Even if there were tabulated data for 131I adherence to various plumbing materials — iron, brass, ceramic, and plastics such as polycarbonate or polyvinyl chloride — those values would be useless because wastewater pipes become coated with slime, sludge, and other gunk characteristic of the local environment and the coating thickness changes with time.  The retention of the radionuclide will also depend on the amount or flow rate of the wastewater.

I used to decontaminate 131I from hospital rooms until the U.S. Nuclear Regulatory Commission changed its rules and no longer required patients administered between 30 mCi and 200 mCi 131I to be kept in a hospital isolation room until the measured exposure rate was deemed safe.  Ceramic and stainless steel on sinks and toilets were difficult to decontaminate, but I never found any significant radiation coming through the relatively new plastic plumbing below.

The fate of Iodine-131 discharged to the sewers has been modeled and quantified by Punt et al. [16]. Once discharged to the sewer, Iodine-131 will be transported to the sewage treatment works where it may become associated with sewage solids or discharges to surface waters with treated sewage effluent. During low flow periods, bio-solids settle out in the sewer and it is possible that some of the Iodine-131will be retained in the sewage system. Approximately one third of the iodine activity received at the treatment works is believed to become associated with solids during initial sewage treatment and hence will not be discharged with treated effluent [16].

[16] PUNT, A., WOOD, M., ROSE, D., TITLEY, J., COPPLESTONE, D., Assessment of Radioiodine Discharges to Sewer. Royal Society Of Chemistry- 10th International Symposium, Oxford Press (Sept. 2006).

http://www-pub.iaea.org/MTCD/Publications/PDF/TE-1714_web.pdf

Dear Nathaniel, 

I do believe the article  "Outpatient radioiodine therapy for thyroid cancer: a safe nuclear medicine procedure" is interesting for you.

All the best, 

Wille

Nathaniel, I agree with the responses from Wayne and Mushtaq, but think the case against immediate release of ablative patients is compelling.  Some time ago I was involved in producing a safety report  with the IAEA and this is the conclusion we reached.  We didn't find useful references to the issue you raise, and worked on the assumption that all or most of the activity entering the sewage system is flushed out.  This is probabaly true once a main sewer with high flows is reached.  However, rooms for therapy may, by design, be somewhat isolated, and not have high flow rates in the local sewers.  I suspect the main risk is that the activity adheres to trapped solids rather than to the pipes.  The issues are more likely to be related to local flows, their nature (eg kitchen or more general), layout of pipework than properties of the pipes.  The solution may be some local monitoring and improved flows.  Jim

I agree with most things said here. I would guess that you try a conservative calculation using the number of 1/3 from Mushtaqs Reference.

If the resulting value appears to be exceeding the values you want to reach, than probably local dose rate measurements would be the easiest way to determine more exact values. Performing such measurements,  you could also test what happens to this doses depending on whether the toilets are flushed several times etc.