Radioactive waste

Waste is classified by activity and by the heat it generates. Very-low- and low-level waste, gloves, filters, clothing, decommissioning rubble, with under 4 GBq per tonne of alpha emitters, is 90% of the volume and 1% of the radioactivity; it goes to near-surface repositories. Intermediate-level waste, resins, reactor components, sludges, generating no more than 2 kW per cubic meter, is about 7% of the volume and 4% of the radioactivity. High-level waste, spent fuel or the glass from reprocessing, which does generate heat above 2 kW/m³, is only 3% of the volume and 95% of the radioactivity. All the vitrified high-level waste in the world would occupy about 29,000 m³, a building three meters tall on a soccer field.

What makes that 3% manageable is that its activity falls with time. Short-lived fission products dominate at first and vanish within decades: in 40 to 50 years the radioactivity of spent fuel drops by over 99%. What remains, plutonium and long-lived minor actinides, decays over thousands to hundreds of thousands of years, and that is the horizon the repository has to guarantee. About 263,000 tonnes of spent fuel are in storage worldwide, two thirds in pools and the rest, increasingly, dry.

The path is staged. Freshly discharged, the fuel goes to the plant's pool, under water that cools and shields it, for at least one year and in practice five or ten. Then it moves to dry casks: welded or bolted steel cylinders filled with helium, inside concrete or steel overpacks that shield and are cooled only by air convection, with no pumps. The first was licensed at Surry, Virginia, in 1986, and today it is the norm in the United States, Germany, Argentina, which has stored Embalse's fuel that way since the 1990s, and much of the world. It is a solution for decades, not millennia.

The final destination is the deep geological repository: burying the waste hundreds of meters down in stable, dry rock with no circulating water, and relying on engineered barriers and the rock itself for the hundred thousand years required. Onkalo, next to the Olkiluoto plant in Finland, is the world's first for spent fuel. Posiva has been excavating it in granite since 2004, at about 400 to 450 meters depth, using the Swedish KBS-3 method: twelve fuel assemblies in a boron-steel basket inside a copper cylinder, placed in a hole lined with bentonite that swells with moisture and seals. It received its construction license in November 2015, ran trials with empty canisters in 2024 and awaits the operating license to start disposing of real fuel. It cost about 1 billion euros, with 4 billion estimated for its hundred years of operation and closure. Sweden (Forsmark) and France (Cigéo) are following the same path.