Ancient Nuclear Waste Is Teaching Us About Radioactive Storage Today

By Nathaniel Scharping | August 13, 2018 2:00 pm
The Dukovany Nuclear Power Station in the Czech Republic. (Credit: zhangyang13576997233/Shutterstock)

The Dukovany Nuclear Power Station in the Czech Republic. (Credit: zhangyang13576997233/Shutterstock)

There are 99 nuclear reactors currently operating in the United States. The power they generate is free of carbon dioxide emissions, but as a byproduct, they also generate small amounts of nuclear waste in the form of spent uranium.

Even after the uranium in the fuel reactors is spent it remains radioactive, and that means storing it is difficult. Controversy over a permanent storage facility deep within Yucca Mountain in Nevada has left most of reactor waste stored on-site at nuclear power plants in ponds or casks. So understanding what happens to spent uranium in storage, both now and in the future, is a pressing issue for researchers. Scientists want to understand exactly how radioactive materials behave over longer periods of time.

That’s difficult, because we’re essentially generating the data as we go — there’s no precedent for humans making and trying to store nuclear waste. To get data on what happens to spent nuclear fuel over long periods of time, researchers are turning to a unique place on Earth — the oldest, and only, known natural nuclear reactor.

Nature’s Own Reactor

Under the Franceville basin in the African nation of Gabon, the Oklo nuclear reactor is the result of a confluence of natural events. Around two billion years ago, enough uranium was lumped together by geological processes to kick off nuclear fission — the process by which particles split off from radioactive elements, releasing energy. Groundwater nearby acted as a moderator, slowing neutrons down so that they were more likely to hit atomic nuclei and cause fission reactions. The reactor ran in spurts – half an hour on, two and a half hours off – as the surrounding water heated and cooled for tens of thousands of years.

Finally, it exhausted its supply of fissile U-235, and it shut down. The region is still slightly radioactive, though, and the products of nuclear fission are scattered all throughout the area.

A diagram of the geology of the Oklo nuclear reactor. 1. Nuclear reactor zones 2. Sandstone 3. Ore layer 4. Granite (Credit: MesserWoland/Wikimedia Commons)

A diagram of the geology of the Oklo nuclear reactor. 1. Nuclear reactor zones 2. Sandstone 3. Ore layer 4. Granite (Credit: MesserWoland/Wikimedia Commons)

A natural reactor is pretty much impossible today. That’s because U-235, the less-stable isotope of uranium can lead to sustainable fission reactions, no longer exists in high enough concentrations to kick off fission. Too much of it has decayed. Today, U-235 makes up around .72 percent of all uranium on Earth, while back then it hovered around 3 percent.

But ever since the reactor shut down, the products of nuclear fission have been sitting in the ground, and it gives researchers a chance to see what happens to nuclear waste over long periods of time.

“What we have is a really incredible natural analogue for nuclear waste storage,” says Evan Groopman, a research physicist at the United States Naval Research Laboratory. “Because these samples are two billion years old, so they’ve completely decayed away, they’re no longer hot like current nuclear waste from anthropogenic reactors. So they’re a really great test bed for doing tests of how these different elements and fission products actually behave over a long period of time.”

Additionally, in the two billion years those elements have been sitting in the ground, a lot has happened geologically to disrupt and alter the area. Understanding exactly how they’ve changed will also inform our nuclear waste storage plans.

“There have been lots of different geological processes that have gone on over time,” Groopman says. “Both intrusion of groundwater, which can cause things to move about be lost and also local volcanic intrusions which can heat up the surrounding area and cause loss of different elements as well. It’s relevant because earthquakes and volcanoes could also conceivably strike our nuclear waste storage facilities, and we want to know what might happen if they do.”

Nuclear Forensics

Groopman led a team that put several cores from one of the most reactive sites at Oklo through their NAUTILUS (NAval Ultra Trace Isotope Laboratory’s Universal Spectrometer) mass spectrometer to analyze their chemical makeup. As radioactive materials decay, they go through a chain of elements, and sometimes different isotopes of those elements. Some isotopes are more dangerous to humans than others, and knowing which show up, and under what conditions, will inform future storage plans for nuclear waste.

In this study, the scientists were looking to see what happened to cesium, a common, and dangerous, product of uranium fission. Cesium-134 and cesium-137 were found in the environment after the Chernobyl and Fukushima meltdowns, and they’re both highly radioactive. Looking at the core, though, they say saw that much of the cesium was captured by another element, ruthenium, around five years after the reactor stopped fission, and held there for the next two billion years. That’s much better than what happens to the cesium produced by our reactors today, which doesn’t bind chemically easily, and ends up sitting in storage casks, ready to burst free into the environment should a rupture occur (and which has happened before). Cesium is often found in the environment as salt, as well, meaning that it can dissolve in water be readily transported.

Thanks to their new data, however, the researchers may have found a path to safely storing this dangerous cesium. Ruthenium itself isn’t a good option, unfortunately — it’s one of the rarest elements on Earth, but it does lie in the same group as other more common elements. Iron, for example, is chemically similar in some ways, and could offer a means of capturing radioactive cesium, according to Groopman.

In the future, he’s hoping to examine the molecular structure of the ruthenium that’s holding onto the radioactive cesium to better understand how the two elements are bound together. This will help them understand whether other things like iron might be adapted to hold onto the radioactive elements in spent nuclear fuel.

Radioactive fuel rods continue to pile up today, and long-term solutions for storing them safely are still a ways off. Knowing exactly what will happen to our waste in the near and far future is critical if we want to continue using nuclear power.


[This post has been updated to clarify that nuclear reactors produce spent, not depleted, uranium as a byproduct.]

CATEGORIZED UNDER: Environment, Space & Physics
  • daledewitt

    Kinda mute as a concern since the thrust in Gen 4 is to deploy burner reactors to take advantage of the old inefficient reactor waste.

    • GRLCowan

      Moot. Not really, because while future reactors can convert, say, plutonium-240 into fission products, they cannot convert fission products into anything stable. Only time can do that.

      If we are to continue sending salt-laden ships’ pantries over the sea, is it critical to understand, exactly, the near and far future of salt that escapes those pantries, or might conceivably escape them, and end up in the ocean? *No!?*

      Then why would similar drop-in-the-ocean considerations not apply to manmade radioactivity that escapes, or might escape, into the environment? Scharping’s last paragraph is big-money-placating cowardice.

      • daledewitt

        I’m no expert on nuclear, my thing was and is organic chem. From my cursory reading of the burner reactors is that isotope remaining from burners are vastly reduced and short lived in the 300 year category. We must do our homework to grasp the rewards of fission energy.

  • Rhotel1

    Depleted Uranium is not created in a nuclear reactor. This sentence and the following bit as well are flat out wrong – what is the supposed expertise of either this author or their editor or fact checker? This is supposed to be about science, not bad science fiction.

    “The power they generate is free of carbon dioxide emissions, but as a byproduct, they also generate small amounts of nuclear waste in the form of depleted uranium.”

    Depleted Uranium is highly purified naturally occuring Uranium-238. It is not created in a nuclear reactor and spent fuel is not called depleted uranium. Go back and redo the beginning of this or maybe even go talk with some nuclear engineers.

  • Joffan

    Natural uranium is very low radioactivity. Depeleted uranum is a byproduct of fuel enrichment and even lower radioactivity – this is NOT spent fuel. Spent fuel is what comes out of reactors after power production; this is high radioactivity. It is also an encased mineral, so nothing is “floating freely” in the immensely strong spent fuel storage casks.

    As far as long term storage goes, cesium-137 is not a concern, because it decays away to almost nothing in under 500 years. It’s really only an issue if there is a massive accident at an operating reactor – Chernobyl and Fukushima. So although I’m sure the research going on here is interesting and potentially useful in some way, it doesn’t really change the already-safe nature of spent fuel storage.

    By contrast nuclear weapons waste, as at Idaho, is nasty stuff that was often generated in a tearing hurry. Often chemically toxic and corrosive liquids were involved which became strongly radioactive. Cleaning up the after-effects of these processes is hard and slow.

  • BeholdersEye

    No worries, after 2 billion years we’re not going to have Earth anyways, you will be frying in the bloaded Sun…all’s good…

  • Steven Curtis

    It is very disingenuous to draw any analogy from the reference “and this has happened before” to spent fuel casks breaching. 55-gallon drums are far less robust than reinforced concrete casks surrounding steel alloy nuclear fuel assembly material. This inflammatory comment degrades the credibility of your article considerably. While the study may be prudent, recycling the spent fuel to help burn excess plutonium (isotope 239, not 240) while using fast reactor technology of the next generation of nuclear reactors is far more promising. NevadansCAN is an organization promoting this and other ideas for moving nuclear power into the next generation as the common-sense carbon-free energy producer.

  • Dean Hildreth

    It’s seems apparently obvious to me, that earth should just launch vehicles to be consumed by our sun, containing nuclear waste. Explain how I may be mistaken, if you can. Other than agreements by international organizations, not to do so…
    Disposal of nuclear waste, into our closet nuclear furnace…
    What am I not seeing?

    • Jimy Goh

      There would be few catches…
      We are talking about ejecting very heavy loads from Earth to Sun, that takes very heavy rockets and loads of fuel – not exactly ideal eco solution.
      Also factor in that rocket carriers occassionally fail and fall back to ground, including their cargo. You don’t want this stuff to land in your neighbourhood, nor in the ocean on far side of the Earth.


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