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.     

Ab initio based rate theory model of radiation induced amorphization in β-SiC

An ab initio informed rate theory framework for a multicomponent system is developed and used to model radiation induced amorphization in β-SiC. Based on the published modeling and experimental studies we propose three possible energy landscapes (ELs) for defect recombination in SiC. We demonstrate that defect ELs have a dramatic effect on the shape of the dose to amorphization vs. temperature curve and on the critical temperature to amorphization T_cr. In the no-barrier EL model, T_cr is correlated with the mobility of silicon interstitials, while in the recombination and trapping models T_cr is governed by the rate of defect recombination. We conclude that both the defect migration barrier and the defect recombination barrier are key parameters to consider when modeling radiation resistance of SiC, and possibly of other multi-component covalent materials.     

Ion irradiation damage effects in δ-phase Y6W1O12

Polycrystalline Y₆W₁O₁₂ samples were irradiated with 280 keV Kr²⁺ ions to fluences up to 2 × 10²⁰ ions/m² at cryogenic temperature (100 K). Ion irradiation damage effects in these samples were examined using grazing incidence X-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (TEM). The pristine Y₆W₁O₁₂ possesses rhombohedral symmetry (structure known as the δ-phase), which is closely related to cubic fluorite structure. GIXRD and TEM observations revealed that the irradiated Y₆W₁O₁₂ experiences an ordered rhombohedral to disordered cubic fluorite transformation by a displacement damage dose of ∼12 displacements per atom (dpa). At the highest experimental dose of ∼50 dpa, the uppermost irradiated region was found to be partially amorphous while the buried damage region was found to contain the same fluorite structure as observed at lower dose.     

Composition of reactor radiation—statistically significant radiation damage factor of diamond

Conclusions  

Radiation damage effects in the uranium-bearing δ-phase oxide Y6U1O12

Ion irradiation damage effects in delta (δ) Y₆U₁O₁₂ were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Experimental results revealed no amorphization transformation occurs in Kr-ion irradiated Y₆U₁O₁₂ to a maximum displacement damage dose of ∼50 displacements per atom at cryogenic temperature. Density functional theory calculations indicate that δ-Y₆U₁O₁₂ possesses a relatively low cation antisite formation energy, which may help to explain the observed resistance of δ-Y₆U₁O₁₂ to irradiation-induced amorphization of δ-Y₆U₁O₁₂.     

Microstructural influence on high temperature creep flow of Zr-1%NbO alloy in near-α, (α + β), and β temperature ranges in a high vacuum environment

Uniaxial tensile creep tests were carried out at 650-1100 °C in a high vacuum environment on Zr-1%NbO tubes with various microstructures. The effect of microstructure on creep flow in the (α + β) temperature range is significant (the creep rate being modified by up to three orders of magnitude) under stresses lower than 10 MPa, that is, for stress values of one order of magnitude lower than those characteristic of prototypical Loss-of-coolant-accident (LOCA) conditions. Under stresses higher than about 20 MPa, this effect is much smaller. No transformation-induced plasticity was detected from anisothermal creep tests, once the creep strain was thoroughly taken into account to process experimental strain vs. time data.     

Radiolysis studies of aqueous κ-carrageenan

The effects on N₂O and N₂ gas on the radiation degradation yield of aqueous kappa (κ-) carrageenan were investigated. The G_d of solution saturated with N₂O solution was expectedly much higher than in air (1.7 and 1.2 × 10⁻⁷ mol J⁻¹). On the other hand, a lower G_d of 1.1 × 10⁻⁷ mol J⁻¹ was obtained from κ-carrageenan solution saturated with N₂.The rate constant of reaction of OH radicals with sonicated and irradiated κ-carrageenan were determined using e-beam pulse radiolysis. The rate constant of OH interaction with sonicated κ-carrageenan decreased with decreasing molecular weight. On the other hand, the OH interaction with irradiated κ-carrageenan decreased but did not vary significantly with decreasing molecular weight. Metal ion (Na⁺) induced conformational transition into helical form decreased the rate constant of OH reaction with κ-carrageenan. Likewise, the G_d in aqueous form was affected by the conformational state of κ-carrageenan. The helical conformation gave a lower G_d (7 × 10⁻⁸ mol J⁻¹) than the coiled conformation (G_d = 1.2 × 10⁻⁷ mol J⁻¹).     

Effects of γ-ray irradiation on the C-V and G/ω-V characteristics of Al/SiO2/p-Si (MIS) structures

The effect of the ⁶⁰C_o (γ-ray) exposure on the electrical characteristics of Al/SiO₂/p-Si (MIS) structures has been investigated using capacitance-voltage (C-V) and conductance-voltage (G/ω-V) measurements. The MIS structures were stressed with a bias of 0 V during ⁶⁰C_oγ-sources irradiation with the total dose range from 0 to 25 kGy. The C-V and G/ω-V characteristics were measured at 500 kHz and room temperature before and after ⁶⁰C_oγ-ray irradiation. The results indicated that γ-irradiation caused an increase in the barrier height Φ_B, interface states N_ss and depletion layer width W_D obtained from reverse bias C-V measurements. The series resistance R_s profile for various radiation doses was obtained from forward and reverse bias C-V and G/ω-V measurements. Both C-V and G/ω-V characteristics indicate that the total dose radiation hardness of MIS structures may be limited by the decisive properties of the SiO₂/Si interface to radiation-induced damage. After γ-irradiation, the decrease in capacitance of MIS structure results in the increase in the semiconductor depletion width.     

Kinetic studies of the α-β phase transition in the Zr1%Nb cladding for nuclear reactors

The α-β phase transition of a zirconium alloy doped with 1 mol% of niobium (E110 alloy) is investigated using differential scanning calorimetry. The onset and endpoints of the transition are identified from the measured heat flow signal and from the integration of the observed peak the extent of the α-β phase change is calculated as a function of temperature. The experiment has been performed at different heating rates and a shift of the onset temperature with increasing heating rate was observed. From the dataset the equilibrium transition curve has been extrapolated and compared with other types of zirconium-based cladding materials.     

元素分析-同位素质谱测定δ15N和δ34S的信号强度响应特征

自然界中不同类型样品C、N和S元素含量以及C/N和C/S的差异极大,导致元素分析-同位素比值质谱仪(EA-IRMS)联机系统的进样量和检测信号强度的变化范围极大。进样量和信号强度对EA-IRMS测定δ¹⁵N和δ³⁴S的影响将导致样品测量结果具有较大的不确定性。为应对样品中目标元素含量低或样品稀缺等限制,本研究重点关注EA-IRMS测定δ¹⁵N和δ³⁴S的信号强度依赖性特征。结果表明,EA-IRMS在小元素进样量或弱信号强度条件下测定δ¹⁵N和δ³⁴S需要进行空白校正。在1.71～561.93μg N和3.74～100.62μg S的进样条件下,同位素标准物质δ¹⁵N和δ³⁴S的测定误差变化范围分别为0.18‰～0.54‰和0.21‰～0.88‰,测定精度变化范围分别为0.24‰～0.57‰和0.27‰～0.54‰。该联机系统对高C/N和C/S(>200)木材标准物质测定时,其δ¹⁵N平...     

Nucleation and growth of self-interstitial atom clusters in β-SiC during irradiation: Kinetic Monte-Carlo modeling

In order to clarify formation kinetics of self-interstitial atoms (SIA) clusters in cubic silicon carbide (β-SiC) during irradiation, the nucleation and growth process of SIA-clusters have been investigated by a kinetic Monte-Carlo (KMC) simulation technique. It has been found from the KMC simulations that the formation kinetics of SIA-clusters in β-SiC during irradiation is classified into the following two types, depending on temperature. At relatively high temperatures, the thermal stability of an SIA-cluster is crucial for the nucleation and growth of the cluster, in which the composition of the cluster is almost stoichiometric. In contrast, at relatively low temperatures where the cluster thermal stability is no longer crucial, even an SIA-cluster far from stoichiometric composition is formed.     

Partitioning of dopant cations between β-tricalcium phosphate and fluorapatite

Mixed crystalline phase composite ceramics offer the possibility of partitioning defect species between the phases as well as occupancy of specific sites within a given phase. Here we use atomic scale simulations to study the site preference of an extensive range of divalent and trivalent substitutional ions across the five cation sites in β-tricalcium phosphate (β-TCP) and the two cations sites in fluorapatite (FAp). This study indicates that in β-TCP small dopant species occupy the smaller of the five cation sites and vice versa. Conversely, in FAp, small divalent species occupy the nominally larger Ca(1) site while larger cations occupy the Ca(2) site. Partition energies between the two phases indicate that divalent species strongly segregate to β-TCP as do Al³⁺ and Ga³⁺, whereas all other (larger) trivalent ions exhibit little preference.     

Molecular dynamics evaluation of the impact of Ga, He, and vacancy concentration on the mechanical properties of Ga-stabilized δ-Pu

The effects of self-irradiation on the mechanical properties of Ga-stabilized δ-Pu are investigated by means of classical molecular dynamics simulations using a modified embedded-atom method interatomic potential. The impact that variations in the concentration of Ga, He, and vacancies have on the elastic properties (Young’s modulus, Poisson’s ratio, shear modulus, and bulk modulus) of Ga-stabilized δ-Pu is determined through CMD simulation. The effect that variations in these concentrations have on plastic flow is assessed at strain rates of 10⁷-10¹⁰ s⁻¹. Comparison is made to experimental data.     

Radiation resistance of photodiodes based on indium monoselenides under γ-irradiation

The influence of γ-irradiation (E = 3 MeV) over a large dose range 0.14-140 kGy on the electrical and photoelectric parameters of p-n-InSe and intrinsic oxide-p-InSe photoconvertors has been investigated. The detected changes in current-voltage characteristics, photoresponse spectra, open-circuit voltage, and short-circuit current for the structures are explained by the formation of radiation-induced point defects. A comparison to silicon photodiodes irradiated at analogous conditions has been carried out. On the basis of the absence of essential changes of the characteristics of the homo- and hetero-junctions based on III-VI layered semiconductors even at the maximum irradiation doses these junctions are recommended as radiation-resistant photodetectors for operation under γ-irradiation.     

Fine-structure effects on the polarization of Kα1 radiation from heavy, highly-charged ions

Radiative decay of heavy, few-electron ions following electron capture is studied within the framework of the density matrix theory combined with the multiconfiguration Dirac-Fock approach. Special attention is paid to the linear polarization of the decay X-ray photons. Detailed calculations have been carried out, in particular, for the Kα₁ (1s2p_3/2^1,3P_1,2 →  1s²¹S₀) transition in helium-like uranium U⁹⁰⁺ ions. For this line, the characteristic radiation is almost unpolarized as a result of the superposition of its fine-structure ¹P₁ → ¹S₀ and ³P₂ → ¹S₀ components which are strongly linearly polarized in perpendicular directions.     

XPS analysis of UxCe1−xO2±δ and determination of oxygen to metal ratio

The chemical states of U and Ce in the solid solutions of UO₂ and CeO₂ are studied using the X-ray photoelectron spectroscopy. A detailed analyses on U 4f and Ce 3d photoelectron peaks revealed the presence of Ce³⁺ and U⁵⁺/U⁶⁺ states in the mixed oxides. The oxygen to metal ratios in different compositions of mixed oxides were estimated from the quantity of different chemical states of U and Ce present in mixed oxides.     

A steam generator model identification and robust H∞ controller design with ν-gap metric for a feedwater control system

A robust H_∞ controller for the feedwater system of KSNP (Korean Standard Nuclear Power Plant) vertical U-tube type steam generators (UTSG) is presented herein. As the first step of the controller development, a precise thermal–hydraulic model for the steam generator is built. A series of model experiments are performed using the developed thermal–hydraulic model in order to acquire the input–output data sets which represent steam generator characteristics. These data sets are utilized to build simplified steam generator models for control through a system identification algorithm, Simple Process Models. Among the developed steam generator models, the representative models for the designated power ranges are selected by a criterion of ν-gap metric. The representative robust controllers for the selected models are designed utilizing the loop-shaping H_∞ design technique. Finally, the robustness and performance of the proposed controllers are validated and compared against those of PI (proportional–integral) controller. The validation results demonstrated that the proposed H_∞ robust controller has a superior robustness and an enhanced control performance.     

An analytical multigroup benchmark for (n, γ) and (n, n′, γ) verification of diffusion theory algorithms

The angular flux for the “rod model” describing coupled neutron/gamma (n, γ) diffusion has a particularly straightforward analytical representation when viewed from the perspective of a one-group homogeneous medium. Cast in the form of matrix functions of a diagonalizable matrix, the solution to the multigroup equations in heterogeneous media is greatly simplified. We shall show exactly how the one-group homogeneous medium solution leads to the multigroup solution.     

A study of radiation induced reactions in super critical water

It is known that water exists in supercritical state above 374℃ and 22.1MPa, where gas phase and liquid phase are merged into a single phase. In the supercritical state, the density is controllable by changing the pressure. The properties such as the ionic product, solubilities of salts, gas and organic compounds, and dielectric constant of supercritical water are very different from those of water at room temperature.