UN3321-3333: Fissile Radioactive Material Classifications Explained

Why This Matters — Even If You Never Ship Fissile Material

Here's the reality: the vast majority of radioactive material shippers will go their entire careers without using a UN number in the 3321–3333 range. Fissile material shipping is specialized territory — enriched uranium fuel assemblies, plutonium, MOX fuel — and it lives in a world of NRC Certificates of Compliance, criticality safety analyses, and armed escorts.

So why should you care? Because every time you classify radioactive material for shipping, one of the questions you must answer is: “Is this material fissile?” If you cannot confidently answer “no” — or confirm that your material qualifies for a fissile exception — you cannot use the standard UN2908–UN2919 numbers. That is where understanding fissile material classification becomes practical knowledge.

I have seen it where a shipper at a research facility had a small quantity of enriched uranium reference standards and assumed they could just ship them as UN2910 because the activity was low. The activity was indeed within the limited quantity limits. But enriched uranium above 1% U-235 is fissile material, and they had not evaluated the fissile exceptions. That is a compliance problem, even if the shipment itself posed minimal risk.

Who Needs to Know This

This guide is relevant to:

• Every RAM shipper — You need to understand fissile concepts to confirm your material qualifies for standard (non-fissile) UN numbers
• Nuclear fuel cycle personnel — Fuel fabrication, reprocessing, and enrichment facilities that routinely ship fissile material
• National laboratory staff — Research involving enriched uranium, plutonium, or U-233
• Waste management personnel — Facilities handling waste streams that may contain fissile nuclides
• Compliance auditors — Anyone reviewing RAM shipment documentation for completeness

What Makes Material “Fissile” (49 CFR 173.403)

Under 49 CFR 173.403, fissile material means material containing any of the following nuclides:

The defining characteristic is that these nuclides can sustain a nuclear chain reaction through fission. This is why fissile material has additional transport requirements — the concern is not just radiation dose (which is handled by the base package type) but criticality safety. If enough fissile material accumulates in one location under the right conditions, you could theoretically get an uncontrolled chain reaction.

The most common mistake I see is people confusing “radioactive” with “fissile.” All fissile material is radioactive, but the overwhelming majority of radioactive material is not fissile. Cs-137, Co-60, Ir-192, Am-241 — none of these are fissile. The only time fissile becomes relevant is when you have uranium enriched above natural concentrations or plutonium.

The UN3321–UN3333 Number System

The 49 CFR 172.101 Hazardous Materials Table assigns UN3321 through UN3333 to radioactive material classifications that supplement the basic UN2908–UN2919 series. This range includes both fissile versions of standard package types and a few additional non-fissile classifications for package types (like Type C) not covered in the 2908–2919 range.

Here is the complete list:

Non-Fissile / Fissile-Excepted Entries (within the 3321–3333 range)

Fissile Entries

My approach when I encounter this numbering system is to think of it as a two-axis grid: one axis is the package type (excepted, LSA, SCO, Type A, Type B, Type C, special arrangement) and the other axis is fissile vs. non-fissile. The UN number sits at the intersection of those two axes.

Fissile Exceptions (49 CFR 173.453)

This is where the rubber meets the road for most shippers. 49 CFR 173.453 defines when material that technically contains fissile nuclides is excepted from fissile classification requirements. When material qualifies for an exception, you use the standard non-fissile UN numbers (UN2908–UN2919 or UN3321–UN3323/UN3332) instead of the fissile-specific numbers.

Material is excepted from fissile requirements when it meets at least one of the following criteria:

I worked with a facility that shipped low-enriched uranium (LEU) calibration standards containing about 1.5 grams of material at 4% enrichment. That is a tiny amount, but because U-235 was above 1%, they could not use exception (d). They used exception (a) instead — less than 2 grams of fissile material per package. That is the kind of practical analysis you need to do when fissile nuclides are present. You check each exception in order until you find one that applies.

Criticality Safety Index (CSI) Explained

The Criticality Safety Index (CSI) is defined in 49 CFR 173.403 and is specific to fissile material packages. While non-fissile packages have a Transport Index (TI) based on radiation dose rate, fissile packages have both a TI and a CSI.

The CSI is assigned based on a criticality safety analysis and represents the degree of control needed to prevent nuclear criticality during transport. Here is how it works in practice:

• Every fissile package must have an assigned CSI
• Vehicle limit (non-exclusive use): Total CSI of all packages ≤ 50
• Vehicle limit (exclusive use): Total CSI of all packages ≤ 100
• Storage in transit: Total CSI in any one group ≤ 50, with 6 meters between groups

Think of CSI as the criticality equivalent of TI. The TI controls radiation exposure by limiting package accumulation based on dose rate. The CSI controls criticality risk by limiting fissile package accumulation based on the criticality analysis. A package with a CSI of 10 means no more than 5 of those packages can be in a non-exclusive-use vehicle (because 5 × 10 = 50).

Additional Requirements for Fissile Packages

Fissile packages must meet all requirements for the base package type (LSA, Type A, Type B, etc.) plus additional fissile-specific requirements. That being said, these are the key additional obligations:

• Criticality safety evaluation — The package design must be evaluated for criticality safety per 49 CFR 173.457
• NRC approval — Type B fissile packages require an NRC Certificate of Compliance that specifically covers the fissile contents
• CSI assignment — Must be determined from the criticality safety analysis and documented on labels and shipping papers
• Fissile label marking — The word “FISSILE” must appear on the radioactive label
• Stowage and segregation — CSI-based limits on how many packages can be grouped during transport and storage in transit
• Shipping paper entries — Additional entries including the CSI value and the fissile proper shipping name
• Vehicle controls — CSI limits per vehicle based on exclusive vs. non-exclusive use

Keep in mind that fissile transport is one of the areas where DOT and NRC jurisdiction overlaps most directly. The DOT regulations tell you how to classify and ship fissile material. The NRC regulations govern the package design approval and criticality safety analysis. If you are handling fissile material, you are almost certainly working with both agencies. For more on this overlap, see our guide on NRC vs. DOT jurisdiction.

Real-World Context: Who Actually Ships Fissile Material?

Fissile material shipments are a small fraction of overall radioactive material transport. Here is where they typically occur:

• Nuclear fuel cycle: Fresh fuel assemblies (LEU) moving from fabrication plants to reactors — typically UN3327 (Type A, fissile) or UN3328 (Type B(U), fissile)
• Spent fuel transport: Spent nuclear fuel contains significant fissile content (Pu-239, remaining U-235) and ships as Type B fissile — UN3328 or UN3329
• Research reactors: HEU and LEU fuel elements for research and test reactors
• National laboratories: Enriched uranium and plutonium samples for research, reference standards
• Waste containing fissile material: Certain waste streams from fuel reprocessing or enrichment may contain fissile material requiring classification under fissile package types (UN3327–UN3329) rather than LSA
• Military/defense: Nuclear weapons components, naval reactor fuel (heavily controlled, often under DOE authority)

I have seen fissile shipments come through commercial carrier networks, and it always gets the carrier's attention. A Type B(U) fissile package with a CSI on the label is one of those shipments where the carrier documentation requirements are extensive and the loading controls are strict. That is by design — these shipments deserve that level of scrutiny.

How RadShip Helps

RadShip.com supports fissile material classification in several ways:

• RAMcalc — Identifies when your material contains fissile nuclides and flags the need for fissile exception evaluation
• Guides you through the fissile exception criteria to determine if standard UN numbers apply
• Generates correct proper shipping names including fissile designations when applicable
• Includes CSI documentation on shipping papers for fissile packages

Here's the reality: fissile classification adds complexity to what is already a detailed regulatory process. Having a tool that systematically walks through the fissile determination — first asking “does this material contain fissile nuclides?” and then evaluating the exceptions — prevents the kind of oversight where someone ships enriched uranium under a non-fissile UN number because they did not think to check.

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Common Questions

Is Am-241 fissile?

No. Americium-241 is not a fissile nuclide. Only U-233, U-235, Pu-239, and Pu-241 are considered fissile for transport purposes. Am-241 is one of the most commonly shipped isotopes (smoke detectors, well logging sources), and it is always classified using the standard non-fissile UN numbers. This is the question I get asked most often because people know Am-241 comes from the nuclear fuel cycle, but it is not capable of sustaining a chain reaction on its own.

What if my material has a tiny amount of U-235?

Check the fissile exceptions. Many materials contain trace amounts of U-235. If the enrichment is ≤ 1% by mass and there is no Pu or U-233 present, exception (d) in 173.453 applies and you use the standard non-fissile UN numbers. For natural uranium (0.72% U-235), depleted uranium, and slightly enriched material at or below 1%, you are clearly in fissile-excepted territory.

Do I need an NRC license to ship fissile material?

Generally, yes. Possession of fissile material typically requires an NRC license (or Agreement State equivalent). The transport packaging for Type B fissile material requires an NRC Certificate of Compliance. However, some fissile-excepted shipments can be made under existing license authority. The transport regulations (DOT) are separate from the possession regulations (NRC), but in practice they are deeply intertwined for fissile material.

What is the difference between UN3327 and UN3333?

Physical form. UN3327 is a Type A package containing fissile material in non-special form (normal form). UN3333 is a Type A package containing fissile material in special form — meaning the material has been sealed and tested to remain intact under accident conditions. The A1 vs. A2 activity limits differ between these two classifications.

Checklist: Is My Material Fissile?

Use this checklist every time you classify radioactive material for transport:

• ☐ Does the material contain U-233, U-235, Pu-239, or Pu-241? If no → material is not fissile. Use standard UN numbers (UN2908–UN2919, UN3321–UN3323, or UN3332).
• ☐ Is it natural uranium or depleted uranium? If yes → not fissile for transport purposes. Use standard UN numbers.
• ☐ Does it meet exception (a)? ≤ 2 g fissile nuclides per package → fissile-excepted, use standard UN numbers.
• ☐ Does it meet exception (b)? ≤ 15 g with ≥ 200:1 non-fissile dilution → fissile-excepted.
• ☐ Does it meet exception (c)? ≥ 2,000:1 nonfissile dilution, ≤ 180 g per 360 kg → fissile-excepted.
• ☐ Does it meet exception (d)? U-235 enrichment ≤ 1%, Pu+U-233 ≤ 1% of U-235 mass → fissile-excepted.
• ☐ Does it meet exception (e) or (f)? Homogeneous solutions or low-concentration limits → fissile-excepted.
• ☐ No exception applies? → Material is fissile. Use UN3327–UN3331 or UN3333 based on package type. Assign CSI. Consult criticality safety specialist.
• ☐ Document your fissile determination — Record which exception applies (or that none does) in your shipping records.

My strongest recommendation is to make this fissile evaluation a standard step in your classification process, even if you have never encountered fissile material. It takes 30 seconds to confirm “no fissile nuclides present” for a Co-60 source. But for the one time you get a sample containing enriched uranium, that 30 seconds of habitual checking will save you from a significant compliance issue.

Regulatory References

DOT Requirements:

• 49 CFR 172.101 — Hazardous Materials Table (UN3321–UN3333 entries)
• 49 CFR 173.403 — Definitions (fissile material, CSI, TI)
• 49 CFR 173.453 — Fissile materials — exceptions
• 49 CFR 173.457 — Transportation of fissile material packages — Loss of moderation requirements
• 49 CFR 173.459 — Mixing of fissile material packages with non-fissile or fissile-excepted packages

NRC Requirements:

• 10 CFR Part 71 — Packaging and transportation of radioactive material (NRC package approvals, fissile package standards)

International:

• IAEA SSR-6 (2018 Edition) — Regulations for the Safe Transport of Radioactive Material (international basis for fissile classifications)