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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Interpretation Response #25-0135

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

Interpretation Response Details

Response Publish Date:

Company Name: Atkins Réalis Nuclear Secured

Individual Name: Bradley Scott

Location State: WA Country: US

View the Interpretation Document

Response text:

June 16, 2026

Mr. Bradley Scott
Atkins Réalis Nuclear Secured
2801 George Washington Way
Richland, WA  99354

Reference No. 25-0135

Dear Mr. Scott:

This letter is in response to your October 7, 2025 email requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) as they apply to the definition of "fissile material." Specifically, you request clarification on the scope of exclusions for irradiated natural or depleted uranium as specified in § 173.403.

We have paraphrased and answered your questions as follows:

Q1. In the definition of fissile material under § 173.403, is the exclusion of "natural uranium or depleted uranium that has been irradiated in thermal reactors only" intended to exclude only the U-235 component that may have become slightly enriched during irradiation?

A1. Yes. The definition of fissile material means plutonium-239, plutonium-241, uranium-233, uranium-235, or any combination of these radionuclides. The exclusion of natural or depleted uranium that has been irradiated in a thermal reactor is limited solely to the U-235 component.

Q2. Does this exclusion also apply to fissile isotopes, such as plutonium-239 or plutonium-241, that are created because of the irradiation of natural or depleted uranium in a thermal reactor?

A2. No. See answer A1.
 
Q3. Does the Department of Transportation (DOT) consider U-233, produced from the irradiation of natural thorium within a thermal reactor, to be excluded from the definition of fissile material?

A3. No. See answer A1

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

Sincerely,

Dirk Der Kinderen
Chief, Standards Development Branch
Standards and Rulemaking Division

Regulation Sections

Section Subject
173.403 Definitions