Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 251
their way to a halt. Gamma rays 25 are also released. By catching all of
this energetic output, the surrounding material gets very hot <strong>and</strong> can be
used to make steam.
25: ...veryhigh energy photons
15.4.2 Chain Reaction
235
As we have seen, in order to get fission to happen, we need U <strong>and</strong>
some w<strong>and</strong>ering neutrons. Once fission commences, the breakup of
the nucleus usually “drips” a few spare neutrons, like crumbs left after
cutting a piece of bread. The left-over neutrons provide a replenished
source of neutrons ready to initiate more fission events. Now the door
is open for a chain reaction, in which the neutrons produced by the
fission events are the very things needed to stimulate additional fission
events.
When the nucleus splits, any extra neutrons come out “hot” (high speed),
which tend to bounce off uranium nuclei without sticking. They need
to be slowed down, which is accomplished by a moderator: basically
light atoms 26 that can receive the neutron impact as a sort of damping
medium. Then the main trick is to prevent a runaway that could occur
if too many neutrons become available; in which case it’s a party that
can get out of control. So nuclear plants employ control rods containing
materials particularly effective at absorbing (trapping) neutrons. The
colors of the lower halves of some squares in the Chart of the Nuclides
(Figure 15.4) indicate neutron capture cross section. Boron (
10 B)isa
favorite choice to soak up neutrons <strong>and</strong> tame (or even halt) the reaction.
The goal is to maintain a chain reaction that produces a net balance
of exactly one unabsorbed slow neutron per fission event, available to
235
attach itself to a waiting U nucleus.
26: . . . usually either water or carbon in the
form of graphite
15.4.3 Fission Accounting
The nucleus (uranium in the present discussion) always breaks up
into two largish pieces, possibly accompanied by a few liberated spare
neutrons. Because of the way the track of stable elements curves on the
Chart of the Nuclides, the resultant pieces are likely to be neutron rich,
to the right of the stable nuclei. To underst<strong>and</strong> this, refer to Figure 15.14
<strong>and</strong> the associated caption.
The math always has to add up: nucleons are not created or destroyed
during a fission event. They just rearrange themselves, so the total
number of neutrons stays the same, as does the total number of protons.
After the split, β − decays will carry out flavor changes, but we’ll deal
with that part later.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.