Answer to Question #175 Submitted to "Ask the Experts"

Q

I need information on the design of delay/holding tanks for radioactive waste (e.g., ¹³¹I in a hospital). How long before I can discharge the waste into the main sewerage system?

A

In the United States, patient waste can be discharged directly into the sewer system without the need to store for decay. Part of the reason for this is that patients with "small" doses (less than 33 mCi of iodine-131) are usually sent home and, in some cases, patients with much higher doses can be dosed on an out-patient basis. Their waste cannot be reasonably collected, and waste from patients in hospitals need not be collected either. This is stated in 10 CFR 20.2003(b), which says, "Excreta from individuals undergoing medical diagnosis or therapy with radioactive material are not subject to the limitations contained in paragraph (a) of this section." If, in spite of this, you are still interested in building such tanks, the general rule is that waste stored for decay must be held for 10 half-lives AND until radiation is not distinguishable from background levels. I would recommend not constructing holding tanks because they can be very nasty to maintain, clean, or repair. If you do choose to (or are required to) build them, the actual design should be done by a firm with experience in this area. Considerations should include ease of maintenance, shielding for the ¹³¹I gamma (365 keV), bremsstrahlung from the 606 keV beta, ease of decontamination (i.e., smooth inner surfaces, minimal low-flow pockets, etc.), sealed pumps to avoid contamination from leaking pump seals, and so forth. Designing one is a job for a professional. But it is better to avoid this altogether if at all possible.

Andrew Karam, PhD, CHP