The materials the United States and other countries plan to use to store high-level nuclear waste will likely degrade faster than anyone previously knew because of the way those materials interact, new research shows.
The findings, published today in the journal Nature Materials, show that corrosion of nuclear waste storage materials accelerates because of changes in the chemistry of the nuclear waste solution, and because of the way the materials interact with one another.
“This indicates that the current models may not be sufficient to keep this waste safely stored,” said Xiaolei Guo, lead author of the study and deputy director of Ohio State’s Center for Performance and Design of Nuclear Waste Forms and Containers, part of the university’s College of Engineering. “And it shows that we need to develop a new model for storing nuclear waste.”
The team’s research focused on storage materials for high-level nuclear waste—primarily defense waste, the legacy of past nuclear arms production.
The waste is highly radioactive. While some types of the waste have half-lives of about 30 years, others – for example, plutonium – have a half-life that can be tens of thousands of years. The half-life of a radioactive element is the time needed for half of the material to decay.
The United States currently has no disposal site for that waste; according to the U.S. General Accountability Office, it is typically stored near the plants where it is produced.
A permanent site has been proposed for Yucca Mountain in Nevada, though plans have stalled. Countries around the world have debated the best way to deal with nuclear waste; only one, Finland, has started construction on a long-term repository for high-level nuclear waste.
But the long-term plan for high-level defense waste disposal and storage around the globe is largely the same.
It involves mixing the nuclear waste with other materials to form glass or ceramics, and then encasing those pieces of glass or ceramics – now radioactive – inside metallic canisters.
The canisters then would be buried deep underground in a repository to isolate it.
In this study, the researchers found that when exposed to an aqueous environment, glass and ceramics interact with stainless steel to accelerate corrosion, especially of the glass and ceramic materials holding nuclear waste.
The study qualitatively measured the difference between accelerated corrosion and natural corrosion of the storage materials. Guo called it “severe.”
“In the real-life scenario, the glass or ceramic waste forms would be in close contact with stainless steel canisters.
Under specific conditions, the corrosion of stainless steel will go crazy,” he said. “It creates a super-aggressive environment that can corrode surrounding materials.”
To analyze corrosion, the research team pressed glass or ceramic “waste forms” – the shapes into which nuclear waste is encapsulated – against stainless steel and immersed them in solutions for up to 30 days, under conditions that simulate those under Yucca Mountain, the proposed nuclear waste repository.
Those experiments showed that when glass and stainless steel were pressed against one another, stainless steel corrosion was “severe” and “localized,” according to the study.
The researchers also noted cracks and enhanced corrosion on the parts of the glass that had been in contact with stainless steel.
The experiments also showed that when ceramics – another potential holder for nuclear waste – were pressed against stainless steel under conditions that mimicked those beneath Yucca Mountain, both the ceramics and stainless steel corroded in a “severe localized” way.
With the rising global energy demand, the potential of nuclear energy, as a low-carbon energy resource, is becoming more and more obvious. But if nuclear energy is going to achieve its full potential, four grand challenges such as the maximization of application of available nuclear fuel uranium, the lifetime maximization of today’s nuclear power plants, the resistance towards nuclear proliferation, and the minimization of nuclear waste via reasonable reprocessing and treatment must be fulfilled. New types of materials with unique behaviors and functions are considered to be central to meet all the four challenges [1].
New capabilities in the synthesis and characterization of ma terials with controlled nano-scale structure offer tremendous opportunities for the development of tailored marerials for fabrication of advanced nuclear fuels (with nano-scale control of composition), use of actinides in catalysis, efficient nano-scale sorbents for spent nuclear fuel reprocessing, advanced nuclear waste forms and convenient sensors for detection of radionuclides.
Therefore, some countries with developed nuclear energy programs, such as the USA, attach great importance to the research of nanomaterials and nanotechnologies; nano-scale materials research facilities have been established at six DOE (Department of Energy) national laboratories to undertake leading-edge materials development and testing [1].
It is difficult, almost impossible, to summarize all the latest important results in nanomaterials and nanotechnologies related to nuclear fuel cycle system in one short article.
Thus, the advances we select for discussion are limited to nanostructured actinide materials, such as actinides encapsulated in fullerenes, uranium oxide nanocrystals, actinide nanospheres and nanoclusters, thorium and uranium based metal-organic frameworks (MOFs).
Non-radioactive nanomaterials for nuclear waste disposal and environmental protection are also selectively covered in this review.
In recent years, Chinese radiochemists have achieved significant success in the basic research of nanomaterials and nanotechonologies and their applications in future advanced nuclear fuel cycle.
In this regard, important contributions of Chinese scientists in this field are specifically highlighted. In spite of these considerations, we hope that this article might shed light on the further research directions of the nanoscience and nanotechnology in the nuclear energy field, particularly in the nuclear fuel cycle chemistry.
Actinides encapsulated in fullerenes and neutron radiation-induced novel fullerenes
Fullerenes, as a new class of carbon molecules with hollow cages composed of three-connected networks of carbon atoms, such as the most common C60, are regarded as the first truly molecular form of pure carbon yet isolated.
Since the first demonstration of their existence in 1985, fullerenes have become a source of interest in a variety of life, space, environmental and material sciences, due to their unique structures and properties. Particularly, metal-fullerenes, i.e. fullerenes that contain metal atoms in a cage structure, are attracting much attention in development of new functional materials for special applications.
Many efforts by several investigators have focused on studies of fullerene encapsulating actinides. Guo et al. [3] reported the first synthesis of actinide fullerenes, in which U@C28 was prepared by using the laser and carbon-arc technology. C28 is considered as the smallest fullerene formed with substantial yields.
It has an open shell ground state and behaves as a sort of hollow superatom with an effective valence of 4. Thus, stable closed-shell derivatives of C28 can be obtained by trapping a tetravalent atom inside the cage to make endohedral fullerenes such as U@C28, with an efficiency of production being much greater than for other endohedral metallofullerenes. Akiyama et al. [4] studied the production of metallofullerenes for U, Np and Am.
Two kinds of uranium metallofullerenes, U@C82 and U2@C80 with uranium oxidation state of +3, were prepared by the conventional arc-discharge method, and then purified and identified by HPLC (High Performance Liquid Chromatography). Metallofullerenes for Np, and Am were also successfully obtained.
From the equivalence of the HPLC retention time and similar 5 f characterizations as actinide elements, it was assumed that the major products for Np and Am fullerenes are also M@C82 species with the oxidation state of +3.
The studies quoted above have dealt with the synthesis of actinide endohedral fullerenes, but the detailed structures of actinides in the cage of fullerenes, however, still remain unclear. Recently, Wu and Lu [5] revealed the presence of unprecedented U−U multiple bond in U2@C60 by means of all-electron relativistic density functional computations.
The optimized structures and symmetries of U2@C60 isomers 1a–d derived from the C60 (Ih ) fullerene are depicted in Fig. 1. The U−U multiple bond in this work is dominated by the uranium 5 f atomic orbitals and is very different from the metal-metal bonds in the d- and f -block polynuclear metal complexes.
This finding may open a way for connecting the metal-metal multiple bonding and the fullerene chemistry. However, some authors give different opinions. Infante et al. [6], for example, suggested through a DFT study on endohedral and exohedral U2@C60 systems that U−U binding found in C60 is a calculation artifact due to the small size of the cage. In a larger cavity such as C70 or C84, two U atoms mainly interact with the interior wall of the cage and the U–U bond no longer exists.
Anyway, the differences on the proposed structures of U atoms in C60 will promote further interest in endohedral fullerenes. It is reasonable to believe that the true potential for nuclear waste management using fullerenes can be realized by fostering wider attention and discussion on the actinide endohedral fullerenes.
Inspired by the successful synthesis of actinide fullerene composites, Zhao et al. continued to explore novelstructured fullerenes by neutron irradiation. The first highly selective reaction that produces only C2m -X-C2n (m = n or m = n, X stands for C or O) and no C2m-C2n type fullerene dimer was consequently found. Utilizing a neutron irradiation method, Zhao and co-workers have synthesized C141, C131, C121, and C140O, of which C141 and C131 were prepared and characterized for the first time [7, 8].
The synthesis method is described in Fig. 2. This method is simple and capable of producing new C2m -X-C2n molecules by chemical modification of precursors to introduce different X-atoms into the reaction system prior to the energetic radiation. The irradiation with neutrons of high energy could create reactive sites for covalently bound bridges between fullerenes originally only weakly bound by van der Waals forces.
The results open a new direction and provide a practical approach for polymer sciences of fullerenes and fullerene derivatives. One possible formation mechanism for C2m-X-C2n is proposed as follows. In the irradiation process, energetic neutrons transferred their energy to C70 or C60 cages.
It caused a secondary ionization and induced a dissociation of C70 or C60 molecule to form some fragmental species as well as excited C70 or C60 cages. The reactions among excited C70 or C60 cages (of high reactivity) and nascent fragmental species hence occurred. This process finally resulted in the new molecule C141.
Nanomaterials for nuclear waste disposal and environment remediation
The renaissance of nuclear energy promotes increasing demands for dealing with the troublesome nuclear contamination and the diffusive high concentrated radioactive waste. Especially, after this year’s Fukushima nuclear accident, a large quantity of high concentrated radioactive waste water has been discharged into the ocean, causing a great public concern. This leads to more exigent efforts in removal of radionuclides from aqueous solution for water decontamination and environmental protection.
Under this situation, versatile nanomaterials that can remove the radionuclides and remediate the environment are favored by more and more investigators. Carbon nanotubes (CNTs), as well known, are a novel and important graphitic carbon nanomaterial, and have been widely applied in various scientific areas. CNTs exhibit many noteworthy properties such as strong tensile strength, large elastic modulus, high heat conductivity and electrical conductibility and large surface area [52].
These advantages make CNTs an ideal supporting material for solidphase extraction of radionuclides. Wang et al., for example, firstly reported the sorption of lanthanides and actinides from NaClO4 aqueous solution by using multiwall carbon nanotubes (MWCNTs). It was found that MWCNTs are an effective sorbent for Eu(III) [53], Am(III) [54] and Th(IV) [55, 56], and the sorbent after nuclides sorption is very stable due to the strong complexation of sorbates on the MWCNTs surface.
The authors further fabricated car boxymethyl cellulose grafted MWCNT (MWCNT-g-CMC) by using plasma induced grafting method. The preliminary results suggested that MWCNT-g-CMC is easily dispersed in solution and has much higher adsorption capacity for U(VI) compared to the raw MWCNT, as shown in Fig. 6 [57].
These works highlight the vast opportunities of CNTs applied in separation of radionuclides from aqueous solutions. However, the studies performed at the Institute for Transuranium Elements (ITU) of the European Commission in Karlsruhe and the Institute of Nuclear Sciences of Ege University (INS) do not confirm this high performance of MWNTs for Th and Am (the adsorption capacity of ≤ 10 mg/g vs. ≥ 40 mg/g reported by Wang et al.).
The authors attributed the difference to the pretreatment of the CNTs prior to batch sorption experiments. That is, the pretreatment will enhance the adsorption capacity of CNTs by increasing the degree of purity, providing more hydrophilic surface and increasing the amount of oxygen-bearing functional groups [52]. From a pragmatic point of view, some material issues of these nanomaterials need to be discussed first to better define the potential applications of CNTs. Basically, the reversibility of adsorption and durability mainly determine the reusability and “economics” of the CNTs.
In current stage, the reports about the reusability and durability of CNTs are rare, and it is hard to say that CNTs are more appropriate candidates for environmental remediation compared to inorganic materials such as e.g. clay, zeolite and prussian blue sorbents. Further work is necessary to promote the adsorption capacity, selectivity and radiation stability of CNTs, making them more competitive compared to traditional adsorbents in radionuclides preconcentration and environmental remediation.
Beside CNTs, mesoporous materials, porous molecular sieves with pore diameter between 2 and 50 nm, such as SBA-15 (Santa Barbara Amorphous-15), are also important nanomaterials that have received ever-increasing attention in various scientific areas. Ordered mesoporous carbon and silica materials are novel families of the fascinating porous solids, which have the advantages of large surface area, well-defined pore size, excellent mechanical resistance, non-swelling, excellent chemical stability and radia tion tolerance, as well as extraordinarily wide possibilities of functionalization [58, 59].
These advantages also make the ordered mesoporous carbon and silica compounds attractive for nuclear waste disposal. Vidya et al. [60, 61] reported the entrapment of UO2 2+ in MCM-41 and MCM-48 molecular sieves based on direct template-ion-exchange. It was found that the entrapment of UO2 2+ was facilitated by the large pore size and the high surfactant content in the as-synthesized host materials.
However, these sorbents normally show poor selectivity and slow sorption kinetics for UO2 2+. To promote the sorption selectivity and achieve higher sorption capacity and faster sorption kinetics, mesoporous surface based functionalizations were performed. Li et al. [62] developed a new sorbent for U(VI) by functionalizing ordered mesoporous carbon CMK-5 with 4-acetophenone oxime (Oxime-CMK5) via thermally initiated diazotization.
The U(VI) sorption by Oxime-CMK-5 was also found to be very fast and pH-dependent, and a maximum U(VI) sorption capacity of 65.18 mg/g could be achieved. Fryxell and his coworkers [63, 64] developed self-assembled monolayers on mesoporous supports (SAMMS), in which organosilicates with functional moieties are orderly attached on the surface of MCM-41, as an efficient method for actinides sequestration. Yousefi et al. [65] studied the solid-phase extraction of U(VI) using 5-nitro-2-furaldehyde modified mesoporous silica (MCM-41).
The sorbent exhibits good stability, reusability, high sorption capacity and fast rate of equilibrium for sorption/desorption of U(VI). Recently, we synthesized two kinds of organic functionalized mesoporous silicas, phosphonate functionalized MCM-41(NP10) [66] and amino functionalized SBA-15 (APSS) [67], by co-condensation and grafting method, respectively. Consequently, the synthesized materials were used as sorbents for the removal of U(VI) from aqueous solution. Fig. 7 shows the structure of NP10 and the sorption isotherm for U(VI) in NP10.
It is found that U(VI) sorption has ultra-fast kinetics and the new sorbents offer large sorption capacity. The maximum sorption capacity in APSS, for example, is even more than 400 mg/g at pH 5.3±0.1.
Furthermore, these silica materi als show a desirable selectivity for U(VI) ions over a wide range of competing metal ions. The studies on the sorption mechanism of U(VI) on the organic functionalized mesoporous silicas and on Th(IV) sorption using the organic functionalized mesoporous silicas are still underway in our laboratory. In addition, magnetite nanoparticles (MNPs) have received much attention recently to remove radioactive metal ions due to the easy separation of sorbent from solution by adding a magnetic field. Xu et al. reported the first use of bisphosphonate modified magnetite Fe3O4 nanoparticles to remove UO2 2+ from blood. At pH 7.0, the partitioning coefficient (Kd) is estimated as high as 19.8 L/g, and the sorbent can be easily removed from blood by using a small magnet [68].
However, the Fe3O4 nanoparticles are unstable at acid condition, which limits their application for nuclides sorption from acid nuclear waste water. Qiang et al. developed silica coated magnetic nanoparticles for the separation of acidic nuclear waste, in which the Fe2O3 nanoparticles were coated with the silica, followed by covalent attachment of the actinide-specific chelators to separate nuclear waste under acidic conditions.
The silica coated MNPs are stable even in 1 M HCl solution, and show enhanced actinide separation efficiency compared to the uncoated counterparts [69]. All above mentioned studies have dealt with radionuclides adsorption and separation using nanomaterials. From these works we could reassert that nanomaterials show several interesting aspects for application in nuclear waste disposal and environmental remediation.
However, some nanomaterials are not quite stable under the conditions of ionizing radiation and can not keep their functions for relatively longer period of time in complex chemical environments. Furthermore, the issues on safety and migration behaviors of nanomaterials themselves still need clarification and evaluation.
In these regards, full realization of a true potential of nanomaterials as radionuclide adsorbents requires further studies concentrating on developing high efficient, radiationresistant, renewable and environment-friendly nanomaterials for solid-phase extraction of radionuclides.
More information: Self-accelerated corrosion of nuclear waste forms at material interfaces, Nature Materials (2020). DOI: 10.1038/s41563-019-0579-x , https://nature.com/articles/s41563-019-0579-x
