Answer to Question #758 Submitted to "Ask the Experts"

Q

I worked at a hospital that had some depleted uranium. It was contained in a machine but they said it was leaking radiation. At a meter, the exposure rate was 10 mR/hr. Should this hospital be concerned about people and inhalation problems from this depleted uranium?

A

While you did not say what kind of machine the depleted uranium was
contained in, I presume it is a device used for radiation therapy
within the hospital. Thus it would contain a high intensity, highly
penetrating gamma radiation source. Depleted uranium, because of its
very high density (almost twice as great as lead) and very low
intrinsic radioactivity, is sometimes used to shield gamma radiation
sources of the type that are used for radiation therapy. As most of the
radiation from uranium is beta radiation, the thin steel (or sometimes
other metal) shell that encloses the uranium is adequate to absorb
virtually all the radiation from the uranium. The uranium in turn
absorbs most (but not all) of the high-energy penetrating radiation
emitted by the gamma radiation source. The small fraction that passes
through the shield is often referred to as leakage radiation.
Clearly the hospital should be concerned about people and with limiting
their radiation exposure to levels that are as low as reasonably
achievable, and I suspect that they have indeed done so. Since uranium
is a solid metal, and further is encased or canned in a steel shell,
there is no likelihood of any exposure from inhalation. Inhalation of
uranium is simply out of the question.




The exposure rate of 10 mR/h at a distance of 1 meter from the shield
is likely not from the uranium but mostly from the radioactive
gamma-emitting source used for therapy that is inside the shield—it is
the "leakage." This level of radiation is not imminently dangerous to
health, but it is greater than the level that would be permissible for
full-time occupational occupancy (40 hours per week). Personnel could
be exposed to this level for up to 500 hours in a year or 10 hours each
week without exceeding the federal regulatory limits. Persons who work
around this radioactive source should be provided with personal
dosimeters such as film badges or TLDs to monitor and document their
exposures to ensure that they are not in excess of the legal limits or
still lower limits that may be established by the hospital.



Ronald L. Kathren, CHP

Professor Emeritus

Washington State University