It is the IMO rulebook for dangerous goods on a ship: how to class them, pack them, mark them and stow them. Each substance gets a class, a UN number and packing rules that the carrier and the terminal have to follow.
Class 7
Radioactive Substances
Read the divisions below as a planning note. The class name tells you the main hazard. Packing, labels and where the box may sit on the ship still come from the IMDG Code.
Radioactive substances, class 7
Radioactive material
Class 7 is material that contains radionuclides, once both the activity concentration and the total activity in the consignment sit above the limits in the dangerous-goods table. Below those limits it is not class 7.
A package with the RADIOACTIVE YELLOW III label is treated as a full radioactive consignment. Some low-specific-activity loads in exclusive use do not carry that package label, and the vehicle still needs the RADIOACTIVE placard.
Exclusive use means one consignor has the conveyance to itself. Loading and unloading follow that consignor or the consignee, and the people who do it have the training and equipment for the material. The consignor gives the carrier written instructions with the shipping papers.
Fissile material here means plutonium-238, plutonium-239, plutonium-241, uranium-233, uranium-235, or a combination of them. Unirradiated natural uranium and depleted uranium are outside this definition, as is natural or depleted uranium that was irradiated in a thermal reactor. Further exceptions are in 49 CFR 173.453. A controlled fissile shipment is one with a nuclear criticality transport index above 10 under 49 CFR 173.457.
A highway route controlled quantity in one package is the least of three figures: 3,000 times the A1 value for special-form material, 3,000 times the A2 value for normal-form material, or 1,000 TBq (27,000 Ci). The A1 and A2 values are in 49 CFR 173.435.
A limited quantity stays inside the package limits of 49 CFR 173.425 and the conditions of 49 CFR 173.421. Low specific activity (LSA) material has a limited activity per mass and meets the LSA descriptions. Shielding around it is not counted when the average specific activity is estimated.
A freight container in this class is a reusable box of 1.81 cubic metres (64 cubic feet) or more, lifted with its contents. It is small if one outer side is under 1.5 metres (4.9 feet) or the inside volume is 3.0 cubic metres (106 cubic feet) or less. Anything else is a large freight container.
Questions
What a class does not settle
The class names the main hazard. Many goods also have a second one, such as a poisonous gas that can also burn. The papers and the placards show both, and the separation rules follow whichever hazard is stricter.
It belongs to the substance, or to a small group of similar substances, in the IMDG Dangerous Goods List. It is not one number for a whole class. Two boxes in the same class can have different numbers, packing groups and separation rules.