Nuclear Analysis最新文献

In order to have a deep understanding of the tritium migration and release mechanisms in ceramic breeders, computer simulation of the tritium release behavior of Li₄SiO₄ ceramic breeder was performed by using an improved tritium release model. The influences of various factors, such as surface adsorbed water, water vapor in the purge gas, hydrogen in the purge gas, grain size and tritium production amount, on the tritium release process were systematically investigated. The simulation results have shown that: 1) The surface adsorbed water and water vapor in the purge gas can remarkably facilitate tritium release as tritiated water (HTO) in low temperature region (typically <450 °C); 2) Adding hydrogen to the purge gas is effective to promote tritium release as tritium gas (HT), but the effectiveness is sensitive to surface adsorbed water and water vapor in the purge gas; 3) Large grain size and tritium production amount can relatively weaken the effects of surface adsorbed water and water vapor in the purge gas. In general, the simulation results were consistent with the observations in tritium release experiments, which were helpful to further elucidate the complex behavior of tritium release from Li₄SiO₄.

Uranium oxides are tightly related to the whole nuclear fuel cycle in the nuclear industry. However, the understanding of the chemical states in mixed valence uranium oxides remains scattered and deserves more exploration. The current study aims to recognize the valences of uranium in U₃O₈ by a simple X-ray photoelectron spectroscopy (XPS) investigation. U₃O₈ mixed with TiO₂ was photocatalytically reduced under UV or X-ray irradiation, and the chemical states were in-situ determined by XPS. Under UV irradiation, the shoulder peaks at 380.9 eV (U 4f_7/2) and 392.7 eV (U 4f_5/2) gradually increased, and an enhancement in intensity was also observed at 399.6 eV, which is the characteristic of U(V) satellite. Therefore, it was proved that the shoulder peak originated from U(V), and U₃O₈ consists of U(V) and U(VI). Under the irradiation by X-ray, the main lines of U 4f shifted lower by ∼0.8 eV, and a new satellite at ∼397.6 eV appeared and grew with increasing irradiation time, proving the generation of U(IV). This in turn verified that the shoulder peaks in primary U₃O₈ should be U(V) rather than U(IV). Therefore, it was confirmed by the current study that there are two-U(V) and one-U(VI) in each U₃O₈ unit cell.

The neutron-induced cross sections are of great significance for the design of nuclear devices and advanced reactors for the nuclear energy production. At CSNS Back-n white neutron source, new measurements of the fission cross sections and total cross sections are performed with two sets of Day-one spectrometers based on the multi-cell fast fission ionization chamber (FIC). The neutron-induced ^236,238U fission cross sections relative to ²³⁵U from the fission threshold energy to 200 MeV were measured by using the TOF method and the Fast Ionization Chamber Spectrometer for Fission Cross Section Measurement (FIXM) in the single/double bunch mode of Back-n. The experimental uncertainties are analyzed in detail, and the results from the two modes are consistent. The measured ^236,238U/²³⁵U fission cross section ratios are compared with previous experiments and evaluations. The ^236,238U(n,f) cross sections are obtained based on the standard ²³⁵U fission cross section. The neutron total cross sections of carbon and aluminum in the energy region from 1 eV to 20 MeV have been measured by using the TOF method and transmission method based on the Neutron Total Cross Section Spectrometer (NTOX) in the double bunch mode. The total cross section results after unfolding are in good agreement with the previous measurements as well as the broadening of the ENDF/B-Ⅷ.0 evaluation with Gaussian function within the experimental uncertainty. The present results provide the experimental data for further measurements, relevant evaluations and the design of nuclear system.

With as goal of improving on traditional α-ZrP, four zirconium phosphonate materials were prepared as potential adsorbents for the removal of ⁹⁰Sr from nuclear wastewater. A typical sol–gel method was used, for phosphonates: hydroxy ethylidene diphosphonic acid (HEDP), amino tri-(methylenephosphonic acid) (ATMP), ethylene diamine tetra-(methylene phosphonic acid) (EDTMP), and diethylenetriamine penta-(methylenephosphonic acid) (DETPMP) to give ZrP-HEDP, ZrP-ATMP, ZrP-EDTMP and ZrP-DETPMP, respectively. These materials exhibit a similar crystalline phase to α-ZrP, but have a completely different morphology. After loading of these organophosphonate groups, the original sheet-morphology disappears, and the materials were consistent with smaller particles. However, the loading of organophosphonate groups expands the inter-layer distances. Remarkably, these materials have a stronger ability to remove Sr²⁺, with higher adsorption capacity than α-ZrP, especially ZrP-ATMP due to its wider layer distance. The maximum adsorption capacities for Sr²⁺ are 158 mg g⁻¹, 175 mg g⁻¹, 115 mg g⁻¹ and 76 mg g⁻¹for ZrP-HEDP, ZrP-ATMP, ZrP-EDTMP and ZrP-DETPMP, respectively, while that ofα-ZrP is 55 mg g⁻¹. The higher adsorption capacities of these zirconium phosphonate materials is attributed to their wider interlayer spacing, allowing more room for Sr²⁺ to move.

To study inverse effect of hydrogen isotopes in LaNi₅ for hydrogen isotope separation at ambient temperature, temperature dependence of hydrogen isotope effect in LaNi₅－H(D) system was investigated in the temperature range -70 °C to 3 °C. Isotherms of hydrogen isotopes were tested in an automated Sieverts apparatus with a thermostat, and pressure differences were studied between H₂ and D₂. Isotope exchange process was carried out and H₂－HD－D₂ equilibrium was judged by online chromatography, separation factors were calculated to investigate the thermodynamic isotope effect. In the H₂, D₂ isotherms, average time interval (0.5h) between each equilibrium point was relatively short due to high precision temperature control. From the plateau pressures at a constant hydrogen (or deuterium) concentration in LaNi₅ (2D or 2H atoms/mole of LaNi₅) the partial molar enthalpies, ⊿H, and entropies, ⊿S of formation were calculated. ⊿H and ⊿S kept good accordance with the values from references, which indicated good performance of the Sieverts apparatus. The isotope exchange equilibrium was reached within 1h. The results showed that ratio between H₂ pressure and D₂ pressure, r(H－D) = P(D₂)/P(H₂), dropped as temperature decreased and the inverse effect was found (r(H－D)<1.0) below -5 °C according to the absorption branch of isotherms, whereas the effect was estimated to appear below 60 °C from the desorption branch. The separation factor of the H₂－HD－D₂ mixture, decreased as temperature increased and the inverse effect happened below -20 °C. The inverse effect was affected by the isotherm hysteresis and HD existence.

Estimation of both the surface tritium and tritium in bulk in key materials of fusion reactors is of great importance for tritium safety and the management of tritium-contaminated material. For the further quantitative analysis of tritium in solids, the elaborate BIXS spectra were calculated based on Monte Carlo simulation. Four types of tritium depth profile were considered to evaluate the quantitative estimation method. It is found that the attenuation of X-rays in tungsten depends on both the energy of X-rays and the depth of X-rays. The evaluation of tritium amount in surface layer indicated that the intensity of Ar(Kα) peak could be used to evaluate the surface tritium within 400 nm from the surface in most cases and the deviations were less than 9% in the calculation. The intensity of both W(Lα) X-rays and the high energy X-rays can be employed to roughly estimate the total tritium amount. For linearly decreasing and exponentially decreasing distribution, the maximum calculation deviations were 24.9% and 28.8%, respectively. While for Gaussian distribution, the maximum deviations were 146% and 53%, respectively. And it can also be used for tritium estimation in other materials.

The motion of a single bubble in a 2-D vertical channel filled with stagnant or moving fluid is simulated for various sets of conditions using the computational fluid dynamics(CFD) method. The volume of fluid (VOF) model is applied to track the interface of the bubble. Corresponding results are compared with experimental and theoretical studies, and good agreement is achieved. Two-phase flows composed of various fluids are simulated to check the impact of physical properties on bubble dynamics, and buoyancy is found to have a key influence on the bubble rising behavior, including both the trajectory and the terminal rising velocity. The dimensionless number Bu is introduced to characterize this effect, and a larger Bu leads to a larger bubble rising velocity. Three other dimensionless numbers Eo, We, and Mo are introduced to study the bubble deformation which can be characterized by aspect ratio E, and an approximately linear relationship is found between E, Eo, and bubble size. Inertial force is proved to influence the bubble motion significantly in moving fluid, whose velocity dominates the bubble terminal speed. The simulation of motion for two bubbles is also performed and investigated detailedly.

Two-dimensional (2D) non-van der Waals hematene has increasingly attracted attention in the applications of photocatalysis, spintronic devices, and magnetic storage media. Mechanical properties as well as magnetic and electronic properties tuned via strain in two-dimensional non-van der Waals Fe-terminated (hematene-1) and O-terminated hematene (hematene-2) have been systematically studied using the first principles density functional theory. The stress-strain relationships for 2D hematene-1 and hematene-2 demonstrate the influence of biaxial strain is larger than that of uniaxial strain, and under uniaxial strain, the stress-strain relationships transform from the isotropic and linear elastic behavior under small strain to the anisotropic and nonlinear response under large strain. For hematene-1 under uniaxial and biaxial strain and hematene-2 under biaxial strain, the magnetic moments of Fe1 and Fe2 atoms increase monotonically with applied strain, while for hematene-2 under uniaxial strain, the magnetic moment of Fe1 atom first decreases and then increases with applied strain, and the case is opposite for Fe2 atom. For hematene-1, the band gap decreases with increasing strain from −10% to 7%, while for hematene-2 under uniaxial strain, the spin up band gap decreases with increasing strain, and the case is opposite for the spin down band gap. The present work not only provides a basic understanding of the mechanical properties of 2D hematene but also demonstrates its magnetic and electronic properties tuned by strain engineering for the potential possibility in both spintronic and catalytic applications.

Porous CeO₂ has been of great interest recently, due to its great catalytic efficiency in splitting CO₂ into CO and O₂. Porous CeO₂ microspheres are the key to scaling up such reactions in the fluidized-bed solar thermochemical reactors. In this paper, we report the processing and CO₂ splitting performance of porous ceria microspheres. To fabricate the porous ceria microspheres, homogenous cerium-based sol-gel precursors were synthesized from cerium acetate. The acrylamide (AM) was used as both solidification and pore-generation agent. The polymerization of acrylamide facilitated the conversion of liquid droplets to solid gel microspheres. The optimal AM content was around 20 wt%, ensuring both sufficient porosity and the integrity of the ceria microspheres after thermal decomposition. After heat treatment at 1500 °C for 1 h, porous CeO₂ microspheres were obtained, with an average diameter of ∼800 μm and surface area of 2.9 m²/g. These microspheres had high porosity and good sphericity. The CO₂ splitting performances of these microspheres were characterized using thermogravimetric analysis (TGA). During the thermal cycles between 1000 and 1400 °C, the O₂ yield of porous ceria microspheres was 49 μmol/g, and the CO yield was 88 μmol/g. Compared with porous ceria granules of particle size of ∼800 μm, the porous ceria microspheres exhibited higher O₂ and CO production yields. After thermochemical cycles, the microspheres were not sintered together, and the surface area was slightly reduced to 2.7 m²/g.

A method combining particle-induced X-ray emission spectrometry (PIXE) and elastic backscattering spectrometry (EBS) was used to calibrate the PIXE system at Peking University. The selected thin standard samples were typically 50 μg/cm² and the thicknesses of two thick standard samples are 3 mm. The calibration of PIXE system was divided into the calibration curve measurement of thin standard samples and the external standard method of thick standard samples. Both thin and thick samples cover the commonly used PIXE X-ray energy range 1.4–18.0 keV. GUPIXWIN introduces an Instrument factor H to describe the calibration process. It is found that the measured H(Z) curve can be approximated as a Z-independent line with an average value of $3.30\times {10}^{-4}$. The deviation of multiple measurements of a single element is between 0.42% and 4.75%. For thick samples, the incident particles are protons and ³He⁺, a H value of $\left(4.97\pm 0.01\right)\times {10}^{-4}$ was obtained.