Nuclear waste lasts millions of years - so what do we do with it?

When a nuclear power plant finishes squeezing energy from its fuel, it’s left with waste that stays radioactive for millions of years.

One of the biggest troublemakers in this waste is iodine-129, or I-129. It’s a stubborn isotope that doesn’t break down easily, and if it enters the human body – particularly the thyroid – it can increase cancer risk.

That makes I-129 one of the most closely watched pieces of the nuclear waste puzzle. Right now, countries are taking very different approaches to managing this risk.

Some bury the waste deep underground. Others filter and release parts of it into the environment. And at least one country – France – is letting some of it flow straight into the ocean.

A new study from MIT and several national labs compared these strategies. The researchers found big differences in how much I-129 each method releases into the environment, and how those releases could affect people over time.

The problem with iodine-129

I-129 isn’t something we see, smell, or feel. But it’s a big deal in nuclear waste because it sticks around – its half-life is about 15.7 million years.

Iodine-129 also travels easily through soil and water, which means it can eventually reach people if not contained.

In the U.S., I-129 is treated as a major risk factor in waste storage safety plans. The Environmental Protection Agency (EPA) limits the amount of I-129 allowed in drinking water to just 5.66 nanograms per liter – the lowest limit set for any radioactive element.

Ways to handle nuclear waste

The study compared three ways to manage spent nuclear fuel. Each of these paths results in a different amount of I-129 making its way into the biosphere.

France, which recycles its spent nuclear fuel, discharges about 153 kilograms of I-129 into the ocean each year. That’s within their regulatory limits, but the researchers found that this approach releases about 90 percent of the I-129 from spent fuel into the environment.

The U.S. approach, on the other hand, involves sealing nuclear waste in containers and burying it deep underground.

Even assuming the protective barriers fail after 1,000 years (which is very conservative), this method results in only 2.14 × 10⁻⁸ kilograms of I-129 per gigawatt of energy generated per year being released – over a million-year span.

The third method, which uses filters during reprocessing, cuts the release down to about 0.05 kilograms per gigawatt-year. Most of the captured I-129 is then stored in shallow underground sites.

But this method carries the risk that people might accidentally dig into these sites centuries down the line, after regulatory oversight fades.

Tracking nuclear footprints in water

The team analyzed I-129 in surface waters near known nuclear activity areas: the English Channel, the North Sea, and U.S. rivers near old weapons production sites.

Around France and the U.K., they found low levels of I-129 in ocean water. While not considered harmful, these traces show how far the isotope can spread through the sea.

The researchers also looked at rivers near American nuclear waste sites, like the Columbia River and a site in South Carolina.

In South Carolina, where waste was released into areas with less water flow, they found higher concentrations of I-129 in surface water – highlighting how geography and dilution affect the environmental impact.

Protecting local communities

According to study lead author Haruko Wainwright, the team wanted to quantify the environmental factors and the impact of dilution, which in this case affected concentrations more than discharge amounts.

“Someone might take our results to say dilution still works: It’s reducing the contaminant concentration and spreading it over a large area,” said Wainwright.

“On the other hand, in the U.S., imperfect disposal has led to locally higher surface water concentrations. This provides a cautionary tale that disposal could concentrate contaminants, and should be carefully designed to protect local communities.”

Weighing risks and tradeoffs

There is not a nuclear waste disposal method that is risk-free. France’s method sends most of the I-129 into the ocean right now, but in low concentrations.

The U.S. method locks the waste away long-term, but a failure in the future could release it in a more concentrated way. Filters strike a balance, but shallow burial brings its own risks.

“Putting these pieces together to provide a comprehensive view of Iodine-129 is important,” Wainwright said. “There are scientists that spend their lives trying to clean up iodine-129 at contaminated sites. These scientists are sometimes shocked to learn some countries are releasing so much iodine-129.”

“This work also provides a life-cycle perspective. We’re not just looking at final disposal and solid waste, but also when and where release is happening. It puts all the pieces together.”

The future of nuclear waste handling

Wainwright emphasized that the findings shouldn’t discourage countries from reprocessing nuclear fuel. Countries like Japan, for example, plan to use more filtration systems to capture I-129 before it’s released.

Since I-129 is an internal carcinogen without strong penetrating radiation, shallow underground disposal would be appropriate in line with other hazardous waste, noted Wainwright.

“The history of environmental protection since the 1960s is shifting from waste dumping and release to isolation. But there are still industries that release waste into the air and water.”

“We have seen that they often end up causing issues in our daily life – such as CO2, mercury, PFAS and others – especially when there are many sources or when bioaccumulation happens.”

Wainwright pointed out that the nuclear community has been leading in waste isolation strategies and technologies since the 1950s.

“These efforts should be further enhanced and accelerated. But at the same time, if someone does not choose nuclear energy because of waste issues, it would encourage other industries with much lower environmental standards.”

The full study was published in the journal Nature Sustainability.

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