Radiation damage of α-Al2O3 in the HVEM: II. Radiation damage at high temperature and high dose

The temperature dependence of the apparent displacement threshold energy has been measured for single crystal α-Al₂O₃ in a high voltage electron microscope, over the temperature range 320–1120 K. Below 570 K the threshold energy for observable damage remains constant at 390 ± 10 keV, then rises to a peak of 440 ± 20 keV at 695 K followed by a rapid fall to ~200 keV at 870 K and above. Measurement of the displacement threshold energy, using the optical absorption and luminescence of oxygen and aluminium electron irradiation induced vacancies at several temperatures, gives a value of 440 ± 25 keV for the oxygen ion and 175 ± 25 keV for the aluminium ion. These results show that oxygen and aluminium ions have widely different displacement energies, E_d, of ~75 and 18 eV respectively.     

Radiation damage of α-Al2O3 in the HVEM: I. Temperature dependence of the displacement threshold

The temperature dependence of the apparent displacement threshold energy has been measured for single crystal α-Al₂O₃ in a high voltage electron microscope, over the temperature range 320–1120 K. Below 570 K the threshold energy for observable damage remains constant at 390 ± 10 keV, then rises to a peak of 440 ± 20 keV at 695 K followed by a rapid fall to ~200 keV at 870 K and above. Measurement of the displacement threshold energy, using the optical absorption and luminescence of oxygen and aluminium electron irradiation induced vacancies at several temperatures, gives a value of 440 ± 25 keV for the oxygen ion and 175 ± 25 keV for the aluminium ion. These results show that oxygen and aluminium ions have widely different displacement energies, E_d, of ~75 and 18 eV respectively.     

The determination of the displacement energy in type 316 austenitic steel

The displacement energy E_d in the 110 direction, which is a good approximation to the polycrystalline value in face-centred cubic metals, has been measured in type 316 austenitic steel at high temperatures using a high-voltage electron microscope to both produce and observe loop growth, dislocation flux and void swelling as a function of electron energy. In the loop growth experiments the threshold voltage for displacements was directly measured, in the other methods the normalized results were compared with Oen's calculated cross sections assuming various values of E_d. The latter method assumes that the effect per electron is independent of dose rate, and this has been verified for void-swelling. The results obtained from all the experiments lie in the range E_d = 18 ± 1.5 eV.     

Simulating radiation damage in Ga stabilised δ-Pu

Radiation events in Ga stablised δ-Pu are investigated by means of Molecular Dynamics simulations. Pu 5 at.% Ga is considered using the Modified Embedded Atom Method to govern the atomic interactions. Cascades were initiated with Primary Knock-on Atom (PKA) energies in the range of 0.4-10 keV, with trajectories deduced through comprehensive sampling of a representative set of directions, combined with different Ga atomic positions. The displacement threshold energy, E_d, for Pu and Ga atoms was also determined through similar extensive studies to aid the understanding and interpretation of the cascade results.Values of E_d between 5 and 40 eV were determined for Pu, with Ga PKAs requiring generally more energy to create a defect with E_d between 8 and 70 eV. Low energy collision cascades, initiated with energies in the range of 0.4-1 keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone surrounded by isolated interstitial defects. A feature of these cascades is that the displaced Ga atoms return to lattice sites during the ballistic phase, leading to a lack of Ga-type residual defects. Higher energy cascades show similar features but with the development of an amorphous region at the cascade core of around 5 nm diameter at 5 keV. Quantitatively, the residual number of defects found shows no distinct variation to that for previous work on pure Pu, suggesting the inclusion of Ga does not significantly effect the susceptibility or resistance of Pu to initial cascade development.     

Displacement threshold and Frenkel pair formation energy in ionic systems

Displacement threshold energies (E_d) and Frenkel pair formation energies (E_Fp) are investigated in detail by molecular dynamics computer simulation for three different ionic systems with the same crystal structure, MgO, SrO and NaCl in order to see if there is a functional relationship between them. It is found that there are wide variations in the values of E_d depending on the direction in which energy is imparted to a static atom in the lattice. Large values of E_d are found along the major crystallographic directions and lower values elsewhere. Typically these thresholds are between 5 and 9 times bigger than the Frenkel pair formation energies E_Fp with no observable dependence on mass or ion charge. The differences in the interaction potentials also means that for any given direction, there is only limited correlation between values of E_d in the different systems studied and no quantifiable relationship with E_Fp.     

Deuteron–nucleus total reaction cross sections up to 1 GeV

Total reaction cross sections of deuteron, σ^R_d, are calculated by a microscopic three-body reaction model. The reaction model has no free adjustable parameter and applicable to reactions at various deuteron incident energies E_d and with both stable and unstable nuclei. The predicted σ^R_d are consistent with those evaluated by a phenomenological optical potential for E_d ? 200 MeV in which the potential has been parametrized. A simple formula of σ^R_d up to E_d = 1 GeV, as a function of E_d, the target mass number A and its atomic number Z, is given.     

Low silicon U(Al,Si)3 stabilization by Zr addition

Previous knowledge states that (U,Zr)Al₃ and U(Al,Si)₃ phases with Zr and Si content higher than 6 at.% (7.7 wt%) and 4 at.% (1.4 wt%), respectively, does not partially transform to UAl₄ at 600 °C. In this work, four alloys within the quaternary system U-Al-Si-Zr were made with a fixed nominal 0.18 at.% (0.1 wt%) Si content in order to assess the synergetic effect of both Zr and Si alloying elements to the thermodynamic stability of the (U,Zr)(Al,Si)₃ phase. Heat treatments at 600 °C were undertaken and samples were analyzed by means of XRD, EPMA and EDS techniques. A remarkable conclusion is that addition of 0.3 at.% Si in the (U,Zr)(Al,Si)₃ phase reduces in 2.7 at.% the necessary Zr content to inhibit its transformation to U(Al,Si)₄.     

Theoretical calculation of neutron cross sections for 90,91,92,94,96Zr in the incident energy range between 200 keV and 20 MeV

Neutron cross sections of ^{90,91,92,94,96}Zr were calculated in the incident energy (E_n) range from 200 keV to 20 MeV for the revision of the 4th version of the Japanese Evaluated Nuclear Data Library (JENDL-4.0). The calculation was carried out by using conventional nuclear reaction models such as the spherical optical model, the distorted wave Born approximation, preequilibrium models, and the multi-step statistical model. Parameter values of these nuclear models were adjusted with the aid of experimental cross sections which were published after the JENDL-4.0 evaluation. Cross sections were computed for total, elastic and inelastic scattering, (n, γ), (n, 2n), (n, p), (n, α), (n, nα), and (n, x) = (n, d) + (n, np) reactions, and they were almost consistent with the experimental data. The cross sections were also estimated for the metastable states with the half-life larger than 1 sec. The obtained results well reproduced measured cross sections for the reactions ⁹⁰Zr(n, 2n)^89mZr, ⁹¹Zr(n, x)^90mY and ⁹¹Zr(n, nα)^87mSr.     

Molecular dynamics study of Frenkel pair recombinations in fluorite type compounds

The recombination behaviors of cation and anion Frenkel pairs have been investigated in four fluorite structure compounds: Li₂O, CaF₂, CeO₂ and UO₂. The calculations have been done in the framework of empirical potentials molecular dynamics simulations. They have revealed that the recombination processes are strongly dependent on the configuration (i.e. the sublattice, the distance between the Frenkel pairs and the local topology of the interstitials). Each configuration shows the same recombination process whatever the chemistry (i.e. for the four compounds Li₂O, CaF₂, CeO₂ and UO₂), such that the recombination processes are related to the crystal structure of the materials. Two different recombination regimes have been identified: either spontaneous (i.e. temperature independent) or thermally activated. The recombinations are direct or occur through replacement sequences.     

Stopping of 0.3-1.2 MeV/u protons and alpha particles in Si

The stopping cross sections ε(E) of silicon for protons and alpha particles have been measured over the velocity range 0.3-1.2 MeV/u from a Si//SiO₂//Si (SIMOX) target using the Rutherford backscattering spectrometry (RBS) with special emphasis put on experimental aspects. A detection geometry coupling simultaneously two solid-state Si detectors placed at 165° and 150° relative to each side of the incident beam direction was used to measure the energies of the scattered ions and determine their energy losses within the stopping medium. In this way, the basic energy parameter, E_x, at the Si/SiO₂ interface for a given incident energy E₀ is the same for ions backscattered in the two directions off both the Si and O target elements, and systematic uncertainties in the ε(E) data mainly originating from the target thickness are significantly minimized. A powerful computer code has been elaborated for extracting the relevant ε(E) experimental data and the associated overall uncertainty that amounts to less than 3%. The measured ε(E) data sets were found to be in fair agreement with Paul’s compilation and with values calculated by the SRIM 06 computer code. In the case of ⁴He⁺ ions, experimental data for the γ effective charge parameter have been deduced by scaling the measured stopping cross sections to those of protons crossing the same target with the same velocity, and compared to the predictions of the SRIM 06 computer code. It is found that the γ-parameter values generated by the latter code slightly deviate from experiment over the velocity region around the stopping cross section maximum where strong charge exchanges usually occur.     

Overall Conductivity and Electromotive Force of SrZr0:9Yb0:1O3–a Cell System Supplied with Moist CH4

A proton-conducting ceramic cell for recovering tritium from process streams was investigated for its application to a fusion reactor system. The ceramic cell tested here was composed of a SrZr_0:9Yb_0.1O_3–a tube, one end of which was closed, and Ni/SiO₂ and NiO/SiO₂ porous electrodes. Its anode was supplied with moist CH₄ or H₂ and its cathode with moist O₂. All of the j-V curves obtained by a direct-current method were correlated to the relation V = E ₀ ? jd/σ at 600–700°C regardless of the two different conditions of the CH₄ + H₂O and H₂ + H₂O supply. The rate-controlling step of charged hydrogen ion transfer was determined from the dependences of the overall conductivity σ and the electromotive force E ₀ on the anode H₂O partial pressure and temperature. The E ₀ value under the condition of the CH₄ + H₂O supply was affected by the diffusion of reaction products of CH₄ + H₂O = CO + 3H₂ through the porous anode. On the other hand, the σ value was limited by the oxygen reduction rate at the cathode interface between the ceramic and the Ni electrode regardless of the different conditions between CH₄ + H₂O and H₂ + H₂O. These results were consistent with our results obtained by an alternating-current method. The activation energy of the overall conductivity was 60 kJ/mol.     

Additive Element Effects on Electronic Conductivity of Zirconium Oxide Film

A theoretical study on the electronic structure of zirconium oxide using a molecular orbital method was carried out to investigate the additive element effects on the electronic conductivity of oxide film formed on Zr-alloys. The atomic clusters used were (MZri₁₂O₈)³⁶⁺(M=Zr, 3d-transition metals and alkali metals). To simulate the electron conduction process in the oxide, calculations for a cluster with oxygen vacancy (V0) were also carried out. The energy gap E_g between electron-occupied and empty levels was evaluated, and the electronic conductivity was estimated qualitatively. Opposite effects on the electronic conductivity were found for additions of 3d-transition metals and alkali metals. The latter increased the electronic conductivity by forming impurity levels with small E_Q. The former, however, induced compressive strain in the oxide, resulting in a lowering of electronic conductivity due to widening of the energy gap at the oxygen vacancy.     

Equilibrium and kinetic studies of selective adsorption and separation for strontium using DtBuCH18C6 loaded resin

A crown ether loaded resin was prepared by successive impregnation and fixing the 4′,4′(5″)-di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6) and its molecule modifier, 1-dodecanol, onto the porous silica/polymer composite support (SiO₂-P particles). The characterization of DtBuCH18C6 loaded resin was examined by thermal gravimetry and differential thermal analysis and electron probe microanalysis. The adsorption behavior of Sr(II), Cs(I), Ru(III), Pd(II), La(III), Nd(III), Sm(III), Gd(III), Zr(IV), and Mo(VI) was investigated by the batch method. Furthermore, the column test for Sr (II) was performed. The batch experiments were carried out by varying the shaking times, HNO₃ concentration, and initial concentration of metal ions. A relatively large K _d value above 182 cm³/g for Sr(II) was obtained in the presence of 3 M HNO₃. In contrast, the K _d values of Cs(I), Ru(III), Pd(II), La(III), Nd(III), Sm(III), Gd(III), Zr(IV), and Mo(VI) were considerably lower than 10 cm³/g. The adsorption of Sr(II) was found to be controlled by chemisorption mechanism, and followed a Langmuir-type adsorption equation. The breakthrough curve of Sr(II) had S-shaped profile, and the elution percentage was estimated to be 99.9% by using the eluent of H₂O.     

Effects of water-to-cement ratio and temperature on diffusion of water in hardened cement pastes

Apparent diffusion coefficients (D_a) of water and activation energies (E_a) of diffusion in hardened cement pastes (HCPs) were determined as a function of water-to-cement (w/c) ratio (0.36–0.60) and temperature (293–323 K) using HTO and H₂¹⁸O as tracers. The values of D_a and E_a ranged from 1.1×10^?11 to 1.7×10^?10 m² s^?1 and from 21.5 to 31.3 kJ mol^?1, respectively. No significant difference between the D_a values of HTO and H₂¹⁸O suggests that water predominantly diffuses as H₂O molecule and dissociation of water is not significant even at high pH range in HCP. The values of E_a at low w/c ratio were higher than in bulk liquid water, suggesting a contribution of a different water regime, such as supercooled bulk water. Two simple models consisting of capillary and gel pores were considered to estimate the volume ratio of gel pores to total pores by optimizing the model to fit with the experimental data. The result suggests that HCP has a pore network mostly consisting of capillary pores with some very narrow pores plugged with hydrates, where HTO must diffuse through gel pores. This view of the HCP pore network was made available through analysis of E_a values.     

作为核废料载体锆石缺陷结构的能量学研究

原原子模拟技术拟合锆石某些物理性质的实验值获得了锆平衡结构模型的势能参数。在锆石平衡结构的基础上,结构中基本点缺陷及其形成能和结构无序态及其形成能得到了详细研究。锆石中Pu、U和Th与Zr的类质同象替代方式及其类质同象形成热的研究发现,组分为Pu75mol%-Zr25mol%处存在一不混溶间隔,这表明锆石中放射性核素的载量应小于50mol％。     

Study of ion beam induced mixing during sputter depth profiling of thin films by LEIS

Ion beam induced mixing during sputter depth profiling was studied for tantalum and lead marker layers in silicon with 5 keV neon by low energy ion scattering spectroscopy (LEIS). The diffusion approximation was used to calculate mixing efficiency values (D/JF_d) from decay length measurements. The mixing efficiency values are shown to be sensitive to the preferential sputtering which takes place during ion bombardment. TRIM simulations for Ta/Si are shown to agree with the experimentally determined value for preferential sputtering. Depth profiling at high temperature is shown to separate some of the interrelated mixing mechanisms of radiation enhanced diffusion (RED) and radiation induced segregation (RIS). For the Ta/Si system the mixing efficiency value is observed to remain constant regardless of the 5 keV inert gas ion beam used for depth profiling.     

Effects of gamma-ray irradiation on crevice corrosion repassivation potential of stainless steel in high temperature diluted simulated seawater

The crevice corrosion repassivation potentials (E_R,CREV) of type 304 stainless steel (304 SS) were measured in high temperature (373–553 K), diluted simulated seawater under gamma-ray irradiation, in order to confirm the effects of gamma-ray irradiation on the crevice corrosion behavior of a representative stainless steel in seawater. Overall, for high temperatures, the E_R,CREV values decreased with increasing chloride ion concentration, which was the same as the behavior observed under the non-irradiated condition. The E_R,CREV values measured under gamma-ray irradiation were the same or slightly higher than E_R,CREV values measured under the non-irradiated condition when the [Cl^?] was the same. Consequently, it was confirmed that the threshold potential of crevice corrosion of 304 SS for the gamma-ray irradiation of 1.8 kGy at least did not deteriorate compared with the non-irradiated condition. Under the conditions of this work (seawater composition, [Cl^?] range, dose rate, absorbed dose, flow rate, etc.), the crevice corrosion of 304 SS could be suppressed by maintaining the potential below the threshold potential which was determined approximately as ?0.3 V vs. SHE even for the irradiated condition at temperatures up to 553 K.     

Atomistic simulation of collision cascades in zircon

Defect production in energetic collision cascades in zircon has been studied by molecular dynamics simulation using a partial charge model combined with the Ziegler–Biersack–Littmark potential. Energy dissipation, defect accumulation, Si–O–Si polymerization and Zr coordination number were examined for 10 keV and 30 keV U recoils simulated in the constant NVE ensemble. For both energies an amorphous core was produced with features similar to that of melt quenched zircon. Disordered Si ions in this core were polymerized with an average degree of polymerization of 1.5, while disordered Zr ions showed a coordination number of about 6 in agreement with EXAFS results. These results suggest that nano-scale phase separation into silica- and zirconia-rich regions occurs in the amorphous core.     

Liquid Metal Extraction for Removal of Molybdenum from Molten Glass Containing Simulated Nuclear Waste Elements

Laboratory-scale experiments for removing Mo and MoO3 from molten borosilicate glass were performed using liquid Cu as an extractant. Removal of Mo from the simulated HLW glass containing oxides of Nd, Fe, Zr, Mo, Sn, Ni, Sr, Cd, Ru, and Se was also performed, and the fractions of these elements transferred into Cu were examined. Mixtures of Cu anda ternary SiO₂-B₂O₃-Na₂O glass containing metallic Mo or MoO₃ were heated in an alumina crucible at 1,673K in an Ar environment. The amounts of Mo and MoO₃ added to 10 g of the ternary glass were fixed at 0.1 and 0.15 g, respectively. As for the glass containing metallic Mo, more than 90% of Mo was extracted into liquid Cu. Spherical Cu metal buttons containing Mo formed on the bottom of the crucible when Cu was added at more than 10 times that of Mo on a mass basis. Removal of Mo from the glass containing MoO3 was also achieved by the addition of Si as a reducing agent for the reduction from MoO₃ to Mo. The fraction of Mo extracted into liquid Cu depended on the molar ratio of Si to Cu added to the glass. The fraction increased up to 84% with an increase in the molar ratio of Si/Cu. However, the excess addition of Si may enhance the chemical interaction between the metal phase and the glass phase, and some of the metal phase containing Mo remained in the glass phase without forming a metal button. The optimum molar ratio of Si/Cu that produces the highest removal fraction was found to be approximately 0.5. Almost the same removal fraction of 88% was obtained from the simulated HLW glass under the condition of Si/Cu = 0.5. Nearly 100% of Ru was extracted into Cu with Mo, while Sr, Zr, and Nd were hardly extracted and remained in the glass.