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One of the highest-risk constituents of nuclear waste is iodine-129 (I-129), which stays radioactive for millions of years and accumulates in human thyroids when ingested. In the U.S., nuclear waste containing I-129 is supposed to be disposed of in deep underground repositories, which scientists say will sufficiently isolate it.
France routinely releases low-level radioactive effluents containing iodine-129 and other radionuclides into the ocean. France also recycles its spent nuclear fuel, and the fuel reprocessing plant discharges about three hundred and thirty-seven pounds of I-129 each year, which is under the French regulatory limit.
A new study by MIT researchers and their collaborators at several U.S. national laboratories quantifies I-129 release under three different scenarios. These include the U.S. approach of disposing spent nuclear fuel directly in deep underground repositories, the French approach of dilution and release, and an approach that uses filters to capture I-129 and disposes of them in shallow underground waste repositories.
The researchers found France’s current practice of reprocessing releases about ninety percent of the waste’s I-129 into the biosphere. They found low levels of I-129 in seawater around France and the U.K.’s former reprocessing sites, including the English Channel and North Sea.
The low level of I-129 in the seawater in Europe is not considered to pose health risks. However, the U.S. approach of deep underground disposal leads to far less I-129 being released, the researchers found.
The researchers also reviewed the effect of environmental regulations and technologies related to I-129 management, to reveal the tradeoffs associated with different approaches around the world.
MIT Assistant Professor Haruko Wainwright is the first author on the paper who holds a joint appointment in the departments of Nuclear Science and Engineering and of Civil and Environmental Engineering. He said, “Putting these pieces together to provide a comprehensive view of Iodine-129 is important. 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.”
Kate Whiteaker is a MIT graduate student who led many of the analyses with Wainwright. Their co-authors on the report are Hansell Gonzalez-Raymat, Miles Denham, Ian Pegg, Daniel Kaplan, Nikolla Qafoku, David Wilson, Shelly Wilson, and Carol Eddy-Dilek. Their study appears today in Nature Sustainability.
Iodine-129 release is often a key focus for scientists and engineers as they conduct safety assessments of nuclear waste disposal sites around the world. It has a half-life of about sixteen million years, high environmental mobility, and could potentially cause cancers if ingested. The U.S. sets a strict limit on how much I-129 can be released by a given source and how much I-129 can be allowed in drinking water. The limit is five and sixty sixth hundredths nanograms per liter, the lowest such level of any radionuclides.
Wainwright said, “Iodine-129 is very mobile, so it is usually the highest-dose contributor in safety assessments”.
For the report, the researchers calculated the release of I-129 across three different waste management strategies by combining data from current and former reprocessing sites as well as repository assessment models and simulations.
The authors defined the environmental impact as the release of I-129 into the biosphere that humans could be exposed to and its concentrations in surface water. They measured I-129 release per the total electrical energy generated by a one-gigawatt power plant over one year, denoted as kg/GWe.y.
MIT Nuclear Science and Engineering
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