Tritium-producing Burnable Absorber Rods Research Articles

Cluster dynamics simulations of tritium and helium diffusion in lithium ceramics

Tritium (T) and He diffusion in LiAlO2 and LiAl5O8 phases influences the performance of tritium producing burnable absorber rods (TPBARs) by affecting the gas release, swelling and thermal conductivity of Li-bearing ceramic pellets. Frenkel pair defects and clusters created by irradiation can attract T and He interstitials and form clusters of the type HeixLi,HeixAl,HeixO,TixLi,TixAlandTixO,1≤x≤4 in a Li, Al or O vacancy site (notation denotes x He or T atoms in a 1 Li, 1 Al or 1 O vacant site). The concentration and mobility of each of these clusters collectively contribute to the diffusion of the He and T gases in LiAlO2 and LiAl5O8. In this work, free energy cluster dynamics simulations implemented in the Centipede code, are used to obtain the concentration and diffusivities of these clusters which are then used to calculate the total diffusivity of T and He gases in LiAlO2 and LiAl5O8. The results show that diffusivity of T is at least one order of magnitude higher in LiAlO2 as compared to that in LiAl5O8 whereas He diffusion is 2–13 orders of magnitude higher in LiAlO2 as compared to that in LiAl5O8. There is a higher concentration of highly diffusive species (T interstitials and Ti03Li for the case of tritium and Hei01Li, Hei02Li and Hei03Li for the case of He) in LiAlO2 than in LiAl5O8 which increase the total diffusion of T and He in LiAlO2.

Non-equilibrium molecular dynamics studies of thermal diffusion of hydrogen isotopes in low concentration zirconium hydrides

Tritium permeability in zirconium-based tritium getter critically impacts tritium storage and environmental safety during operation of tritium-producing burnable absorber rods (TPBARs). Previous experiments indicated that during irradiation operation, the hydrogen equilibrium pressured is increased. Further experimental and modeling studies suggested that the enhanced tritium release observed for reactor scale assemblies might be related to a thermal diffusion known as the Soret effect. A direct measurement of the Soret factor, however, has not been performed. To improve TPBAR and other nuclear applications, here we have applied two non-equilibrium molecular dynamics methods to study thermal diffusion of hydrogen isotopes in low-concentration zirconium hydrides. One of the methods produces sufficiently converged results to distinguish crystal orientation, isotope type, and concentration effects. With this method, crystal orientation, isotope type, and concentration effects are discussed.

Study of Tritium Diffusivity in Pure and Sn-Defective Zr: A First-Principles Density Functional Theory Approach

Zirconium alloys (e.g., zircaloy-4) are used as tritium (3H) getter materials in tritium-producing burnable absorber rods (TPBARs) owing to their ability to capture 3H and chemically convert them into metal hydrides. Understanding of 3H diffusion mechanisms in zircaloy is crucial for the optimal design of material performance in nuclear technology. Here, we perform first-principles density functional theory calculations to study the 3H diffusion mechanism in pure and impure Zr with a low concentration of tin (Sn) atoms to determine the impact of the presence of Sn on the movement of 3H atoms through the material. First, we calculated the diffusion barriers for 3H in pure Zr by taking different migration pathways. We then introduced a low concentration of Sn impurity and systematically explored the impurity effect on the diffusion barriers for 3H. Using our calculated diffusion energy barriers, we further obtained the diffusion coefficients and analyzed the results by comparing them with the experimental and previously calculated values. A diffusion coefficient of the order of 10–8 m2/s is predicted. We also found that the presence of a Sn impurity could reduce the diffusivity up to 4 orders of magnitude. Our results could serve as guidelines for further experimental investigations.

Ab Initio Simulations of Tritium Diffusion in Al-Rich Iron Aluminide Coating Phases

Surface coatings of steels used in extreme conditions and corrosive environments generally aim to provide protection and increased durability. In the case of tritium-producing burnable absorber rods (TPBARs) used in nuclear reactors, a 316 stainless steel has been coated with Al. Scanning transmission electron microscopy (STEM) characterization of the coating found three Al-rich (>60 atom % Al) iron aluminide alloys identified as hexagonal FeNiAl5, monoclinic Fe4Al13, and orthorhombic Fe2Al5. Density functional theory simulations using nudged elastic band have been performed to investigate the diffusion of interstitial tritium in each Al-rich iron aluminide phase. While FeNiAl5 and Fe4Al13 can be viewed as the stacking of two layers, the structural peculiarity of Fe2Al5 is that channels of variable Al vacancy content are present along the c-axis. Therefore, three stoichiometries for Fe2Alx phase, namely, Fe2Al4, Fe2Al5, and Fe2Al6, have been considered to evaluate the impact of Al vacancy concentration on tritium diffusion behavior. Altogether, we found that at 600 K, tritium diffusion decreases from a faster rate in the channels of Fe2Alx phases (DT ≤ 10–11 m2·s–1) to Fe4Al13 (DT ≈ 10–12 m2·s–1), and finally in FeNiAl5 (DT ≈ 10–13 m2·s–1). We also find that interstitial tritium generally diffuses faster in Fe–Al coating phases than in the tritium breeding material γ-LiAlO2 (DT ≈ 10–14 m2·s–1) but slightly slower than in 316 stainless steel (DT ≈ 10–10 m2·s–1).

Molecular dynamics study of grain boundary and radiation effects on tritium population and diffusion in zirconium

Tritium population thermodynamics and transport kinetics critically define the tritium storage performance of zirconium tritides that can be used for a variety of nuclear applications including tritium-producing burnable absorber rods. Both thermodynamic and kinetic properties can be sensitive to grain sizes of materials and can be significantly altered by irradiated defects during operation under the reactor environments. A thorough experimental characterization of how these properties evolve under different reactor conditions and different initial grain structures is extremely challenging. Here molecular dynamics simulations are used to investigate tritium population and diffusion in zirconium with and without different planar symmetric and asymmetric tilt grain boundaries and irradiated defects. We found that in addition to trapping tritium, the most significant effect of planar grain boundaries is to increase tritium diffusivity on the boundary plane. Furthermore, fine grain structures are found to mitigate the change of tritium diffusivity due to irradiated point defects as these point defects are likely to migrate to and sink at grain boundaries.

Molecular dynamics study of radiation effects on tritium diffusion in zirconium tritides (δ-ZrT2-x)

Tritium diffusivity in zirconium tritides is key to tritium storage in tritium-producing burnable absorber rods. Experiments struggle with assessing tritium diffusivity in reactor environments because irradiated defects alter diffusion. Here molecular dynamics simulations are used to investigate the irradiation effects on tritium diffusion in δ-ZrT2-x. We found that for defect-free δ-ZrT2-x, diffusivity can be described by a single Arrhenius equation for composition range x = 0.01–0.07. The irradiated vacancies slightly increase diffusivity although have no effects on diffusion energy barriers. Interestingly, irradiated self-interstitial atoms substantially increase diffusivity and reduce the energy barriers due to a lattice expansion effect.

Neutron irradiation induced changes in isotopic abundance of 6Li and 3D nanoscale distribution of tritium in LiAlO2 pellets analyzed by atom probe tomography

Tritium, a radioactive isotope of hydrogen is produced by neutron irradiation of 6Li enriched LiAlO2 pellets in Tritium producing burnable absorber rods (TPBARs). Three-dimensional nanoscale mapping of Tritium in irradiated pellets is critical to understand its spatial variation and its association with various microstructural features in the pellets. Spatially resolved analysis of 6Li isotopic enrichment in such ceramic pellets before and after irradiation can provide insights to heterogeneities in Li isotopic distribution. However, Li being a light isotope, its spatially resolved compositional or isotopic analysis is a challenging task to most analytical methods. Here we used atom probe tomography, to evaluate the Li isotopic enrichment in both the LiAlO2 matrix and the Li deficient secondary phase LiAl5O8 before irradiation. These results were then compared with 6Li isotopic ratio measured by APT analysis of neutron irradiated pellets to clearly demonstrate the unique capability of APT to quantitatively analyze the isotopic enrichment of 6Li as well as for other light elements present in these materials. Evidence for heterogenous nanoscale distribution of 3H and O3H within irradiated pellet microstructure is also provided, which may be attributed to 3H trapping in irradiation induced vacancy clusters or voids within the LiAlO2 matrix. The characterization results given here, now prove the feasibility for using APT for analyzing nanoscale spatially resolved isotopic enrichment of Li as well as other light elements such as Tritium in TPBARs. This work also paves way to adopt APT for analyzing light elements such as Li and tritium in materials used for nuclear fusion reactor applications, geochemistry, astrophysics as well as for energy storage applications.

Effects of carbon impurity on tritium diffusion and helium formation in γ-LiAlO2 pellets: A first-principles study

Gamma-phase lithium aluminate (γ-LiAlO2) in the form of annular pellets is used as a breeding material in tritium-producing burnable absorber rods (TPBARs) for the production of tritium by thermal neutron irradiation of 6Li. Significant radiation damage occurs in γ-LiAlO2 due to neutrons, and tritium and helium (He) nuclei, which recoil through the crystal. The radiation-damaged crystal is rich with vacancies and impurities such as carbon (C) that can act as tritium trapping sites. The presence of C impurities can hinder 3H diffusivities significantly. Here, we studied the effect of C impurities on 3H diffusivity and energy barriers in γ-LiAlO2 using first-principles density functional theory. In addition, we studied the He diffusion in an undefective γ-LiAlO2. We calculated the energy barrier for diffusion in γ-LiAlO2 to be 0.98 eV for substitutional 3H in the presence of a C impurity. At 600 K, the corresponding diffusion coefficient was calculated to be 7.98×10-14m2/s. Similarly, we calculated the energy barrier for He diffusion to be 0.12 eV, which was along the crystallographic c-direction. The corresponding diffusion coefficient for He at 600 K was calculated to be 6.8×10-7m2/s. This work may open further theoretical and experimental avenues on impurity effects and release of He from γ-LiAlO2.

Probing the Radial Chemistry of Getter Components in Light Water Reactors via Controlled Electrochemical Dissolution.

Getters are among the key functional components in the tritium-producing burnable absorber rods (TPBARs) of light water reactors (LWRs) and are used to capture the released tritium gas. They are nickel-plated zircaloy-4 tubes that, upon exposure to irradiation or tritium in the light water reactors, undergo alteration in structure, chemical composition, and chemistry. Understanding the radial tritium distribution is key to gaining insight into the evolution of new chemistry upon irradiation to predict getter performance. The holy grail is to develop a method akin to selectively peeling off the layers of an onion in an effort to get a radial map of elements and particularly tritium across the getter. Toward this goal, the overall aim of this work is to establish a correlative technique that can be used to determine radial tritium distribution across getters. To this end, this work specifically focuses on the validation of a correlative method for controlled radial dissolution of nickel-plated getters. Here, pristine getters as well as getters loaded with different mass ratios of hydrogen and deuterium are used as the nonradioactive surrogates of tritium, the idea being that the methodology can be readily extended to tritiated getter components. Here, the surface nickel layers as well as the bulk zirconium layers are sequentially dissolved in a controlled, uniform way using voltage-assisted electrochemical dissolution techniques. The dissolution is complemented by periodic elemental analysis of the electrolyte solution during and post dissolution. This is complemented by microscopic analyses on the exposed surfaces to provide a correlative technique for a complete picture of the radial distribution of various elements across the getter

Microstructural classification of unirradiated LiAlO2 pellets by deep learning methods

Microstructural features and defects can greatly impact material properties and performance in a wide range of application areas. Recognition and characterization of microstructural features is essential to the understanding and prediction of material performance under various operational conditions, including irradiation. In this work, we tested a collection of Deep Convolutional Neural Network (DCNN) architectures that have been optimized for image segmentation and selected the best performer to obtain pixel-level classification of the main microstructural features in unirradiated LiAlO2 pellets, including grains, grain boundaries, voids, precipitates, and zirconia impurities. LiAlO2 is an important material that is used as a tritium producer for the Tritium Sustainment Program. While LiAlO2 pellets have been employed in tritium-producing burnable absorber rods (TPBARs) for years, comprehensive microstructural analysis of unirradiated LiAlO2, and therefore time-dependent tritium release from the material during irradiation, has not been established. A full understanding of unirradiated LiAlO2 microstructure and how it evolves as a result of neutron irradiation is necessary to produce an integrated performance model to predict in-reactor behavior as well as to target strategic experiments. This work aims at developing a fast and quantitative analysis method to classify various microstructural features in unirradiated LiAlO2 pellets that are visualized by scanning electron microscopy (SEM).Given classification results obtained, statistical analysis was then carried out to evaluate the performance of the DCNN classification and to describe the properties of the microstructural features as a whole, based on standard aggregation and spatial point-process methodology. Our results show improved performance over a baseline heuristic approach. Also, the computational efficiency of the computer-aided analytical method allows for quantitative characterization of a larger volume of SEM images than was previously possible using manual segmentation.

First-principles study of the tritium reaction and diffusion on the γ-LiAlO2 (1 0 0) surface with carbon impurity

Carbon (C) deposits have been experimentally found in numerous locations throughout the tritium-producing burnable absorber rods (TPBARs) on virtually every component. It is important to consider the typical C impurity in γ-LiAlO2 pellets investigation. Using the first-principles calculations, we explore the possible C impurities on γ-LiAlO2 (1 0 0) surface and their effect on tritium (T, 3H) reaction and diffusion. Our results show that the C prefers to adsorb on the outmost O-Li bridge site or substitute the outmost Al site. The adsorption C could not stay on the surface for a long time, due to the strong attraction with 3H. With increasing the number of 3H from bulk to the surface, the 3H and C atoms can form tritiated methane (CT4) and desorb from the surface. Therefore, CT4 is an extra product by C impurity from γ-LiAlO2 pellets in TPBARs. For the T/OT diffusion on γ-LiAlO2 (1 0 0) surface with C substitution, the T diffusion is always avoided by the O bonding with the substitutional C, because of the strong C-O covalence. Similarly, the OT diffusion is apart from the substitutional C, due to the repulsion of anion dangling-bond on C. Thus, T/OT diffusion is hampered by C-substituted Al site.

First-principles study of the surface properties of γ-LiAlO2: Stability and tritium adsorption

γ-LiAlO2 enriched with 6Li isotope exhibits excellent performance under neutron radiation and is used in tritium-producing burnable absorber rods (TPBARs) to produce tritium (T, 3H) in pressurized water reactor (PWR). To better understand the 3H formation and diffusion on the surfaces, the first-principles calculations are performed to study the stability of γ-LiAlO2 surface and the 3H adsorption on the surface. By considering all possible symmetrical stoichiometric and nonstoichiometric low-index surfaces, two most stable surfaces, one stoichiometric (100) and one nonstoichiometric (101), were identified. The 3H adsorption and diffusion on the most stable (100) surface were further explored, and the obtained results show that there are four stable sites (three O and one Al atop) for 3H adsorption. The calculated Bader charges of atoms reflect that 3H adsorption on the top of O site is a reduction process, while on the top of Al site is an oxidation one. Under low adsorption coverage of 1/4 per surface unit cell, 3H atoms are adsorbed on the most stable OA atop site. With increasing 3H diffused from bulk to surface, the AlG site is the second choice for 3H adsorption, and then the OB site. The coverage of 2/4 is the most favorable 3H adsorption state due to the counteracted charge transfer. When the coverage is up to 4/4, the interaction between 3H atoms leads to partial formation of T2 molecule that desorbs from surface. Therefore, T2 molecule should be the main product from the perfect surface of γ-LiAlO2 at high 3H coverage.

Density functional theory study of the point defect energetics in γ-LiAlO2, Li2ZrO3 and Li2TiO3 materials

The defect energetics in γ-LiAlO2, Li2TiO3, and Li2ZrO3 materials used in tritium-producing burnable absorber rods and fusion solid breeder applications was investigated using density functional theory calculations. A comprehensive analysis of the charged defects was performed for cation and anion vacancies, interstitials, antisite defects, and 3H interstitial and substitutional defects to understand the defect structures at different charge states and their stability as a function of the electron chemical potential across the electronic band gap of crystals at operating temperature and oxygen partial pressure conditions. Through investigation of the local bonding configurations and projected electronic density of states of the 3H related charged defects, including 3H interstitial (3Hint), 3Hint bound with an oxygen interstitial, and substitutional 3H defects on the Li, O, and M (M = Al, Ti, and Zr) sites (3HLi, 3HO, and 3HM), the 3H states of these defects were distinguished, i.e. triton (3H+), neutral 3H (3H0), and tritide (3H−). The two stable 3H related defects under the Li2O-LiOH rich condition at T = 1000 K, between P(O2) = 10−5 and 10−25 atm were found to be 3H interstitial (3Hint) with charge of +1 and neutral 3HLi (substitutional 3H on the Li site) in γ-LiAlO2, Li2ZrO3, and Li2TiO3 materials. For Li2TiO3, the stability of the 3HO with charge of +1 and 3HM with charge of −3 becomes comparable to that of 3Hint atm at P(O2) = 10−25 atm. Overall, our results indicate the energy cost to form the stable 3H defect species (3Hint with charge of +1 and neutral 3HLi) in γ-LiAlO2 material are higher than those of Li2TiO3 and Li2ZrO3 materials at T = 1000 K, between P(O2) = 10−5 and 10−25 atm. Interactions between the point defects and the 3H species as a function of the electron chemical potential across the band gap of the materials were also examined. All the three materials exhibit strong interactions between the 3Hint and the O vacancy, Li vacancy, and O interstitial defects with the strongest interactions found in the case of γ-LiAlO2 phase, indicating a higher tendency to trap 3H at such point defects in γ-LiAlO2.

Tritium Diffusion Pathways in γ-LiAlO2 Pellets Used in TPBAR: A First-Principles Density Functional Theory Investigation

With superior thermophysical and thermochemical properties, γ-LiAlO2 has high compatibility with other blanket materials and is used in the form of an annular pellet in tritium-producing burnable absorber rods (TPBARs) to produce tritium by thermal neutron irradiation of 6Li. In radiation damaged γ-LiAlO2, different types of vacancies, defects of its constituent elements, and other trapping sites hinder the diffusion process of tritium. In this study, the first-principles density functional theory approach is used to study the diffusion mechanisms of tritium defect and its species, such as interstitial and substitutional tritium defects, oxygen–tritium vacancy defects, and interaction of tritium with oxygen vacancies in defective and nondefective γ-LiAlO2. The obtained results provide an understanding of how such defects hamper the diffusivity and solubility of tritium. By calculating several different diffusion pathways, our results show that the smallest activation energy barrier is 0.63 eV for substitu...

In Situ TEM Multi-Beam Ion Irradiation as a Technique for Elucidating Synergistic Radiation Effects.

Materials designed for nuclear reactors undergo microstructural changes resulting from a combination of several environmental factors, including neutron irradiation damage, gas accumulation and elevated temperatures. Typical ion beam irradiation experiments designed for simulating a neutron irradiation environment involve irradiating the sample with a single ion beam and subsequent characterization of the resulting microstructure, often by transmission electron microscopy (TEM). This method does not allow for examination of microstructural effects due to simultaneous gas accumulation and displacement cascade damage, which occurs in a reactor. Sandia’s in situ ion irradiation TEM (I3TEM) offers the unique ability to observe microstructural changes due to irradiation damage caused by concurrent multi-beam ion irradiation in real time. This allows for time-dependent microstructure analysis. A plethora of additional in situ stages can be coupled with these experiments, e.g., for more accurately simulating defect kinetics at elevated reactor temperatures. This work outlines experiments showing synergistic effects in Au using in situ ion irradiation with various combinations of helium, deuterium and Au ions, as well as some initial work on materials utilized in tritium-producing burnable absorber rods (TPBARs): zirconium alloys and LiAlO2.

Isotope Effects on Hydrogen Getter Rate

Tritium Producing Burnable Absorber Rods (TPBARs) use nickel plated, zirconium tubing for gettering tritium. However, during the development phase of the getter design only protium gas was used to determine getter rate. The verification phase of getter performance (product acceptance testing) also used only protium gas. Thus, isotope mass effects on getter rate were not evaluated. A modified getter rate test using both protium and deuterium gas was performed to determine isotope mass effects. Test procedures were developed to measure isotope effect on getter rate versus temperature while minimizing experimental uncertainties with respect to surface cleanliness and variability of test results between different specimens.