Answer to Question #10341 Submitted to "Ask the Experts"

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