Answer to Question #1637 Submitted to "Ask the Experts"

Q

We have a small bottle containing approximately 50 grams of reagent grade thorium nitrate that we estimate to be approximately 30 years old. The bottle has been stored safely away from employee work areas. Are there any significant health risks associated with the manner of storage of this material? As we have no use for this product what are your recommendations for proper disposal of this material?

A

There should not be any serious external exposure risks associated with this quantity of ²³²Th and decay products. However, given the age of the material, the ²²⁸Ra, with a 5.7 year half-life, has re-equilibrated. Thus, assuming the bottle is sealed well, each of all the decay products will decay for each ²³²Th alpha decay. And there are some highly penetrating gamma rays in this decay chain. To keep external exposure as low as reasonably achievable (ALARA), you've done the right thing and should not store the bottle in or near a position where someone may be working continuously. Nevertheless, the most significant hazard with this thorium nitrate material is the potential internal exposure, should it become airborne. In fact, the occupational annual limit of intake is very low for soluble (Class W) ²³²Th, at 0.001 microcuries (µCi), and the respective derived air concentration is 5×10^-3 µCi/ml. Thus, this is not a material to simply dispose of in the normal trash collection. Also, many solid-waste facilities now use sensitive radiation-monitoring systems on their transfer facilities, landfills, and incinerators. This is technically a "generally licensed" small quantity of "source material," but it may be included on your institution's broad-scope Nuclear Regulatory Commission (NRC) or Agreement State license. See the NRC's regulations in 10 CFR 40. I'd recommend placing it in a sealed plastic bag and contacting your institutional radiation safety officer. Alternately, a commercial low-level radioactive waste broker should be contacted for assistance in disposal. A list of broker and online links can be viewed on the HPS Affiliate Member's Buyer's Guide—under the section "LLRW and Mixed Waste Service."

David J. Allard, CHP