Crushed concrete from legacy nuclear facilities could play a far greater role in safely managing radioactive land than previously understood.
Research published in ACS ES&T Water and conducted by scientists from The University of Manchester, United Kingdom National Nuclear Laboratory and Clemson University, and funded by the Nuclear Decommissioning Authority, examined how crushed concrete interacts with strontium‑90, a mobile radioactive contaminant found at nuclear legacy sites such as Sellafield and Hanford.
The team found that, under conditions similar to those expected in shallow, on‑site disposal environments, concrete can react and become a long‑term sink for strontium-90, particularly when exposed to air or treated with phosphate.
Professor Katherine Morris , BNFL Research Chair at The University of Manchester and senior author of the study, said:
"Our work shows that crushed concrete doesn't just act as an inert waste material – it can actively remove strontium from solution and hold onto it in forms that are stable over long timescales. That's important for understanding how lightly contaminated concrete could be applied on site to minimise radionuclide transport."
The research team used concrete sourced from the UK's Nuclear Decommissioning Authority and tested how it behaved when mixed with synthetic groundwater containing either stable strontium or trace levels of radioactive strontium‑90. Experiments ran for three months under two contrasting conditions: air‑limited, representing sealed or low‑oxygen (sub-surface) environments, and air‑equilibrated (air-exposed), representing disposal scenarios where air is present.
In air‑equilibrated systems, the crushed concrete removed around 82% of strontium from solution within three months, compared with only 14% under air‑limited conditions. This difference was linked to the formation of calcite, a calcium carbonate mineral that forms as concrete reacts with carbon dioxide in air. Strontium can substitute for calcium in calcite, locking it into the mineral structure.
X‑ray absorption spectroscopy confirmed that strontium was partially incorporated into newly formed calcite in these air‑exposed systems, providing a mechanism for long‑term removal of strontium-90 from groundwaters.
The team also tested two phosphate treatments – one where phosphate was added during the experiment, and one where the concrete was pre‑treated with phosphate. Both approaches increased strontium uptake, even when air was limited.
In air‑equilibrated phosphate systems, up to 98% of strontium was removed from solution within 48 hours. Microscopy showed that poorly crystalline calcium phosphate coatings formed on the concrete surface, providing additional sites for strontium to sorb or incorporate over long timescales to allow radioactive decay to stable Zr.
Strontium‑90 is a key contaminant at many historic nuclear sites because it is relatively mobile in groundwater. Significant volumes of lightly contaminated concrete are generated during decommissioning, and on‑site disposal is increasingly being explored to manage this material.
The findings suggest that, when concrete is crushed and exposed to air – as would occur during recycling or shallow burial – natural carbonation processes can significantly enhance strontium retention. Phosphate treatments could further improve performance, particularly in areas where air access is limited.
Professor Morris added: "These results give us a clearer picture of what happens when concrete waste interacts with groundwater over time. By understanding the mechanisms that trap strontium, we can better support safe, evidence‑based decisions about on‑site disposal and long‑term radioactively contaminated land management."
Journal: ACS ES&T Water
Full title: Strontium Interactions with Crushed Concrete Waste: Implications for Management of Radioactively Contaminated Land
DOI: 10.1021/acsestwater.6c00365
URL: https://doi.org/10.1021/acsestwater.6c00365
