Environmental Engineering Reference
In-Depth Information
Technical Note
.
: Nuclear Power Primer
A nuclear power reactor is mainly characterized by three
things: fuel, moderator, and coolant. The work horse of today
s
nuclear energy supply is the light water reactor (LWR). There
are other kinds of reactors using different fuel, moderators, and
coolants, but these are not widely used for energy production.
In an LWR, the fuel is enriched uranium (U). Natural uran-
ium comes in two isotopic forms (the isotopic number is the
total number of protons and neutrons in the nucleus), U-
'
andU-
.OnlytheU-
is
ssionable and this makes up only
% of natural uranium. The LWRs of today use uranium
enriched to about
.
, and enrichment today is
done mainly with gas centrifuges. When a gas containing the
two uranium isotopes is spun at high speed, the heavier isotope
tends to move toward the outside. The gas near the center is
taken off and goes to another centrifuge where it is further
enriched. Each centrifuge gives only a tiny enrichment and a
“
%to
%ofU-
of thousands of gas centrifuges is used to enrich the
uranium to the necessary
cascade
”
% level. The same process can be
continued beyond the level required to produce reactor fuel to
produce the
-
% enrichment desired for nuclear weapons.
Themoderator controls the energy of the neutrons from the
fission process. When a neutron is captured in U-
,the
nucleus will split (
(fission) into two lighter nuclei, releasing a
large amount of energy plus a few more neutrons.
The probability of neutron capture in a given nucleus depends
on the energy of the neutron. Fission neutrons tend to have
high energy, but the probability of capture and
is large for low-energy neutrons. The moderator, which in an
LWR is ordinary water, controls the neutron energy. Neu-
trons collide with the proton nucleus of the hydrogen in water
and lose energy at each collision, quickly reaching a
low enough energy tomake capture onU-
ssion inU-
highly probable.
The coolant in a reactor takes away the heat generated in
the
fission process,
limiting the temperature rise in the
