Migration of elements found in nuclear waste may get a boost from natural proteins

Past and present nuclear activities (energy, research and weapon tests) have increased the need to understand the behavior of radioactive materials in the environment.

Nuclear wastes containing actinides (such as plutonium, americium, curium and neptunium) are particularly problematic as they remain radioactive and toxic for thousands of years.

However, when compared to more classic contaminants, little is known about the speciation (i.e., the chemical form) of these elements in the environment. Since environmental samples are really complex, scientists typically study actinide environmental chemistry using simple models, like small organic binding agents and inorganic ions.

Lawrence Livermore National Laboratory (LLNL) scientists and collaborators at Penn State University have found that natural proteins, called lanmodulins (LanM), render certain actinides more soluble under environmental conditions, hence making those radioactive elements more prone to migrate from their initial location. The research appears in the journal Environmental Science & Technology with a cover highlight pending.

LanM proteins are secreted by ubiquitous microorganisms called methylotrophic bacteria and exhibit both high affinity and a remarkable selectivity for certain actinides and also some nuclear fission products called lanthanides.

“The LanM family of proteins was only discovered five years ago and as such, the impact of such natural compounds on radioactive waste was not previously considered in risk-assessment studies,” said LLNL scientist Gauthier Deblonde, lead author of the study. “The new study clearly shows that LanM proteins prevent certain radioactive elements, notably americium and curium, from interacting with the minerals naturally present in the ground, and thus render the radioelements more soluble in groundwater and more prone to migrate from their original source.”

Minor actinides, like americium and curium, are major contributors to the long-term radiotoxicity of nuclear fuels and other radioactive waste. Understanding their interactions with natural binding agents and minerals is key to evaluating their transport behavior in the environment.

“Monitoring biological binding agents, including metalloproteins and their biogenerators should be considered during the evaluation of nuclear waste repositories and the risk assessment of contaminated sites,” Deblonde said.

In recent years, there has been a renewed interest in using nuclear energy for electricity production due to its low greenhouse gas emissions, small land footprint and non-intermittent operation. However, with these nuclear fission reactors come long-lived, radioactive waste comprised of fission products, plutonium and minor actinides such as neptunium, americium and curium that must be managed appropriately.

Disposal in deep underground repositories is largely seen as the safest option for the long-term management of spent fuels and nuclear waste. These repositories are meant to isolate the nuclear material for thousands of years -- the time scale required so that the radioactivity returns to a level similar to that of naturally radioactive ores.

Once stored in these repositories it is important to consider the chemical reactions between radionuclides and the underground natural environment (minerals, groundwater, metal chelators as well as microorganisms) that are expected to take place. Up to now, the major drivers controlling the chemistry of radionuclides in the environment have been considered to be the minerals present in the vicinity of their source location.

“Studying the chemistry of radioelements under environmentally relevant conditions is critical to assess the long-term behavior of nuclear waste,” Deblonde said. “This study is really meant to raise awareness among the radiochemistry and geochemistry communities about this new class of natural chelators whose environmental behavior is yet to be robustly studied in the presence of actinides and other radioactive materials.”

Other LLNL researchers involved in the project include Keith Morrison, Mavrik Zavarin and Annie Kersting as well as Joseph Mattocks and Joseph Cotruvo Jr. from Penn State. This work was funded by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy’s National Nuclear Security Administration.