Fissile Material by Air: IATA Exemptions, CSI Limits, and Storage Rules

Why Fissile Air Transport Rules Exist

All the other radioactive material transport rules are about radiation dose — keeping external dose rates low and preventing contamination. Fissile material adds a fundamentally different hazard: criticality. If enough fissile material accumulates in the right geometry with the right moderation, it can sustain a nuclear chain reaction. The consequences of an uncontrolled criticality are far more severe than a radiation exposure event.

The fissile transport rules exist to ensure that no plausible combination of packages — even under accident conditions — could result in a criticality. This is why fissile packages have the CSI label, aircraft loading limits, and the 6-meter spacing requirement. Each control is a layer of defense against accumulation.

The most common mistake I see is shippers confusing “radioactive” with “fissile.” All fissile material is radioactive, but the overwhelming majority of radioactive material is not fissile. Co-60, Ir-192, Cs-137, Am-241, Tc-99m — none of these are fissile. The fissile rules only apply when you have one of the four fissile nuclides above threshold quantities.

Who Needs to Know This

This applies to anyone who:

• Ships enriched uranium (fuel assemblies, research reactor fuel, UF₆)
• Ships plutonium (sealed sources, mixed oxide fuel, waste)
• Handles U-233 or Pu-241 in any form
• Needs to determine whether fissile material qualifies for an exemption
• Plans aircraft loading for consignments containing fissile packages

What Counts as Fissile Material?

Only four nuclides are fissile (IATA DGR §10.3.7.1.1):

• Uranium-233
• Uranium-235
• Plutonium-239
• Plutonium-241

The following are excluded from the fissile definition:

• Natural uranium or depleted uranium that is unirradiated
• Natural uranium or depleted uranium irradiated in thermal reactors only
• Material with less than 0.25 g total fissile nuclides
• Any combination of the above

The Six Fissile Exemptions

Material that contains fissile nuclides above the 0.25 g exclusion threshold can still ship as “fissile-excepted” if it meets one of six exemption provisions (IATA DGR §10.3.7.2). Fissile-excepted packages don't need the FISSILE CSI label or multilateral package design approval.

Exemption 1: Low Enriched Uranium (≤ 1% U-235)

Uranium enriched to a maximum of 1% U-235 by weight, with total Pu and U-233 not exceeding 1% of the U-235 weight. The fissile nuclides must be distributed homogeneously, and metallic/oxide/carbide forms must not form a lattice arrangement (IATA DGR §10.3.7.2.1).

Exemption 2: Uranyl Nitrate Solution (≤ 2% U-235)

Liquid solutions of uranyl nitrate enriched to a maximum of 2% U-235, with total Pu and U-233 not exceeding 0.002% of uranium weight, and a minimum nitrogen-to-uranium atomic ratio (N/U) of 2 (IATA DGR §10.3.7.2.2).

Exemption 3: Low-Mass Enriched Uranium (≤ 5% U-235, ≤ 3.5 g/package)

Uranium enriched up to 5% U-235, with no more than 3.5 g of U-235 per package and a consignment limit of 45 g of fissile nuclides total (IATA DGR §10.3.7.2.3).

Exemption 4: Small Mass (≤ 2 g/package)

Any fissile nuclide with a total mass not greater than 2 g per package, with a consignment limit of 15 g of fissile nuclides total (IATA DGR §10.3.7.2.4).

Exemption 5: Moderate Mass (≤ 45 g, Exclusive Use)

Any fissile nuclide with a total mass not greater than 45 g, transported under exclusive use on the aircraft (IATA DGR §10.3.7.2.5). This is the most permissive mass exemption but requires exclusive use.

Exemption 6: Competent Authority Approved

Fissile material that meets specific subcriticality requirements and has been approved by the competent authority through multilateral approval (IATA DGR §10.3.7.2.6). This allows higher quantities under controlled conditions.

In my experience, exemptions 3 and 4 are the ones most commonly used by shippers handling small research quantities or calibration sources. Exemption 1 covers most commercial low-enriched uranium fuel. The key thing to remember is the consignment limits — even though the material is “fissile-excepted,” you can't put unlimited amounts on one shipment.

CSI Limits and Aircraft Loading

For fissile material that is not exempted, each package carries a Criticality Safety Index (CSI) that determines how many packages can be loaded together. The CSI is calculated as 50 divided by the number of packages that can be safely grouped under accident conditions (IATA DGR §10.6.2.8.4).

Aircraft and Freight Container CSI Limits

The total CSI on an aircraft or in a freight container must not exceed the limits in IATA DGR Table 10.9.B:

When the CSI sum on board an aircraft exceeds 50 (under exclusive use), storage must maintain at least 6 meters from other groups of fissile packages (IATA DGR §10.9.3.5.2).

The 6-Meter Spacing Rule

Any group of fissile packages stored in transit must be limited to a total CSI of 50, and each group must maintain at least 6 meters of spacing from other fissile groups (IATA DGR §10.9.3.5.1). This applies to:

• Storage areas at cargo terminals
• Transit facilities between flights
• Loading configurations on the aircraft when CSI exceeds 50 under exclusive use

The 6-meter rule is straightforward in concept but can create logistical headaches at busy cargo terminals. If the terminal already has a group of fissile packages in storage, your packages need to be placed at least 6 meters away. At smaller facilities with limited space, this can delay acceptance until the existing packages clear out.

Fissile Material and Package Types

Fissile material that is not exempted must be shipped in packages specifically approved for fissile contents. The UN number changes to reflect fissile status (IATA DGR §10.3.7.1.2):

• UN3327: Type A package, fissile, non-special form
• UN3333: Type A package, special form, fissile
• UN3328: Type B(U) package, fissile
• UN3329: Type B(M) package, fissile

Each fissile package requires the FISSILE CSI label in addition to the standard radioactive category label (I-White, II-Yellow, or III-Yellow). The CSI label must be placed adjacent to the hazard labels on two opposite sides of the package. The CSI value from the package design approval certificate goes in the box on the label.

How RadShip.com Helps

RadShip.com simplifies fissile material classification:

• RAMcalc — Automatically evaluates all six fissile exemptions and tells you whether your material qualifies as fissile-excepted or requires a fissile-classified package
• Calculates fissile mass from activity and specific activity when mass is not directly entered
• Supports enrichment percentage input for uranium nuclides and N/U ratio for uranyl nitrate solutions
• Determines the correct fissile UN number (UN3327, UN3328, UN3329, or UN3333)

My recommendation: if you're shipping fissile material for the first time, run your scenario through RAMcalc to see which exemption applies (if any) before committing to a package type. Many shippers are surprised to learn that their small research quantities qualify for exemption 4 (the 2 g per package provision), which simplifies the entire process considerably.

Try it free for 7 days.

Common Questions

What nuclides are considered fissile material?

Only four: Uranium-233, Uranium-235, Plutonium-239, and Plutonium-241. Natural and depleted uranium (unirradiated) are explicitly excluded, as is any material with less than 0.25 g total fissile nuclides.

Can fissile material be shipped by air?

Yes. Fissile material can be shipped by air either as fissile-classified packages (requiring CSI labels, multilateral package approval, and aircraft CSI limits) or as fissile-excepted packages if the material meets one of the six exemption provisions.

What is the CSI limit per aircraft?

50 for non-exclusive use. The total sum of CSI values must not exceed 50 on any aircraft (passenger or cargo) under normal transport conditions. Under exclusive use, there is no CSI limit, but 6-meter spacing from other fissile groups must be maintained.

What is the 6-meter spacing rule?

Groups of fissile packages must be kept 6 meters apart. Each group is limited to a total CSI of 50. The 6-meter spacing applies to storage areas, transit facilities, and aircraft loading when total CSI exceeds 50 under exclusive use.

Can fissile-excepted material be shipped without a CSI label?

Yes. Material that qualifies for a fissile exemption does not need the FISSILE CSI label. However, most exemptions still have consignment mass limits (e.g., 15 g for exemption 4, 45 g for exemptions 3 and 5) that must be observed.

Summary: Your Fissile Air Shipment Checklist

Before shipping fissile material by air, verify:

• ☐ Confirm material contains one of the four fissile nuclides above the 0.25 g threshold
• ☐ Evaluate whether any of the six fissile exemptions apply
• ☐ If fissile-excepted: verify the consignment mass limit is not exceeded
• ☐ If fissile-excepted: confirm only one exemption provision is used per consignment
• ☐ If fissile-classified: obtain multilateral package design approval
• ☐ If fissile-classified: affix FISSILE CSI label adjacent to radioactive hazard labels
• ☐ Verify total CSI on the aircraft does not exceed 50 (non-exclusive use)
• ☐ Declare fissile exemption paragraph or CSI value in Sequence 3 of the DGD
• ☐ Include fissile package design approval certificate with the shipment
• ☐ Coordinate with airline on 6-meter spacing for storage in transit
• ☐ Use the correct fissile UN number (UN3327, UN3328, UN3329, or UN3333)

Regulatory References

IATA (Air Transport):

• IATA Dangerous Goods Regulations §10.3.7 — Fissile material definitions and exemptions
• IATA DGR §10.6.2.8 — Requirements for packages containing fissile material
• IATA DGR §10.9.3.5 — Transport and storage requirements for fissile material
• IATA DGR Table 10.9.B — TI and CSI limits for freight containers and aircraft
• IATA Dangerous Goods Regulations — Official publication page

DOT/NRC (Ground Transport and Package Approval):

• 49 CFR 173.453 — DOT fissile material exemptions (ground transport equivalent)
• 10 CFR Part 71 — NRC fissile package design requirements

Related RadShip Guides:

• UN3321–3333: Fissile Classifications
• Competent Authority Approvals and Notifications
• Shipping Radioactive Material by Air: IATA Overview
• The DGD: Class 7 Guide
• Marking and Labeling for Air Shipment
• UN2912/2913: LSA and SCO