Answer to Question #1347 Submitted to "Ask the Experts"

Q

I am working with a titration that employs the use of a 0.1M concentration of thorium nitrate. I recognize that this material is radioactive and am in need of precautions to take while working with the powder and then the solution that will be used in the analysis.

A

Thorium nitrate is radioactive and, as a refined compound of ²³²Th, technically "source material" under the U.S. Nuclear Regulatory Commission's (or equivalent Agreement State) rules and regulations. Per 10 CFR 40, small quantities (that is, less than 15 pounds) may be transferred and utilized for research under a "general license." Thus, no "specific license" with respective to additional radiation protection conditions is required for use. However, that does not relieve one from the radiation protection criteria in 10 CFR 20. These regulations relate to internal and external dose limits for workers and the public. All the NRC's regulations can be found online.

As far as precautions, if possible you should consult a health physicist, as thorium nitrate is quite soluble and the airborne concentration limits quite low. Depending on the age of the material, it may have in-growth of its short-lived decay products. There are nine alpha, beta/gamma emitting decay products, but the first radionuclide ²²⁸Ra (with a 5.75 year half-life) determines the state of equilibrium. It takes about 25 to 30 years for all the decay products to reach full equilibrium, once removed through chemical purification. A small quantity of thorium would not present a significant external radiation exposure hazard. These materials should be easily detected with a Geiger counter and exposure estimates made from such readings. Nonetheless, working with powders to produce a 0.1 M solution might cause an airborne/inhalation/internal exposure/dose concern.

I'd recommend working in a good fume hood with at least 100 linear feet per minute open-face velocity. Surveys of the hood with wet filter smear should be made after to ensure no residual surface contamination is present. You should check on disposal of the solutions, as your institution may have low-level radioactive waste acceptance criteria and/or sewering restrictions.

David J. Allard, CHP