Waste classification

Hana's next task is to help sort the waste from a steam generator replacement and a decommissioning study at a TVA site. The list runs to hundreds of lines: protective clothing, filters, ion-exchange resins, activated reactor internals and contaminated concrete. "Every item has to go into a class," Amara says, "because the class decides how it is packaged, which disposal site will take it and what it costs. Get the class wrong and you either spend far too much on something harmless or send something hazardous where it does not belong."

Why classify waste

Radioactive waste varies enormously. Some is barely distinguishable from ordinary rubbish; some is so radioactive that it must be handled remotely and generates enough heat to need cooling. Classification groups wastes with similar hazards so that each group can be managed in a way that matches the risk. The main factors are how radioactive the waste is, which is measured by its activity concentration in becquerels per gram, how long its hazard lasts, which depends on the half-lives of the nuclides it contains, and whether it produces significant heat.

A useful principle is that the disposal facility must isolate the waste for as long as it remains hazardous. Waste that decays to harmless levels in a few decades can go into a near-surface facility designed to last a few hundred years. Waste containing long-lived nuclides at significant concentrations needs a deeper facility, and high-level waste and used fuel need a deep geological repository that isolates them for hundreds of thousands of years.

The IAEA scheme

The IAEA's General Safety Guide GSG-1, Classification of Radioactive Waste, published in 2009, defines six classes that many countries use as a reference. Exempt waste meets the criteria for clearance and can be released from regulatory control. Very short-lived waste can be stored for a few years until it decays and then cleared; it typically comes from medicine and research. Very low-level waste has slightly more activity and can go to a landfill-type facility with limited controls. Low-level waste contains limited amounts of long-lived nuclides and can go to engineered near-surface facilities. Intermediate-level waste contains enough long-lived nuclides that it needs greater isolation, typically at depths of tens to hundreds of meters, but produces little heat. High-level waste is so radioactive that its decay heat must be considered in disposal design; it goes to a deep geological repository.

The United States uses a scheme defined largely by the origin of the waste rather than its properties. High-level waste is defined by law as used fuel and the highly radioactive waste from reprocessing. Transuranic waste, managed by the DOE, contains alpha-emitting transuranic nuclides with half-lives over 20 years at concentrations above 100 nanocuries per gram; it comes mainly from defense programs and goes to the Waste Isolation Pilot Plant in New Mexico. Low-level waste is everything else not in those categories, other than uranium mill tailings, which are regulated separately as byproduct material. The NRC divides commercial low-level waste into Classes A, B and C under 10 CFR 61.55, with a further category, greater-than-Class C, for waste too active for near-surface disposal.

Classification in practice

In practice, classification depends on the nuclides present and their concentrations, which are found by measurement, sampling and calculation. Many difficult-to-measure nuclides, such as nickel-63, carbon-14 or technetium-99, are estimated from easily measured key nuclides such as cobalt-60 or cesium-137 using scaling factors derived from sampling. Waste generators then compare the inventory with the acceptance criteria of a storage or disposal facility.

# Clearance and classification by activity concentration (illustrative thresholds, Bq per gram).
# Real limits depend on the nuclide and the national rules; this shows the logic only.
waste = [("lab gloves and paper", 0.3), ("ion-exchange resin", 4e4),
         ("pressure tube section", 3e6), ("used fuel", 1e10)]
def classify(a):
    if a < 1: return "clearance level: may be released from regulatory control"
    if a < 1e3: return "very low-level waste"
    if a < 1e5: return "low-level waste"
    if a < 1e8: return "intermediate-level waste (needs shielding, long-lived content)"
    return "high-level waste or used fuel (heat-generating)"
for name, a in waste:
    print(f"{name:24s} {a:9.1e} Bq/g -> {classify(a)}")

lab gloves and paper       3.0e-01 Bq/g -> clearance level: may be released from regulatory control
ion-exchange resin         4.0e+04 Bq/g -> low-level waste
pressure tube section      3.0e+06 Bq/g -> intermediate-level waste (needs shielding, long-lived content)
used fuel                  1.0e+10 Bq/g -> high-level waste or used fuel (heat-generating)

The output places protective clothing at clearance levels, ion-exchange resin in the low-level class, a section of pressure tube in the intermediate class and used fuel in the high-level class. The thresholds in the code are illustrative only. Real criteria use nuclide-specific limits; for example, a waste with a modest concentration of a long-lived alpha emitter may be classed as intermediate even if its total activity is lower than that of a short-lived low-level waste.

Volumes and activity

A striking feature of radioactive waste is the inverse relation between volume and activity. Low-level waste makes up most of the volume of radioactive waste but a tiny fraction of the activity. High-level waste and used fuel make up a small fraction of the volume but almost all of the activity. In the IAEA's global figures, more than 95 % of the volume of solid radioactive waste is very low-level or low-level waste, while high-level waste and used fuel account for well over 95 % of the activity.

In the United States, commercial low-level waste is disposed of at four operating sites: Barnwell in South Carolina, Clive in Utah, Richland in Washington and Andrews County in Texas. Disposal is organized through regional compacts under the Low-Level Radioactive Waste Policy Act of 1980 and its 1985 amendments. Most of the volume is Class A waste, which goes mainly to Clive, while Classes B and C are a small share of the volume.

Waste acceptance criteria

Every storage or disposal facility has waste acceptance criteria, which set out what it will accept and in what form. Typical criteria include limits on the total activity and on the concentrations of specific nuclides, the absence of free liquids and of materials that could react, burn or explode, limits on dose rates at the package surface, requirements for the strength and durability of the container, and complete documentation of the contents. A waste generator must show, through measurements and records, that each package meets the criteria before it is shipped. A package that does not meet them must be treated, repackaged or sent elsewhere.

Acceptance criteria link classification to practice. A waste might be low-level by the national scheme but still be refused by a particular facility because it contains too much carbon-14 or a reactive chemical. This is why waste characterization begins as early as possible, ideally when the waste is created, and why generators keep detailed records that may be needed for decades.

US disposal sites set acceptance criteria based on their NRC or Agreement State licenses, and the NRC's rules in 10 CFR Part 61 also set requirements for waste form stability and package labeling, including the requirement that Class B and C waste be stable for 300 years. WIPP has its own detailed criteria for transuranic waste, including limits on liquids, gas generation and the radionuclide content of each drum.

Mixed and special wastes

Some wastes are both radioactive and chemically hazardous. These mixed wastes, such as contaminated solvents, lead shielding or mercury from old instruments, must meet the rules for both radioactive and hazardous waste, which makes them among the most difficult and expensive to manage. Other special wastes include sealed radioactive sources from medicine and industry, which can have high activity in a very small volume, and naturally occurring radioactive material from industries such as oil and gas, phosphate fertilizer and mineral processing. Sealed sources that are no longer used are returned to their suppliers where possible, and national programs collect disused sources so that they do not become orphan sources outside regulatory control.

The sum-of-fractions rule is worth noting because it is widely used. In the example, the fractions are 1.2 × 10⁴ ÷ 4 × 10⁴ = 0.3 for cesium and 250 ÷ 100 = 2.5 for carbon-14, giving a sum of 2.8. Even if the carbon-14 had been just within its limit, the two fractions together could still exceed one. The rule ensures that a package with several nuclides, each close to its limit, does not exceed the overall safety basis of the facility.

Exemption and clearance

Not all material from a nuclear site is radioactive waste. Clearance levels, set by regulators based on IAEA guidance, define the activity concentrations below which material can be released for ordinary use or disposal, because the resulting doses to anyone would be trivial, typically of the order of 10 microsieverts a year. Clearance allows large volumes of decommissioning concrete and metal to be recycled or disposed of conventionally, saving disposal capacity for genuinely radioactive waste. Releasing material requires careful measurement and documentation, and is sometimes contentious with the public even when doses are trivial.

Classification of used fuel

Used fuel occupies a special place in classification. In Canada, Finland and Sweden, which do not reprocess, used fuel is the high-level waste. In countries that reprocess, used fuel is considered a resource until it is reprocessed, and the high-level waste is the vitrified residue. In the United States, used fuel is legally classed as high-level waste for disposal, though it is commonly called spent nuclear fuel. These different definitions matter in international comparisons and treaties, such as the Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management, which covers both and requires each party to report regularly on its inventories and programs.