figure 26-11 depicts a fission chain reaction.

In an atomic bomb, two subcritical portions of fissionable material are brought together

to form a critical mass. A nuclear fission explosion results. A tremendous amount of heat

energy is released, as well as many radionuclides whose effects are devastating to life and

the environment. The radioactive dust and debris are called fallout.

NUCLEAR FISSION REACTORS

In a nuclear fission reactor, the fission reaction is controlled by inserting materials to

absorb some of the neutrons so that the mixture does not explode. The energy that is

produced can be safely used as a heat source in a power plant.

26-15

1026 CHAPTER 26: Nuclear Chemistry

Kr

n

U

Ba

Sr

Xe

n

n

n n n n n n

n

92

36

(^9236) Kr
(^9236) Kr
235
92
(^23592) U
(^23592) U
141
56
(^14156) Ba
(^14156) Ba
(^23592) U
144
54
90
38
Figure 26-11 A self-propagating nuclear chain reaction. A stray neutron induces a single
fission, liberating more neutrons. Each of them induces another fission, each of which is
accompanied by release of two or three neutrons. The chain continues to branch in this way,
very quickly resulting in an explosive rate of fission.
The launching of the nuclear
submarine Hyman G. Rickoverinto
the Thames River in Connecticut
(August 27, 1983).