What is the significance of half-life in radiation control?

In
a nutshell, the radiological half-life is important in radiation control
because long-lived radionuclides, once released, are around for longer time
periods than are shorter-lived species.

Long-lived
radionuclides released to the environment will be present for longer times than
short-lived nuclides. This may require longer term surveillance of the
environment to ensure that a person is not exposed significantly. If exposure
does occur, for example, by ingestion of contaminated food or water or by
inhalation of airborne activity, longer-lived radionuclides may present a
greater internal dose threat than shorter lived radionuclides, although the
rate of biological elimination is also an important factor that must be
considered. Similarly, long-lived radionuclides in the environment may pose a
longer term external dose threat; for example radionuclides released in an
accident at a nuclear facility may deposit on the ground or, if in a building,
on building internal sources, and these depositions may result in dose rates
produced at some distance from the contaminated surfaces. Long-lived
radionuclides, especially gamma emitters may pose an external dose threat to
individuals in the area. Short-lived radionuclides may also pose a short-term
external dose threat, but one may take advantage of radioactive decay to reduce
this hazard so that the area may not be a problem after a reasonable decay
period.

These
types of considerations play an important role in decommissioning of sites that
have been retired from active use of radioactivity but which remain
contaminated with radionuclides. The contamination may have to be mitigated
before the site can be sold or reused for another purpose. The potential impact
of long-lived radionuclides may be much greater than that of short-lived
radionuclides. Short-lived activity may be greatly reduced by radioactive decay
over moderate time intervals, whereas long-lived contamination may necessitate
massive cleanup action, including demolition of buildings and removal of
contaminated building materials as well as possible large-scale removal of
contaminated soil on the site.

Similar
considerations apply to the area of high-level radioactive waste disposal. Among
the most restrictive radionuclides, from the point of view of requiring
long-term assurance that the waste containment method will suffice for very
long times, are the very long lived transuranic radionuclides, such as ²³⁹Pu.

There
are other examples of how long-lived radionuclides impact radiation control
actions, but the above should be sufficient for you to get the idea.

George
Chabot, PhD