Nuclear waste is managed through a multi-step process: it’s first cooled in water pools at reactor sites, then moved into heavy-duty dry storage containers, and ultimately destined for permanent burial deep underground in stable rock formations. The United States alone holds over 90,000 metric tons of spent nuclear fuel from commercial power plants, with roughly 2,000 metric tons added each year. No country has yet begun permanently disposing of this waste, though Finland is on track to be the first in 2026.
What Nuclear Waste Actually Is
Most of the nuclear waste people worry about is spent fuel: uranium that has been used in a reactor until it’s no longer efficient at generating electricity. This high-level waste is intensely radioactive. Ten years after removal from a reactor, a typical spent fuel assembly still emits radiation at more than 10,000 rem per hour, well above the roughly 500 rem whole-body dose that would be fatal to a person in a single exposure. That extreme radioactivity fades over time, but not quickly enough to simply leave the material sitting around.
The other category is low-level waste, which includes everything from contaminated protective clothing and filters to medical equipment and tools exposed to radiation. Radioactivity in low-level waste ranges from barely above natural background levels to significantly higher readings from components inside a reactor. Because it poses far less danger, low-level waste is typically buried in shallow, near-surface facilities rather than deep underground.
Step One: Cooling in Water Pools
When fuel rods come out of a reactor, they are extremely hot, both thermally and radioactively. They go directly into spent fuel pools, which are deep basins of water built right at the reactor site. The water serves two purposes: it absorbs heat and it shields workers from radiation. Fuel must cool in these pools for at least one year before it can be moved anywhere else, though in practice most fuel stays in pools for several years.
Step Two: Dry Cask Storage
Once fuel has cooled enough, it can be transferred into dry cask storage. Each cask is a massive steel and concrete container. The spent fuel is sealed inside with an inert gas, which prevents corrosion and eliminates the need for water. These casks rely entirely on passive cooling, meaning they dissipate heat through natural air circulation with no pumps, fans, or power supply required. That makes them resilient during power outages or natural disasters.
Dry cask storage was designed as an interim solution, something to bridge the gap until permanent repositories were ready. But because no permanent disposal site exists in most countries, thousands of casks now sit on concrete pads at reactor sites across the United States and elsewhere. This approach is considered safe for decades, but it leaves the long-term problem unresolved.
The Long-Term Plan: Deep Geological Disposal
The scientific consensus for permanently handling high-level waste is deep geological disposal: burying it hundreds of meters underground in stable rock formations where it can remain isolated for hundreds of thousands of years. The idea rests on a layered defense system. The waste is first sealed inside corrosion-resistant metal canisters. Those canisters are then surrounded by compacted bentonite clay, a material chosen for a specific set of properties. Bentonite has extremely low permeability, which limits groundwater from reaching the waste. It swells when wet, sealing any cracks that form. It conducts heat well enough to prevent overheating, and its chemical structure can trap and hold radioactive particles that might otherwise migrate.
Countries including Spain, Switzerland, Belgium, Finland, France, and Canada plan to use bentonite barriers in their repository designs. Studies have confirmed that highly compacted bentonite remains mineralogically stable under the temperatures and oxygen-free conditions expected in a repository, even after extended testing periods.
Finland is the farthest along. Its ONKALO facility, carved into bedrock on the country’s western coast, began test operations in August 2024. Posiva, the company operating the site, aims to start actual disposal of spent nuclear fuel in 2026, making it the world’s first operating deep geological repository. No other country is close to this milestone.
Why the Waste Stays Dangerous So Long
Spent fuel contains a complex mix of radioactive isotopes, each decaying at its own rate. In the first few decades, the most intense radiation comes from short-lived fission products like cesium-137 and strontium-90, both with half-lives around 30 years. These are responsible for most of the heat and radiation danger in recently discharged fuel.
After several hundred years, those short-lived isotopes have largely decayed, and the remaining radioactivity is dominated by long-lived elements. Plutonium-239 has a half-life of 24,100 years. Neptunium-237 lasts 2.1 million years. Some fission products persist even longer: iodine-129 has a half-life of 16 million years. After 10,000 years, spent fuel is about ten thousand times less radioactive than it was one month out of the reactor, which sounds reassuring until you consider it’s still significantly radioactive. It takes many hundreds of thousands of years before the radioactivity drops to the level of the original uranium ore that was mined.
This timeline is precisely why deep geological disposal exists. No human institution can reliably guard something for a quarter of a million years. The rock itself has to do the job.
Recycling Spent Fuel
Some countries don’t treat spent fuel as pure waste. Reprocessing separates the still-usable uranium and plutonium from the rest of the spent fuel, and that recovered material can be fabricated into mixed oxide (MOX) fuel for use in reactors. This reduces both the volume of high-level waste and the amount of plutonium requiring disposal.
Only France and Russia currently operate commercial-scale reprocessing facilities. China has a small-scale plant with another under construction. India has modest reprocessing capacity producing about 400 kilograms of plutonium annually. Most of the roughly 30 countries with nuclear power programs do neither reprocessing nor MOX fabrication. About 10 percent of the world’s reactors are licensed to use MOX fuel, but MOX makes up only around 5 percent of new nuclear fuel globally. Reprocessing is expensive and raises proliferation concerns, since it isolates plutonium that could theoretically be diverted for weapons use.
Transmutation: Shortening the Clock
One technology that could change the waste equation is transmutation, which uses fast reactors to bombard long-lived radioactive elements with neutrons, converting them into isotopes that decay far more quickly. Research published in Nature’s Scientific Reports suggests that recovering and transmuting certain long-lived waste components could reduce their potential toxicity after 1,000 years to roughly one-hundredth of what it would otherwise be. Removing these elements from the waste would also shrink the footprint needed for a disposal site.
The most ambitious concept involves a single fast reactor that simultaneously breeds new fuel and transmutes waste, addressing both the fuel supply and the disposal problem. This technology works in principle but has not yet reached commercial scale anywhere in the world.
Transporting Waste Safely
Nuclear waste does move between locations, and the containers used for transport are heavily regulated. The NRC requires transport casks to survive a sequence of simulated accidents before they’re approved: a 30-foot drop onto an unyielding surface, a 4-foot drop onto a 6-inch steel pin (simulating impact on a sharp object), full immersion in an engulfing fire, and submersion in water. Normal-condition tests include a 4-foot drop to simulate the cask falling during loading. These tests are designed to ensure the cask maintains its shielding and containment even in severe, unlikely crash scenarios.
The Scale of the Problem
With over 90,000 metric tons of commercial spent fuel already accumulated in the U.S. and no permanent repository in operation, the waste backlog grows every year. Finland’s progress at ONKALO offers proof that deep geological disposal can move from concept to reality, but it took decades of site selection, public engagement, and regulatory review to get there. Most other countries with nuclear programs are still in early planning stages for their own repositories. Until those facilities open, spent fuel will continue sitting in pools and dry casks at reactor sites, safe for now but awaiting a permanent home.