Most naturally occurring radioactive materials and many fission
products; undergo radioactive decay through a series of transformations
rather than in a single step. Until the last step, these
radionuclides emit energy or particle with each transformation
and become another radionuclide. Man-made elements, which
are all heavier than uranium and unstable, undergo decay in
this way. This decay chain, or decay series, ends in a stable
nuclide.
For example, uranium-238 decays through a series of steps
to become a stable form of lead. Each step in the illustration
below, indicates a different nuclide. Only a few of the steps
are labeled, and the numbers below each label indicate the
length of the particular radionuclide's half-life. Uranium-238
has the longest half-life, 4.5 billion years, and radon-222
the shortest, 3.8 days. The last radionuclide in the chain,
polonium-210 transforms to lead-210, a stable nuclide.
Radionuclide decay chains are important in planning for
the management and disposal of radioactive materials and
waste and for site cleanup. As radioactive decay progresses,
the concentration of the original radionuclides decreases,
while the concentration of their decay products increases
and then decreases as they undergo transformation.
Ingrowth
The increasing concentration of decay products and
activity is called ingrowth. The illustration below
shows ingrowth when the decay product is stable and
the original radionuclide is replaced. In this situation,
the activity decreases with decay of the original
radionuclide.
Original Radionuclide
concentration decreases
as radioactive decay progresses
If the decay products are not stable, their decay
contributes to the total activity and makes planning
for radiation protection more complex.
In the case of a radioactive waste repository, the
mix of radionuclides in the waste will change over
time. The amount of radiation being released
can actually rise over time as successive radioactive
decay products undergo decay. The radiation protection
standards set for a repository must take into account
varying levels of radioactivity as successive iterations
of radionuclide ingrowth take place, even though the
process continues over thousands of years.
How do scientist know how much radioactivity
there will be?
The pattern of ingrowth varies according to the relative
length of the half-lives of the original radionuclide and
its decay products. Under certain conditions, decay products
undergo transformation at the same rate they are produced.
When this occurs, radioactive equilibrium is said to exist.
Whether equilibrium occurs depends on the relative lengths
of the half-live of radionuclides and their decay products.
Using equations that account for half-lives, the rate of
ingrowth, whether equilibrium occurs, the original amount
of radionuclide, and the steps in its decay chain, scientists
can estimate the amount of activity that will be present
at various points.
The importance of understanding decay
chains is illustrated by the ingrowth of radon-222 during
decay of uranium-238. Uranium was distributed widely in
the earth's crust as it formed. Given the age of the earth,
uranium's slowly progressing decay chain now commonly produces
radon-222. It is radioactive and has several characteristics
that magnify its health effects:
Radon is a gas. It can seep through soil and cracks
in rock into the air. It can seep through foundations
into homes (particularly basements), and accumulate into
fairly high concentrations.
Radon decay emits alpha particles, the radiation that
presents the greatest hazard to lung tissue.
Radon's very short half-life (3.8 days) means that it
emits alpha particles at a high rate.
During exposure assessments, we pay close attention to
the potential for radon generation. In designing cleanup
standards for uranium mill
tailings sites, we targeted radium-226, which decays
to radon-222, rather than the radon-222 alone. The radium-226
continue to generate radon-222 during its much longer half-life.
Radon and uranium miners
A higher than expected level of lung disease in uranium
miners helped call attention to the effects of radon-222.
The miners worked long hours in enclosed spaces, surrounded
by uranium ore and radon that seeped out of the rock. Health
workers expected to see health problems in the miners that
would reflect direct exposure to radiation. Instead, the
predominant health problems were lung cancer and other lung
diseases.
First the health workers suspected the dust itself. They
knew that high concentrations of small particles, such as
coal dust, asbestos, or cotton fibers, could damage workers'
lungs. However, close examination of the uranium-238 decay
chain identified radon-222 as the most likely culprit.
This led to regulations in two areas: 1) improved ventilation
in uranium mines and 2) limits on the amount of radon ventilated
from the mines to the ambient air.
Why Are Some Atoms Radioactive? 
