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