A1 and A2 Values: Radionuclide Limits and Package Types

A1 and A2 values for radionuclide transport are the maximum radioactive activities allowed in a single Type A package: A1 applies to “special form” material (sealed sources and solid pieces built to stay intact), and A2 applies to normal form material (liquids, gases, powders, and any solid that isn’t special form). The specific number for each radionuclide is listed at 49 CFR 173.435, in terabecquerels and curies.¹ If your shipment’s activity fits within the applicable value, a Type A package will do. If it doesn’t, you need a Type B package.

Every other packaging threshold in the radioactive transport regulations, from excepted quantities up through Type C air-transport limits, is expressed as a multiple or fraction of these two numbers. Getting the A1 or A2 determination right is the first substantive decision in preparing a Class 7 shipment.

Special Form Versus Normal Form

The regulatory definitions are short but consequential. A1 is the maximum activity of special form radioactive material permitted in a Type A package. A2 is the maximum activity of any radioactive material that isn’t special form (and isn’t low specific activity material or a surface-contaminated object) permitted in a Type A package.²

Special form material is either a single solid piece or a sealed capsule that can only be opened by destroying it, with at least one dimension of 5 millimeters or larger.² The point is dispersal resistance. If the outer container fails in an accident, special form material stays put rather than scattering into the environment, so only external dose matters. That’s why A1 values are almost always higher than A2 values for the same isotope.

Qualifying as special form is not a paperwork exercise. The material must pass a defined set of physical tests: a drop from 9 meters onto an unyielding target, a percussion test equivalent to a 1.4-kilogram weight falling one meter, and a heat test at 800°C for 10 minutes. Long, slender sources also face a bending test.³ If it fails any of them, it isn’t special form, and the A2 value applies.

Normal form is the residual category: any radioactive material that hasn’t been demonstrated to qualify as special form.² Powders, solutions, gases, and sealed sources that haven’t been qualification-tested all fall here. When the physical form is uncertain, the conservative choice is to use A2.

Finding the A1 or A2 Value for Your Radionuclide

The full table sits at 49 CFR 173.435 and covers every commonly transported radionuclide, with values in terabecquerels and approximate curie conversions.¹ The terabecquerel figures are the regulatory numbers; the curies are for convenience.

A few examples show the pattern:

Cobalt-60: A1 = 0.4 TBq (11 Ci), A2 = 0.4 TBq (11 Ci). Identical, because the dominant hazard is penetrating gamma regardless of physical form.
Cesium-137: A1 = 2.0 TBq (54 Ci), A2 = 0.6 TBq (16 Ci). The gap reflects inhalation and ingestion risks if dispersed cesium is released.
Iridium-192: A1 = 1.0 TBq (27 Ci), A2 = 0.6 TBq (16 Ci). Common in industrial radiography.
Iodine-131: A1 = 3.0 TBq (81 Ci), A2 = 0.7 TBq (19 Ci). The wide gap reflects thyroid uptake risk on release.
Molybdenum-99: A1 = 1.0 TBq (27 Ci), A2 = 0.6 TBq (16 Ci) internationally, with a separate domestic A2 of 0.74 TBq (20 Ci).¹

Some entries carry footnotes indicating that daughter nuclides with half-lives under 10 days are already built into the parent’s value, so you don’t add them separately.

Unlisted Radionuclides

If a radionuclide isn’t in the main table, default values from 49 CFR 173.433 apply, and they are deliberately conservative:

Beta or gamma emitters only: A1 = 0.1 TBq, A2 = 0.02 TBq.
Alpha emitters with no beta, gamma, or neutron emission: A1 = 0.2 TBq, A2 = 0.00009 TBq.
Neutron emitters or unknown composition: A1 = 0.001 TBq, A2 = 0.00009 TBq.⁴

The last two categories are extremely restrictive. Identifying the specific radionuclide almost always yields a more workable limit from the main table.

Below A1 and A2: Exempt Quantities

There is a tier beneath the A values entirely. A separate table at 49 CFR 173.436 lists activity concentrations (Bq/g) and consignment activity totals (Bq) below which material is exempt from Class 7 transport regulations.⁵ Material below these levels doesn’t need radioactive labels, specialized containers, or hazmat shipping papers.

How A1 and A2 Determine the Package Type

Once you know the applicable value, the packaging decision follows.

Type A

A Type A package may contain up to A1 of special form material or up to A2 of normal form material.⁶ Type A containers are engineered to survive normal transport conditions (handling, stacking, vibration, weather) but are not designed to remain leak-tight after a severe crash. Most medical isotope shipments and many industrial sources move in Type A packages because their activity fits within the limits.

Type B

When activity exceeds A1 (for special form) or A2 (for normal form), the shipment requires a Type B package.⁷ The NRC reviews and certifies Type B designs under 10 CFR Part 71, and the containers must survive hypothetical accident conditions including high-speed impact, extended fire exposure, and water immersion. Spent fuel casks and high-activity industrial sources travel in Type B packages, which are substantially heavier and more expensive than Type A alternatives.

Type C for Air Transport

Air transport imposes an additional ceiling. Type B packages carried by aircraft are limited to 3,000 A1 or 100,000 A2 (whichever is lower) for special form, or 3,000 A2 for normal form. Shipments above those thresholds require Type C packaging, which is engineered for the impact and fire energies of an aircraft crash. Type C shipments are rare and typically involve high-activity sealed sources that must move by air for time-sensitive reasons.

Excepted Packages and LSA Material

Below Type A, the regulations create lower-risk tiers defined as fractions of A1 or A2. Excepted packages for instruments and articles are capped at 10⁻² of the applicable value for solid special form material; limited-quantity packages for the same material are capped at 10⁻³. Liquids in limited-quantity packages face a tighter 10⁻⁴ of A2 ceiling.⁸ At these fractions, packaging requirements are minimal and hazmat papers and radioactive labels are not required.

Low Specific Activity (LSA) material is a separate track. LSA shipments rely on the diluted nature of the activity rather than container strength, must keep external radiation below 10 mSv/h at 3 meters from the unshielded material, and travel in Industrial Packages rated IP-1, IP-2, or IP-3 depending on the LSA subcategory and physical form.⁹ IP-1 meets basic design standards, IP-2 must also prevent dispersal and radiation increases under test conditions, and IP-3 must meet Type A package standards.¹⁰

Shipping a Mixture: The Sum of Fractions

When a package holds more than one radionuclide, each isotope’s activity cannot be checked against its own limit in isolation. Instead, divide each isotope’s activity by its A1 or A2 value and add the results. If the total is one or less, the package is within Type A limits.⁴

For special form material, sum (activity of isotope 1 ÷ A1 of isotope 1) + (activity of isotope 2 ÷ A1 of isotope 2), and so on. For normal form, use A2 values. A package containing both special and normal form material combines both calculations into one total. A sum above one means either a Type B container or a smaller quantity.

When the identities and proportions of the mixture are known, you can also compute a single blended A1 or A2 value for the mixture using the inverse of the sum of each isotope’s activity fraction divided by its individual A value. The two methods give the same result. The blended approach is more convenient for repeated shipments of the same mixture.⁴

Where the Numbers Come From

A1 and A2 values are not chosen administratively. They come from a dose-modeling framework called the Q System, developed under the International Atomic Energy Agency’s transport safety standards.¹¹ The system models a worst-case transport accident and calculates how much of each radionuclide can be present without exposing a nearby person to more than 50 mSv effective dose or 500 mSv to the skin.

The Q System evaluates five exposure pathways, each producing its own threshold activity:

QA, external gamma: activity that would deliver 50 mSv effective dose from gamma at a distance.
QB, external beta: activity that would deliver 500 mSv to the skin from beta.
QC, inhalation: activity that would deliver 50 mSv effective dose if inhaled.
QD, contamination and ingestion: activity that would deliver 500 mSv to the skin from contamination, or 50 mSv effective dose from subsequent ingestion.
QE, submersion: exposure from being surrounded by a radioactive gas cloud; applies to noble gases only.

The A1 value is set to the lower of QA and QB, because special form material won’t disperse and only external dose applies. The A2 value takes the lowest of all five, because normal form material can scatter, become airborne, or contaminate surfaces. That difference in scope is why A2 is generally lower than A1 for the same radionuclide.

The system originates in the IAEA’s Regulations for the Safe Transport of Radioactive Material (SSR-6), which most national regulators adopt.¹² That common baseline is why a package certified in one country can generally cross borders without re-evaluation, and it’s why the U.S. numbers in 49 CFR 173.435 line up with the values used elsewhere.

1
eCFR. 49 CFR 173.435 – Table of A1 and A2 Values for Radionuclides
2
eCFR. 49 CFR 173.403 – Definitions
3
eCFR. 49 CFR 173.469 – Tests for Special Form Class 7 (Radioactive) Materials
4
eCFR. 49 CFR 173.433 – Requirements for Determining A1 and A2 Values for Radionuclides
5
eCFR. 49 CFR 173.436 – Exempt Material Activity Concentrations and Exempt Consignment Activity Limits for Radionuclides
6
eCFR. 49 CFR 173.431 – Activity Limits for Type A and Type B Packages
7
U.S. Nuclear Regulatory Commission. Transportation of Radioactive Materials – Activity Limits
8
eCFR. 49 CFR 173.425 – Table of Activity Limits, Excepted Quantities and Articles
9
eCFR. 49 CFR 173.427 – Transport Requirements for Low Specific Activity (LSA) Class 7 (Radioactive) Materials and Surface Contaminated Objects (SCO)
10
eCFR. 49 CFR 173.411 – Industrial Packages
11
International Atomic Energy Agency. Determination of Quantities of Radioactive Material – the Q System for the Calculation and Application of A1 and A2 Values
12
International Atomic Energy Agency. Regulations for the Safe Transport of Radioactive Material