The United States has accrued a vast amount of nuclear waste over the last 80 years as a result of the government’s defense programs and domestic nuclear power production. Cementitious and glass waste forms can be used to safely stabilize much of this nuclear waste, but a few radioactive constituents are difficult to immobilize in these waste forms, either because they are not compatible or because of production barriers. As the world’s energy appetite continues to increase, nuclear power remains a viable carbon-free energy source, and it is important to identify better solutions for managing these problematic elements in the waste.
It is the goal of the Center for Hierarchical Waste form Materials (CHWM) Energy Frontier Research Center (EFRC) to discover new materials that can be tailored at the atomic level to effectively immobilize these troublesome radioactive elements. However, conducting laboratory experiments using radioactive elements, such as curium and americium, is expensive and requires a secure research environment. For these reasons, researchers are using their chemical intuition to carry out experiments with surrogate elements that are similar—but are not or only mildly radioactive—and that can be used to develop new potential waste forms. Combining this approach with computational screening techniques provides a powerful investigative tool for speeding up the discovery of new nuclear waste forms.
Harnessing Chemical Reasoning
“Chemical reasoning,” or “chemist’s intuition,” is where a scientist uses knowledge and observations from known reactions and chemical and elemental properties to propose new materials, just as an architect calls upon previous experience when designing a new building. Because compounds incorporating the difficult-to-handle elements are relatively unknown, chemists can look to crystals containing elements that have similar atomic sizes and charges to provide a starting point for a target product. In the laboratory of the EFRC-CHWM Director Hans-Conrad zur Loye, graduate students and postdocs are investigating the crystal growth of new waste forms by drawing on their extensive understanding of crystal chemical principles to propose new compositions to sequester curium and americium. Using elemental surrogates that mimic these materials, the group is exploring the low-temperature crystal growth of new lanthanide fluoride materials. These surrogate compositions can then be vetted for their capability to incorporate the radioactive elements using computational models to rank the likelihood of successful synthesis. Crystals that computations suggest are stable are then targeted for synthesis in the laboratory. First, the reaction conditions are optimized using the surrogate elements. The synthesis “recipe” is then sent to Savannah River National Lab (SRNL) to be used for the preparation of the actual americium and curium crystals. Similarly, new rhenate oxide crystals are being investigated as a template to accommodate technetium, a problematic volatile radioactive waste element.
Prof. zur Loye and his team have successfully used this approach to synthesize single crystals of a new plutonium silicate. Silicate minerals comprise approximately 90% of the Earth’s crust and are known to be robustly stable. His team found that both A2MSi3O9 and A2MSi6O15 (A = Na, K, Rb, Cs and M = metal) crystals had been previously synthesized for both uranium and thorium. Using chemical reasoning, his team proposed that these crystals would be stable with plutonium substituted for uranium or thorium. To test their hypothesis, prototypical crystal structures for A2PuSi3O9 and A2PuSi6O15 were evaluated with computational models; the results indicated that the most stable crystal was Cs2PuSi6O15, based on a known crystal of Cs2USi6O15. The team carried out the uranium synthetic procedure, substituting plutonium for uranium, which resulted in the crystal that was computationally predicted. “This was the first time that we were able to target a new composition with the knowledge, provided to us by computations, that this composition would be stable and, hence, synthesizable. The proof of the pudding is in the eating and the proof of this process was the successful growth of single crystals of Cs2PuSi6O15 and the determination of its crystal structure to validate the computational results,” added zur Loye.
Creating A Nuclear Materials Database
The discovery of a new plutonium crystal is the basis for CHWM’s latest approach to discover new waste forms, creating a database of calculated compounds to rank the likelihood for synthesis of new compounds to sequester radionuclides. “We want to harness our computational methods to streamline the discovery of new waste forms such that experimentalists make, at least qualitatively, what our models predict will be the best alternatives,” says Prof. Theodore Besmann, who is CHWM’s Deputy Director and the PI in charge of a new material structural database being developed for these systems.
Computational structural databases, the mass computational evaluation of hypothetical compounds to predict novel materials, have been well established in various fields. Databases contain information for molecules or crystals that allow researchers to rank the likelihood of creating a target product. Popular databases such as The Materials Project and The Open Quantum Materials Database (OQMD) have been used to design products ranging from battery components to efficient catalysts. The CHWM crystal database is an extension of OQMD, using the same computational approaches for calculating crystal stabilities.
The first iteration of the CHWM database focused on using known cerium and thorium arsenates (AsO43-), molybdates (MoO43-), phosphates (PO43-), and vanadates (VO43-) as prototypical crystals for cerium, thorium, uranium, neptunium, and plutonium. In total, 640 crystals were initially screened. “Based on the calculations we were able to target and successfully hydrothermally grow a new uranium phosphate crystal, K2U(PO4)2, further confirming this approach as a viable route for materials discovery,” stated zur Loye. Both Besmann and zur Loye plan to grow the database to include a multitude of actinide and lanthanide elements, as well as elements such as technetium, which is one of the most difficult radioactive isotopes to handle. Because of its breadth of crystal structures, the database could have an impact outside of nuclear waste forms. Although one of the major motivations for the database is to expand known materials to immobilize radioactive waste constituents, the database will contain crystal and chemical information that can be used elsewhere. “Many of the materials being developed by CHWM can also be applied to water decontamination and treatment applications,” states Jake Amoroso, CHWM member and research scientist at Savannah River National Laboratory
Outlook for Waste Form Discovery
Discovering new materials that can function as nuclear waste forms is critical for increasing the processing throughput of the nation’s nuclear waste. However, the cost and experimental protocols that accompany research with radioactive elements hinder the pace at which new potential waste forms can be identified, synthesized, and studied. Coupling chemical intuition with computational methods is a promising approach to significantly streamline scientific discovery. The creation of the CHWM waste form database along with the development of these new computational approaches will accelerate nuclear waste form research and will provide a foundation for material discovery beyond nuclear applications.
