Radioactive Wastes
Part 1: Background
Information
At Duke University, radioactive
wastes (and other hazardous materials) are stored and monitored by the
Environmental Programs Division (EPD) of the Occupational and Environmental
Safety Office. These wastes include, among others,
- iodine-131 (131I),
a high-energy emitter of gamma and beta radiation, from Duke Hospital;
- phosphorus-32 (32p),
a pure beta emitter, from medical research labs;
- mixed wastes containing
both 125I (a low-energy gamma emitter) and 131I,
also from research labs.
EPD stores all isotopes
with a half-life of 90 days or less until they have decayed enough for
disposal. For medical wastes, "disposal" means incineration because
of the possibility of infectious substances. The standard guideline is
to store materials for 10 half-lives. However, some materials decay below
background radiation levels in less than 10 half-lives, and others are
still quite radioactive after 20 half-lives.
The construction of storage
facilities depends on the energy levels of radiation being emitted. For
example, 131I is shielded by two feet of solid concrete,
which reduces radiation by a factor of 10, but does not completely contain
it. In contrast, 32p can be completely shielded with
a half-inch of high density polyethylene.
- If a radioactive material
is stored for 10 half-lives, by what factor is the radiation reduced? Record
your answer in your computer algebra worksheet.
