Fissionable material, that is, material with the ability to
fission, occurs in some isotopes of heavy elements. The most useful
ones are uranium-235 (U-235) and plutonium-239 (Pu-239).
In brief, when fission occurs, an atom of nuclear fuel (and
we're talking about the fission of nuclear fuel here) splits. This
splitting yields what are called fission fragments, and the atom
splits approximately in two. Note that there are several options as
regards what the atom splits into. It can split into "A" and "B" or
it can split into "C" and "D" or a few other resultants. But
regardless, the fission fragments recoil after fission occurs, and
most of the energy of this recoil, which is kinetic energy on the
atomic scale, is expressed as heat (thermal energy).
The fuel in a reactor, whatever it is, is tightly sealed in a
metal jacket (cladding). The atoms of the fuel are being held
rigidly, and when fission occurs, the recoil of the fragments is
"contained" in the fuel itself. This mechanical energy gives rise
to the appearance of thermal (heat) energy. The lion's share of
energy released by fission is carried off in the recoil of the
fission fragments, which is kinetic (mechanical) energy. Said
another way, the fission fragments can't "go anywhere" in the fuel
matrix, and the kinetic energy they come away with after fission is
captured in the fuel and appears as heat.
There are also free neutrons released, and they carry off
kinetic energy like the fission fragments. These neutrons are
slowed down in the moderator to increase the chances that they will
be captured by other fuel atoms and cause other fission reactions.
They will continue the chain and cause more fissions following
neutron capture events. Electromagnetic radiation in the form of
gamma rays is also produced in nuclear fission. It must be shielded
against. In review, most of the energy of fission appears in the
kinetic energy of the fission fragments, and that kinetic energy is
converted into heat within the fuel element.
A nuclear reactor is a core made up of an assembly of fuel
bundles, which are made of fuel elements, usually using enriched
uranium as the nuclear fuel. In the pressurized water reactor, this
assembly is inside a pressure vessel, as water is used as the
primary coolant, and also the moderator. It can be ordinary water
or heavy water. We also see some reactor designs that use graphite
as a moderator. Also in the reactor are the control rods.
The primary coolant is the heat transfer medium. It carries heat
out of the core and into the steam generator and back to the core
in a closed loop. The reactor is made to reach criticality on start
up when control rods are pulled. The chain reaction within the fuel
will produce a steady power output as a result of nuclear fission,
and this will release heat. The heat is used to produce steam in a
steam generator, and the steam is feed to a conventional steam
turbine/generating unit to generate electric power.
For those investigators attempting to trace the transformations
of energy, nuclear energy (the binding energy that holds atomic
nuclei together) is converted into electromagnetic and kinetic
energy in fission. The electromagnetic energy, which appears as
gamma rays, is largely lost as we cannot "capture" and "use" it.
The kinetic energy (mechanical energy) of the fission fragments is
converted into thermal energy (heat) because the fission products
are "trapped" in the fuel matrix and cannot "fly free" as they
would in air. The thermal energy created in the fuel bundles heats
the fuel, and the primary coolant picks up that heat and transports
it to a steam generator. The steam generator turns secondary water
into steam, and the steam is piped to a turbine. The thermal energy
of the steam is converted into mechanical energy in the turbine,
and the mechanical energy is transferred into a generator. The
generator converts the mechanical energy into electrical
(electromagnetic) energy, and that is the useful product we derive
from nuclear fission.
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