What is radioactivity?
When speaking about nuclear processes, the first terms that usually come to mind
are alpha, beta, and gamma decay. These are the three modes of natural radioactivity.
As a neutron in the nucleus decays into a proton, anti-electron neutrino, and an
electron, a beta particle is released. Since an electron is not a nuclear particle,
it must be expelled from the nucleus. Subsequently it behaves exactly like any "normal"
electron meaning in that it has the same mass and charge. During beta decay the
atom's atomic number increases by 1, while its number of nucleons remains unchanged.
An alpha decay occurs when an excited nucleus emits four nucleons - two protons
and two neutrons. This particle carries a positive charge of +2. Gamma decay occurs
when an excited nucleus "settles" to a lower energy level and releases its extra
energy in the form of a burst of electromagnetic radiation having energies ranging
from 41.4 keV to 414 keV. Gamma radiation always accompanies atomic fission when
an excited daughter nucleus, or decay product, is formed.
Nuclear fission refers to when an unstable radionuclide breaks down into two smaller
daughter nuclei. This is usually done through the absorption of either a thermal
(slow) neutron or a fast neutron. The terms
fissile and fertile
are used to describe this sensitivity. Fertile nuclei
are fissionable when they are struck by a neutron. A fissile nucleus is one which,
when it naturally fissions, produces one or more neutrons along with other radioactive
nuclides that can subsequently emit alpha, beta, or gamma radiation.
There are three
of uranium: 234
U (0.005%), 235
U (99.275%). 238
U has a fertile nucleus which, when
it absorbs neutrons having energies at 1 MeV or above1
U which through subsequent beta decay produces 239
an extremely fissile material.
U is a naturally occuring fissile nucleus which, when it absorbs a
low energy, or thermal neutron, fissions about 85% of the time1
Otherwise, it becomes an excited 236
U and fissions into a variety of
daughter nuclei centering around the atomic masses of 95 and 137. A common pair
of fragments are 140
Xe and 94
2 fast neutrons
. These daughter nuclei eventually decay into 94
Ce. Nuclear power plants in the United States, France, Spain,
use uranium dioxide (UO2
) that is enriched at less than 5% in the 235
A common radioisotope found in spent nucelar fuel is
. It has a half-life of 29.1 years and is
as a radioactive tracer since it chemical behavior is similar to
that of calcium. Moreover, the heat generated by its decay has been converted into
electricity for remote portable power supplies. Another common radioactive isotope
. It has a half-life of 30 years and is used in a myriad of industrial
gauges. Its gamma rays are sometimes used for irradiation in cancer therapy as well
as in some industrial applications.2
The term half-life corresponds to the statistical length of time it takes for half
of a radioactive sample to decay. In the above diagram, various half-lives in seconds,
minutes, days, and hours are denoted with a T
. A half-life does not mean
that the all of the radioistope is no longer present from the original sample.
There are various categories of equilibrum in nuclear processes. As the original radioisotopes disintegrate, attention must be paid to the growth of decay products and the relative
stability of the combined activity rates. There are two types of equilibrium states
between the activity of original radionuclide and its decay products: transient,
where both decay at approximately the same rate as shown on the left-hand graph; and, secular, where the half-life
of the original is much greater than its decay products and the activity levels
off as shown by the horizontal line on the right-hand graph.
Natural and Enriched Uranium
The activity of uranium varies with its composition. In the case of natural uranium
ore, secular equilibrium exists for millions of years between both 238
U and their decay products. In 2009
, Australia, Kazakhstan,
and Canada accounted for 52% of the world’s uranium production.
When uranium ore is initially processed (reference the
Shockwave video Responsible uranium extraction in Niger
) into yellowcake,
), its activity rate
is drastically modified. Note on the following graph that the activity initially
changes from ore's value of 175 Bq/gram, in the top uranium ore graph, to 25 kBq/gram on the bottom natural uranium graph. Eventually the activity rate doubles within 1 year to 50 kBq/gram, and finally reaches 175 kBq/gram within 1000 years. A becquerel(Bq) represents an activity rate of one disintegration/sec while a kBq represents a rate of 1000 disintegrations/sec.
of enriching uranium hexafluoride use gaseous diffusion
or a gas centrifuge. Both methods require expensive capital outlays for equipment,
but the power required to operate the gas centrifuges is less expensive. Once the
uranium-238 is enriched to 3-4% uranium-235, the activity rate for the enriched uranium fuel (UF6
originally processed from yellowcake is shown below.
The enriched UF6
powder and fabricated into uniform, ceramic
The pellets are next inserted into corrosive-resistance, Zircaloy-tubes and sealed
by a "special welding process2
in an inert (helium
atmosphere as a cover gas and the helium is sealed into the tubes at about 400
psi pressure when manufacturing" is finished.2
The fuel rods are then
grouped together to form fuel assemblies, or bundles.
Once placed inside a reactor, the fuel assemblies are rotated three times, at 18-month intervals. When their fuel is exhausted, the asemblies are removed from the reactor and moved to a cooling pond where they spend up to
. During this time they can emit radiation at a
rate of up to
5% of their original heat levels
. After being removed from the cooling pond,
they are sealed in steel or reinforced concrete
for long term storage. Notice in the chart provided below that the activity rate for the spent nuclear fuel (SNF) has numerous additional padioactive pollutants from its time in the reactor.
There are four possible results when a gamma photon passes through matter. The simplest
of these possibilities is that the photon passes through the object without hitting
A second options is that the photon will hit an atom and be completely absorbed
in which case two reactions may occur depending on the binding energy of the electron
and the energy of the gamma photon. If the the energy of the gamma photon is less
than the binding energy, then the electron will move into an excited state but remain
attached. If the energy of the gamma particle is more
than the binding energy, then the photoelectric effect
occurs and the electron is ejected. The kinetic energy
of the electron can be determined by subtracting
the binding energy of the electron from the energy of the gamma photon. The photoelectric
effect is most likely to take effect when the difference between the binding energy
and photon’s energy is small meaning the gamma photon must have low energy
or the electron must have a high binding energy.
Due to this, the photoelectric effect occurs most frequently when the energy of
the gamma particle is less than half a megaelectron volt (MeV).
A third possibility is that the photon will hit an atom and be scattered. Depending
on whether or not the photon deposits any energy, two things can happen. If the
photon deposits no energy
, which only occurs when the photon has little energy,
then the gamma particle changes directions while losing very little energy. If the
photon does deposit energy in an electron, however, the electron is ejected and
the photon is scattered in a random direction with less energy than before which
is known as Compton scattering
. Compton scattering occurs most frequently
when the gamma photon has between 0.5
MeV and 3.5 MeV. Depending on the energy of the emitted photon, it will go through
either the photoelectric effect or Compton scattering until all of its energy is
Finaly a process called pair production can occur. In pair
production, the gamma particle interacts with the nucleus of an atom and converts
energy into matter. Two particles
are produced: an electron and a positron each with
a mass equal to a rest mass energy of .51 MeV. This is why pair production cannot
even occur unless the gamma photon has at least 1.02 MeV
. It rarely occurs, however, until the energy levels
reach several MeV. The excess energy is converted into kinetic energy that is divided
evenly among the two particles. The two particles then zoom off through space either
attracting electrons for the positron and repelling electrons for the electron until
the positron is captured by an electron and the two particles are destroyed. When
this happens two more photons with .51 MeV are produced and they undergo Compton
scattering or the photoelectric effect until they lose all of their energy.