Since the Atomic Age of the 20th century, millions of liters of high-level radioactive waste (HLW) have been generated with the production of plutonium for the nuclear weapon programs. The same kind of HLW can also result in reprocessing the used commercial nuclear fuels, even though most of these used fuels in the US have not been reprocessed. These HLWs, mixes of liquid, sediment, and sludge, are composed of unstable nuclides that undergo spontaneous decay processes accompanied by the emission of energetic particles or photons. The various modes of decay encompass alpha, beta, gamma radiation, electron capture, proton emission, neutron emission as well as cluster radioactivity and spontaneous fission – each characteristic for a given radionuclide. Alpha decay of transuranic elements (TRU) releases alpha-particles with several MeV of kinetic energy and changes the remaining mass of recoil to different elements carrying several tens of keV kinetic energy. Beta decay releases either an electron or positron from certain radionuclide transforming it to an isobar of the nuclide [1], [2]. Due to the large volume and high activities of these nuclear wastes, as well as the associated political and public-policy issues, effective disposal, and isolation of these radioactive wastes from the biosphere is an increasingly urgent and important topic in nuclear science and engineering. The nuclear waste form is the material designed to encase and isolate radioactive isotopes. It ensures safe containment and minimizes the release of harmful radionuclides into the environment. Common nuclear waste forms include glass, ceramics, specialty alloys and the mixture of the above. The decay of encapsulated radionuclides and the resulting radiation and thermal effects can cause modifications to important material properties, leading to either deterioration or improvement of the waste form performance in the very long desired lifetime of service, therefore, evaluation of radiation effects in the waste form designed for encapsulating various radioactive isotopes is a very important task [3], [4], [5], [6], [7]. In addition, the high cesium loading capacity, excellent structural stability, and demonstrated low-cost synthesis routes make hollandite titanates [8] a promising host material for use in beta-voltaic-based nuclear batteries [9]. Further verification of their tolerance to ionizing radiation will be essential for confirming their suitability in such applications.
Alpha-decay of the TRU elements such as Pu, Am, Cm are principal sources of radiation in HLW with a production of energetic alpha-particles (4.5–5.5 MeV), energetic recoil nuclei (70–100 keV), and some gamma-rays. The alpha-particle will deposit its energy by ionization process, while the alpha-recoil ion will transfer energy with the elastic collision processes and result in potential displacement damage, volume swelling, rearrangement or transition of the surrounding material structure [10]. Alpha-decay is generally dominant for a longer time due to the long half-lives of the actinides and their daughter products. To prevent human exposure over extensive periods, it is critical to determine an effective geological disposal method for long-lived actinides such as 239Pu that has a half-life of 24,100 years [11]. The radiation effect resulting from the alpha-decay of TRU exacerbates damage accumulation in waste forms over extended periods. One technique employed for simulating and studying such effects is charged-particle irradiation. Alpha-particle irradiation simulations are achievable through utilizing either sealed actinide alpha-sources, or particle accelerators, with high energy He ions to simulate the alpha-particles and heavy-ions (e.g., Kr, Xe, Pb) to simulate the recoils resulted from the alpha-decay events [12]. Since the majority of displacement damage is caused by the heavy recoils, heavy ions alone has often been used for the simulation study.
137Cs and 90Sr, the major fission products in the HLW stream that undergo beta-decay, are the primary sources of radiation during the initial 500-year duration of storage. The transmutation of cesium (Cs) into barium (Ba) is accompanied by changes in both ionic radius and valence. Specifically, the Cs+ ion undergoes radioactive decay to yield Ba2+, with a consequential reduction in its associated ionic radius by approximately 20 %. Their decay engenders energetic beta-particles and gamma-radiation to cause ionization in the material structure that instigates self-heating, charge defects, covalent and ionic bond rupture, permanent defects from radiolysis, and so on [13]. In accordance with alpha-decay simulation with ion beams, the utilization of electron irradiation allows exploration into the effects of ionization or electronic excitations from beta-particles and gamma-rays on waste forms.
SYNROC (Synthetic Rock) solidification method was proposed in 1978 [8], which used the principle of isomorphic substitution in mineralogy to incorporate radionuclides into the crystal structure of artificial minerals to prevent their loss into groundwater, for permanent disposal of HLW in a geological repository. Artificial rock is a thermodynamically stable mineral solid solution prepared by high-temperature solid-phase reaction. The radionuclide integrates into raw mineral materials at high temperature to generate the expected mineral phase and occupy its lattice positions [14]. Researchers have shown that the waste-forms of the artificial rocks could present superior thermodynamic and geological stability, chemical durability, and radiation resistance than those of the glass waste forms [15], [16], [17], [18].
Among all the artificial rocks, such as hollandite, pyrochlore, zirconolite, perovskite, etc. hollandite-type structure shows the unique properties in effectively immobilizing radioactive alkali and alkaline earth elements (e.g., Cs and Ba) in high-level wastes (HLW) [19], [20], [21]. However, due to the ability to form water soluble compounds, the high volatility at elevated temperatures, and the mobility in many host materials, Cs is always considered as one of the most challenging fission product radionuclides to immobilize. Thus, hollandite-type nuclear waste-form has attracted much attention and research [22], [23], [24], [25]. The hollandite group of minerals has the general formula AxByC8−yO16, in which A-sites are usually occupied by large-size cation (Ba2+ or Cs+) which presented as white dots in Fig. 1a, and B and C-sites the blue dots (Fig. 1a) which are occupied by relatively small-size cation (Ti4+, Ga3+, Cr3+, Al3+, Fe3+ or Zn2+). Further, oxygen (red dots in Fig. 1a) surrounds these cations with octahedral configuration forming a cage to inhibit the free migration of the large Ba2+ and Cs+ cations [25], [26]. Spatially, four adjacent octahedrons are connected at the corner, forming a tunnel parallel to the short crystal axis. Depending on the radius ratio of the A-B site cations, the cross-section of the tunnel may either exhibit square corresponding to tetragonal crystal structure, or distorted rhombus corresponding to monoclinic crystal structure [27].
Hollandite’s capability to withstand radiation has been investigated in previous studies through various methods such as electron, neutron, and alpha irradiations [13], [24], [28]. To simulate the alpha-recoil damage, accelerated heavy ions including Kr ions have also been used for mineral specimens containing Zn-hollandite [23], [28]. Hollandite’s behavior under ionizing radiation has been investigated through electron-beam radiation experiments in transmission electron microscopes and Van de Graaff accelerator-based methods in past research [13], [29], [30]. Although a variety of hollandite structures have been synthesized, including those with Al-, Zn-, and Ga- substitutions [19], [28], [30], [31], there is limited information on Fe-substituted hollandite. The emphasis of many research endeavors has been centered around investigating the thermodynamic stability and chemical durability aspects [32], [33], while electron and ion radiation impacts have not received as much attention.
In this work, a series of Ba1.33−xCsxFe2.66−xTi5.34+xO16 hollandites (as shown in Table 1) with increasing Cs content and simultaneously reducing Ba content were synthesized by solid-state reactions. In this hollandite system, the small cations at B and C sites were chosen as Ti4+ and Fe3+, because the addition of the Ti4+ and Fe3+ could effectively inhibit the formation of a secondary phase, thus enhancing the Cs incorporation. In previous research [32], the crystal chemistry, Cs retention and thermodynamic stability of these materials have been investigated by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and sublattice-based thermodynamic simulations, respectively.
The crystal structure of this hollandite system and its microstructural stability under both ionizing and displacement radiation conditions were further investigated by transmission electron microscopy (TEM). High-angle annular dark-field (HAADF) and integrated differential phase contrast (iDPC) performed in an aberration-corrected STEM were used to obtain atomic-resolution images of the hollandites. Electron and ion beams were employed to simulate the beta- and alpha-decay of Cs and TRU radionuclides that might be encapsulated in the hollandite complex structure. To enhance comprehension of experimental outcomes, density functional theory (DFT) simulations were conducted.