Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 255
paragraph explains the discrepancy, but should be considered nonessential
reading. The fission process typically results in a few spare
neutrons. Each left-over (unbound) neutron deprives us of at least 8MeV
in unrealized binding potential, 33 <strong>and</strong> the subsequent β − decays from
the neutron-rich daughter nuclei to stable nuclei also release energy
not accounted in Table 15.7. Both of these contribute to the shortfall in
comparing 172 MeV to 210 MeV, but even without this, we got a decent
estimate just using the graph in Figure 15.10.
33: Each missing neutron deprives us of
more than the st<strong>and</strong>ard ∼8 MeV per nucleon,
as neutrons have no penalty for repulsive
electric charge. The 8 MeV per nucleon
is an average over protons <strong>and</strong> neutrons.
15.4.4 Practical Implementations
As we saw above, nuclear fission involves getting fissile nuclei—generally
235
U—to split apart by the addition of a neutron. The following criteria
must be met:
235
1. presence of nuclear fuel ( U);
2. presence of neutrons, provided as left-overs from earlier fission
events;
3. a moderator to slow down neutrons that emerge from the fission
events at high speed;
4. a high enough concentration of nuclear fuel that the slowed-down
spare neutrons are likely to find fissile nuclei;
5. neutron absorbers in the form of control rods that can be lowered
into the reactor <strong>and</strong> act as the main “throttle” to set reaction speed
(thus power output), <strong>and</strong> also prevent a runaway chain reaction;
6. a containment vessel to mitigate radioactive particles (gamma
rays, high-speed electrons <strong>and</strong> positrons) from escaping to the
environment.
Figure 15.16 shows a typical configuration.
control rod actuators
control rods
fuel rods
containment vessel
steam out
water in
Figure 15.16: Typical boiling water reactor
design. A thick-walled containment vessel
holds water surrounding 235U
fuel rods.
The water acts as the moderator to slow
neutrons <strong>and</strong> also circulates around the
rods to carry heat away, boiling to form
steam that can run a st<strong>and</strong>ard power plant.
Control rods set the pace of the reaction
based on how far they are inserted into
the spaces between fuel rods. Extra control
rods are poised above the reactor core ready
to drop quickly into the core in case of
emergency—suddenly bringing the chain
reaction to a halt.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.