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Interpretation Response #18-0033

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date:

Company Name: Occupational Services Inc.

Individual Name: Linda Bray

Location State: CA Country: US

View the Interpretation Document

Response text:

August 07, 2018

Linda Bray
Senior Health Physicist
Occupational Services Inc.
6397 Nancy Ridge Drive
San Diego, CA 92121

Reference No. 18-0033

Dear Ms. Bray:

This letter is in response to your March 2, 2018, email requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to custom optical instruments that use Thorium-232 (Th-232) as a filter. You note the following:

  • The Th-232 is an integral part of the lens and is deposited as a fixed coating over the lens surface through a physical heating process.
  • The Th-232 is not a removable (nonfixed) contaminant; it is designed as a part of the finished optical instrument fixed to the surface under normal conditions of transport. 
  • The level of distributed activity on the lens typically exceeds the exemption in § 173.401 based on the size of the lens, and for contamination limits in § 173.403.
  • These lenses do not meet the definition of a sealed source, as the material is not completely encased by an enclosure.

You explain that while it is easy to determine the amount of Th-232 deposited on the lens, it is difficult to determine what is meant by concentration. You state that you would like to weigh each lens individually without any support structures such as holders, rings, or finished devices and then determine the concentration by dividing the Th-232 activity by the weight of the lens. When performing this calculation, the concentration is below the exempt values in § 173.436. Specifically, you ask whether a thin coating containing Th-232 applied to the surface of a lens could use the weight of the lens to determine activity concentration in accordance with the values in § 173.436.

The answer is no. Because the lens is a solid object with a radioactive substance on its surface, it would be more appropriate to consider the lens from a Surface Contaminated Object (SCO) perspective for Class 7 determination using the contamination limits in § 173.403. However, if you were to attempt to determine whether the TH-232 coating meets the definition of radioactive material, then only the mass of the layer of TH-232 coating, not including the mass of the glass lens it is applied to, could be used for the activity concentration calculation as a Class 7 (radioactive material).

I hope this information is helpful. Please contact us if we can be of further assistance.


T. Glenn Foster
Chief, Regulatory Review and Reinvention
Standards and Rulemaking Division

173.401, 173.436, 173.403

Regulation Sections

Section Subject
173.401 Scope
173.403 Definitions
173.436 Exempt material activity concentrations and exempt consignment activity limits for radionuclides