Most radioactive waste ends up buried underground, but the exact destination depends on how dangerous the material is. Low-level waste goes into shallow trenches or engineered vaults near the surface. The most dangerous spent nuclear fuel sits in temporary storage at power plant sites, waiting for permanent deep underground repositories that, with one exception in Finland, don’t exist yet. Around 95% of the world’s radioactive waste by volume is low-level or very low-level, while less than 1% is high-level waste. That small fraction, however, accounts for the vast majority of the radioactivity and the hardest disposal challenge.
How Radioactive Waste Gets Classified
Not all radioactive waste is equally hazardous. The categories determine where each type is allowed to go. Low-level waste includes things like contaminated clothing, tools, filters, and medical equipment. It makes up the bulk of what’s produced and loses most of its radioactivity within a few hundred years or less. Intermediate-level waste, which includes reactor components and certain processing residues, is more radioactive but doesn’t generate significant heat. High-level waste is the material left over from nuclear fuel after it’s been used in a reactor. It’s intensely radioactive, produces substantial heat for decades, and remains hazardous for thousands of years.
The United States doesn’t formally use the “intermediate-level” category. Instead, it splits waste into high-level waste, spent nuclear fuel, transuranic waste (materials contaminated with elements heavier than uranium, mostly from weapons production), and low-level waste. This classification matters because each type has different legal requirements for where it can be stored or disposed of.
Low-Level Waste: Shallow Burial
Low-level waste is the simplest to handle and typically ends up in near-surface disposal facilities. In the United States, sites like the one in Barnwell, South Carolina, use large engineered trenches. The standard trench dimensions at Barnwell are about 100 feet wide, 1,000 feet long, and 22 feet deep, with trench bottoms kept at least 5 feet above the water table to reduce the chance of groundwater contamination. Once filled, these trenches are capped with layers of soil and clay and monitored for decades.
Other countries use concrete-lined vaults or above-ground engineered structures for the same purpose. The goal is straightforward: keep the waste contained long enough for the radioactivity to decay to harmless levels, which for most low-level material happens within a few hundred years.
Spent Nuclear Fuel: Stuck in Temporary Storage
This is where the biggest gap exists between policy and reality. Spent fuel rods pulled from nuclear reactors are the most radioactive waste stream, and in most countries, they’re still sitting at or near the power plants that produced them.
When fuel is first removed from a reactor, it goes into a spent fuel pool, essentially a deep swimming pool lined with steel and concrete. The water cools the fuel and shields workers from radiation. After several years in the pool, once the fuel has cooled enough, it can be transferred into dry cask storage. These are steel cylinders filled with inert gas and surrounded by additional concrete or steel shielding. They sit on concrete pads, often right on the grounds of the power plant.
In the United States, there are currently 63 sites with independent spent fuel storage installations, including 48 operating under general licenses and 15 with specific licenses from the Nuclear Regulatory Commission. This was never meant to be the final answer. U.S. policy, set by the Nuclear Waste Policy Act, calls for permanent disposal in a deep underground geological repository. But no such facility operates in the country yet, so the temporary sites have become long-term holding areas by default. Power plant operators are required to set aside funding for managing spent fuel until the Department of Energy eventually takes it for final disposal.
Transuranic Waste: 2,000 Feet Underground in New Mexico
One category of dangerous waste does have a permanent home in the United States. The Waste Isolation Pilot Plant, known as WIPP, is a deep geological repository carved into a 250-million-year-old salt formation near Carlsbad, New Mexico. It accepts transuranic waste, the material contaminated with plutonium and other heavy elements, primarily from decades of nuclear weapons production.
WIPP stores both contact-handled waste (safe enough for workers to be near briefly) and remote-handled waste (too radioactive for direct contact, requiring shielded containers). The salt formation is ideal because salt is geologically stable, slowly creeps inward to seal around the waste containers, and indicates the area has been free of flowing groundwater for millions of years.
Finland’s Underground Repository: The First of Its Kind
For high-level spent fuel, the world’s first permanent deep geological repository is under construction in Olkiluoto, Finland. Called Onkalo, the facility will entomb spent fuel from all of Finland’s nuclear reactors roughly 450 meters (about 1,500 feet) below the surface in stable bedrock.
The design uses a concept called KBS-3, developed by Swedish and Finnish waste management companies. Spent fuel is sealed in copper canisters, placed in boreholes drilled into the tunnel floors, and surrounded by bentonite clay that swells when wet to form an additional barrier. The IAEA’s director general has called the project “a game changer” for nuclear energy because it demonstrates that permanent disposal is achievable, not just theoretical. Every other country with significant nuclear waste is still in the planning, research, or political negotiation stage for its own repository.
It takes roughly 10,000 years for high-level waste’s radioactivity to decay to the level that would have existed naturally in the uranium ore it originally came from. With proper packaging, backfill, and geological barriers, the goal is to keep these materials isolated from the environment for many tens of thousands of years.
What Used to Happen: Ocean Dumping
Before underground disposal became the standard, radioactive waste was dumped directly into the ocean. The United States was the first country to do this, starting in 1946, and continued until 1970. Other nuclear powers kept dumping for decades longer. The London Convention, which entered into force in 1975, banned ocean disposal of high-level radioactive waste but still allowed dumping of low-level material.
That loophole closed in 1993, after Greenpeace documented a Russian navy ship dumping 900 tonnes of nuclear waste into the Sea of Japan. The resulting international outcry led to an amendment banning all ocean disposal of radioactive waste, along with prohibiting incineration of industrial waste at sea. Today, ocean dumping of any radioactive material is illegal under international law.
Deep Boreholes: A Possible Alternative
Researchers are studying whether some types of waste could be disposed of by drilling narrow boreholes roughly 4 kilometers (about 2.5 miles) into granite bedrock, far deeper than mined repositories like WIPP or Onkalo. An analysis from MIT found that disposing of vitrified defense waste this way would be both technically and economically feasible, with total disposal costs estimated at around $150 per kilogram of waste.
At those depths, the waste would sit in rock that has been geologically stable for billions of years, far below any groundwater that humans could ever access. Copper and high-chromium stainless steel are considered the best canister materials for the extreme conditions found that deep. This approach hasn’t been deployed yet, but it could eventually supplement conventional repositories, particularly for smaller waste volumes from defense programs.