Atkins Réalis Nuclear Secured — Hazardous Materials Safety Interpretation

Page 11200 New Jersey Avenue, SE Washington, DC 20590 U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration 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.#

Page 2Q3. 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#

Page 3Cardez, E. 25-0135 From: INFOCNTR (PHMSA) To: Baker, Yul (PHMSA) Cc: Hazmat Interps Subject: FW: Request for Letter of Interpretation Date: Tuesday, October 7, 2025 15:00:44 Attachments: image001.png Good afternoon, Please see the following interpretation request. Let us know if you need anything else. Best, Aminah From: Brad Scott <brad.scott@atkinsrealisusn.com> Sent: Tuesday, October 7, 2025 11:44 AM To: INFOCNTR (PHMSA) <INFOCNTR.INFOCNTR@dot.gov> Subject: Request for Letter of Interpretation You don't often get email from brad.scott@atkinsrealisusn.com. Learn why this is important CAUTION: This email originated from outside of the Department of Transportation (DOT). Do not click on links or open attachments unless you recognize the sender and know the content is safe. Mr. Shane Kelley Director, Standards and Rulemaking Division U.S. DOT/PHMSA (PHH-10) 1200 New Jersey Avenue, SE East Building, 2nd Floor Washington, DC 20590 Regarding the definition of Fissile Material in 173.403: Is the DOT’s exclusion of “any natural or depleted uranium that has only been irradiated in a thermal reactor” from the definition of fissile material in 49 CFR 173.403 intended to exclude only the U-235 component of that natural or depleted uranium that could become slightly enriched due to the conversion of U-238 to some other isotope, thereby increasing the U-235 enrichment? Or is the intent of DOT to also exclude from that definition of fissile material any fissile plutonium-239 or plutonium-241 that could be created because of the irradiation of natural or depleted uranium in a thermal reactor? Follow up question, if DOT did intend to also exclude from that fissile material definition#

Page 4any fissile plutonium-239 or plutonium-241 that could be created because of the irradiation of natural or depleted uranium in a thermal reactor, does DOT have an opinion on whether U-233 that could be produced from the irradiation of natural thorium within a thermal reactor would also be excluded from the definition of fissile material? Thank you, Bradley Scott Transportation Training Manager T&FS Business Unit AtkinsRéalis Nuclear Secured 2801 George Washington Way, Richland WA 99354 brad.scott@atkinsrealisusn.com Office: 509-940-2071 Mobile: 509-539-8657 See our website at www.atkinsrealisusntraining.com New name. New era. DISCOVER MORE#