Right now, most nuclear waste sits exactly where it was created. About 91,000 metric tons of spent nuclear fuel is stored at roughly 80 sites across the United States, primarily at nuclear power plants and Department of Energy facilities. No country except Finland has begun permanently disposing of its most dangerous nuclear waste underground, though that’s long been the plan. Here’s where nuclear waste actually ends up, how it’s stored, and what comes next.
Not All Nuclear Waste Is the Same
Nuclear waste falls into broad categories based on how radioactive it is and how long it stays dangerous. High-level waste, which includes spent fuel rods pulled from reactors, is intensely radioactive and generates significant heat. It requires isolation from the environment for tens of thousands of years. This is the waste that dominates public concern and policy debates.
Low-level waste covers a much wider range of materials: contaminated clothing, tools, filters, and medical equipment. It’s classified into sub-categories (A through C) based on which radioactive elements are present and their concentration. Most low-level waste can be safely buried in shallow, engineered landfills near the surface. The higher sub-categories need more robust containment, but nothing close to what high-level waste demands.
There’s also a category called transuranic waste, which contains elements heavier than uranium, typically produced during nuclear weapons manufacturing. It’s not as intensely radioactive as spent fuel, but it remains hazardous for extremely long periods. In the U.S., this defense-related waste has its own dedicated disposal site.
Temporary Storage at Reactor Sites
When fuel rods come out of a reactor, they’re extremely hot, both thermally and radioactively. The first stop is a spent fuel pool, essentially a deep, water-filled basin built into the reactor facility. The water serves two purposes: it cools the fuel and acts as a radiation shield. These pools are typically more than 20 feet deep above the top of the fuel rods, with neutron-absorbing plates between each fuel assembly to prevent chain reactions. Backup cooling systems are in place to handle emergencies, including scenarios where normal systems are knocked offline by fires or explosions.
After cooling in the pool for several years, spent fuel can be transferred to dry cask storage. Each cask is a steel cylinder, welded or bolted shut, filled with an inert gas to prevent the fuel from degrading. The cylinder is then surrounded by additional layers of steel, concrete, or other shielding material. These casks sit on concrete pads outdoors at reactor sites. They require no active cooling; air circulates passively around the cask to dissipate heat.
The Nuclear Regulatory Commission has expressed confidence that spent fuel can be stored safely in either pools or casks for at least 60 years beyond the licensed life of any reactor. For a plant that operates for its full initial 40-year term plus a 20-year extension, that amounts to at least 120 years of safe storage. Dry casks themselves are licensed for up to 40 years, with possible renewals of another 40. This storage was always meant to be temporary, a stopgap until a permanent repository opened. Decades later, “temporary” has become the status quo.
The One Underground Repository Operating Today
The Waste Isolation Pilot Plant, known as WIPP, sits in a salt formation near Carlsbad, New Mexico. It’s the only deep geological repository for nuclear waste currently operating in the United States, but it accepts only defense-related transuranic waste, not spent fuel from commercial power plants. The salt beds formed 250 million years ago, and their existence at that depth signals a long absence of flowing groundwater, which is exactly what you want when isolating radioactive material. Over time, the salt slowly creeps inward, encapsulating the waste drums and sealing them in place.
WIPP handles two forms of transuranic waste: contact-handled waste, which workers can be near briefly, and remote-handled waste, which is radioactive enough to require robotic or shielded equipment. The waste itself is a mix of contaminated metals, plastics, rubber, tools, soil, and solidified residues from decades of nuclear weapons production.
Yucca Mountain and the U.S. Policy Stalemate
For high-level waste and commercial spent fuel, the United States designated Yucca Mountain in Nevada as its permanent geological repository back in 2002. The Department of Energy submitted a construction license application to the NRC in 2008. Then politics intervened. In 2010, the DOE moved to withdraw the application. The licensing board denied that request, but Congress stopped funding the NRC’s review after fiscal year 2011. A federal court ordered the NRC to resume its work using leftover funds in 2013, and the agency completed its safety evaluation in January 2015. The adjudicatory proceeding, the formal legal review that would lead to a construction decision, remains suspended. No funds have been appropriated to restart it.
Without Yucca Mountain or any consolidated interim storage facility, all 91,000 metric tons of U.S. spent fuel stays where it is, scattered across those 80 sites in pools and dry casks. Several of these sites are at reactors that have already shut down permanently, meaning communities are hosting radioactive waste with no operating plant and no clear timeline for removal.
Finland’s Solution: ONKALO
Finland is further along than any other country. Posiva Oy, the company responsible for Finland’s spent fuel, has built a facility called ONKALO, carved into granite bedrock on the island of Olkiluoto. The plan is to seal spent fuel inside copper canisters, surround each canister with a thick layer of bentonite clay (which swells when wet and blocks water flow), and place them in tunnels roughly 400 to 450 meters underground. Once a tunnel section is full, it gets backfilled and sealed.
Posiva aims to begin final disposal operations in 2026, which would make ONKALO the first permanent repository for spent nuclear fuel anywhere in the world. Sweden is developing a similar concept in its own granite bedrock, and several other countries, including France, Switzerland, and Canada, are in various stages of site selection or design for deep geological repositories of their own.
Why Deep Underground Works
The logic behind geological disposal is straightforward: put the waste deep enough in stable rock, behind enough engineered barriers, that radioactive material can never reach groundwater or the surface in dangerous concentrations. Repository designs use a “multi-barrier” approach. The waste form itself (often a ceramic or glass) resists dissolving. The canister provides containment. The buffer material (usually bentonite clay) blocks water. And the surrounding rock, whether granite, clay, or salt, provides the final and most enduring barrier.
A critical constraint in repository design is heat. Spent fuel generates enough thermal energy that canisters must be spaced apart to prevent temperatures from exceeding what the bentonite buffer can tolerate. If the clay gets too hot, it can chemically transform into a different mineral that no longer swells or blocks water effectively. This heat limit drives decisions about how large a repository needs to be and how long fuel must cool in surface storage before burial.
Transporting Waste Safely
Moving nuclear waste from storage sites to a repository requires specialized containers tested to survive extreme accidents. The NRC requires transportation casks to pass a sequence of punishing tests: a 30-foot drop onto an unyielding surface, followed immediately by full immersion in a 1,475°F fire for 30 minutes, then water immersion. These tests are cumulative, meaning the same cask must survive each scenario in order without losing containment. The casks are massive, heavily shielded, and designed so that even a catastrophic transportation accident wouldn’t release radioactive material.
Deep Boreholes as an Alternative
Some researchers have explored an approach that skips the large underground tunnels entirely: drilling narrow boreholes about 5,000 meters (roughly 3 miles) into crystalline basement rock and placing waste canisters in the bottom 2,000 meters. The upper 3,000 meters would be sealed with layers of bentonite and cement. This puts waste several times deeper than a conventional mined repository, far below any circulating groundwater.
The concept is appealing in theory but faces real challenges. At those depths, high temperatures and pressure can chemically degrade both the bentonite seals and the cement, potentially compromising their performance over the thousands of years required. Drilling to that depth at the diameters needed is expensive, and the initial borehole at any site would require extensive logging, coring, and testing that drive costs higher. Sites would also need to be screened for upward-flowing groundwater, exploitable natural resources (which could attract future drilling), and any fault zones connecting the disposal depth to shallower rock. No country has moved beyond the research phase with this approach.