Production behavior of irradiation defects in vitreous silica under ion beam irradiation

The production behavior of irradiation defects in vitreous silica was studied by an in situ luminescence measurement technique under ion beam irradiation of H⁺ and He⁺. No apparent difference was observed in the luminescence spectra of specimens of different OH contents. The temperature dependence of the luminescence intensity at 280 and 460 nm was measured, and analyzed by considering the production mechanisms and kinetics of the irradiation defects of oxygen deficiency centers.     

Enthalpy and Gibbs energy of formation of cerium dicarbide

The equilibrium CO pressure over the condensed phase region of CeO₂(s)-CeC₂(s)-C(s) was determined by adopting a method termed as the dynamic effusion MS method, which involves the measurement of the CO effusing out from the sample using a quadrupole mass spectrometer, even during carbothermic reduction of the oxide. The formation of oxicarbide has been ruled out. The Gibbs energies of the reaction, CeO₂(s)+4C(s)=CeC₂(s)+2CO(g), at various temperatures in the range 1350-1550 K were then determined from the equilibrium CO pressures. From the Gibbs energies of the reaction, the Gibbs energy of formation of CeC₂(s) at 298 K was derived. Similarly, from the data on the second and third-law enthalpies of the above reaction, the enthalpy of formation of CeC₂(s) at 298 K was calculated. The recommended Gibbs energy and enthalpy of formation of CeC₂(s) at 298 K are (103.0±6.0) and (120.1±11.0) kJ mol^{− 1}, respectively.     

Ab initio formation energies of Fe-Cr alloys

We have calculated ab initio lattice parameters, formation energies, bulk moduli and magnetic moments of Fe-Cr alloys. The results agree well with available experimental data. In addition to body centered cubic (bcc) alloys, which are representative of ferritic steels used in fast neutron reactors, face centered cubic (fcc) and hexagonal close packed (hcp) phases were considered in order to complete a theoretical database of thermodynamic properties. Calculations were done for the ferromagnetic phase, as well as for a phase with local moment disorder, simulating the magnetic structure at high temperatures. For the latter case, the formation energy of the alloy is strictly positive smooth function of chromium concentration, in agreement with experiments performed at high temperature. In the ferromagnetic case, a negative mixing enthalpy is found for chromium concentrations below 6%. Our observation is consistent with the experimentally observed inversion of the ordering trend, as well as with formation of the chromium rich α^′ phase at Cr-concentrations above 9%, occurring at T<900 K.     

The preparation of fine-grain doped graphite and its properties

Fine-grain doped graphite was prepared by the ball-milling dispersion method for the first time. Such composite has not only high thermal conductivity and excellent bending strength (116 MPa), but also better oxidation resistance at elevated temperature and outgassing properties than those of composite doped with normal size carbides. Correlations between microstructure and properties of such composites are also discussed in detail.     

Laser annealing in combination with mass spectroscopy, a technique to study deuterium on tokamak carbon samples, a tool for detritiation

In this study, a method is presented based on mass spectroscopy to measure the areal density of deuterium on a graphite surface exposed to tokamak discharges. The studied sample was cut from a bumper limiter exposed in the TEXTOR tokamak and annealed by a 1 J Excimer laser (KrF). The energy used was 400 mJ cm⁻², which is below the threshold for ablation, 1 J cm⁻². The release of HD and D₂ was measured by a mass spectroscopy set-up and no other species released from the sample were detected in this experiment. The amount of D released from the sample after 20 laser pulses was measured to 7 × 10¹⁶ D atoms per cm⁻² (for this particular sample) and most of the hydrogen at the surface was released in the first pulse, as checked by nuclear reaction analysis (NRA) techniques, which gave changes of the amount of deuterium before and after laser annealing. The sensitivity in this experiment was 5 × 10¹⁴ atoms per cm⁻² for HD and 5 × 10¹³ atoms per cm⁻² for D₂.     

Sputtering and ion-induced electron emission of graphite under high-dose nitrogen bombardment

The dependence of the sputtering yield Y and the electron emission coefficient γ of isotropic graphites (POCO-AXF-5Q and Russian MPG-LT) on ion fluence and ion incidence angle θ at near room temperatures and the dependence of γ on target temperature under high dose 30 keV N₂⁺ ion irradiation were measured. It was found that Y and γ are stabilized at fluences F?1×10¹⁹ N/cm². A specific target surface topography develops. At steady-state conditions, the N concentration in MPG-LT is 19 at.% and in POCO16 at.%. In the angular range θ=0-80°, Y and γ increase and the angular dependence of Y is slightly stronger than that of γ. Sputtering yields of POCO are 1.5 times higher than those of MPG-LT. The reasons of the difference between the experimental and calculated sputtering yields using the TRIM.SP code are discussed. The dependence of γ on the target temperature manifests a step-like increase at ?250 °C which may be due to radiation induced structure transformation in the modified surface layer.     

Photoluminescence study of swift heavy ion (SHI) induced defect centers in sapphire

Single crystals of sapphire (Al₂O₃: Fe, Ti, Cr) were irradiated at room temperature with different fluence of 100 MeV Ni ions. Photoluminescence (PL) spectra of pristine and irradiated sapphires were recorded at room temperature under 2.8 eV blue excitation. A broad emission band consists of two bands centered at 516 nm corresponding to F₂ defect center and 546 nm corresponding to defect center was observed. The intensity of these defect centers was found to vary with the fluence. defect center develops at low fluence reaching maximum at 5 × 10¹⁶ ions/m² and finally decreasing at higher fluence. The behavior is interpreted in terms of creation of defect centers, their clustering and annihilation.     

Detection of point defects upon ion irradiation by means of precipitate coherency

Transmission electron microscopy has been widely used to investigate the radiation-damage microstructures, but has limitations when observing point defects due to its resolution limit. In the present study, a dilute Cu-Co alloy, which is a typical precipitation-hardened alloy, has been selected to investigate the formation and the mobility of point defects upon ion irradiation, during which the coherent precipitates lose their coherency and exhibit incoherent strain contrast, and the coherency loss can be used to detect the point defects.     

Enthalpy and Gibbs energy of formation of dysprosium dicarbide

The equilibrium CO(g) pressures have been determined over the univariant three-phase field DyO_1.5(s)-C(s)-DyC₂(s) by means of the dynamic effusion MS method, in the temperature range 1360-1590 K. The carbide phase was generated in situ by the carbothermic reduction of the oxide using a quadrupole mass spectrometer, the effusion pressure of carbon monoxide was monitored. The enthalpy of the carbothermic reduction reaction was calculated by second- and third-law methods. By combining the appropriate thermal functions and enthalpies of other constituents from standard thermodynamic tables, the enthalpy of formation of the dicarbide at 298 K has been calculated. The Gibbs energy of formation of the dicarbide in this temperature range has also been calculated. The recommended values of enthalpy and Gibbs energy of formation of DyC₂(s) at 298 K are −(131 ± 19) kJ mol⁻¹ and −(165 ± 6) kJ mol⁻¹, respectively.     

Velocity distribution of helium and neon atoms released from graphite and tungsten limiters in TEXTOR

The release mechanisms of noble gases from plasma-facing components were observed spectroscopically in the TEXTOR plasma boundary by determining the velocity distribution from Doppler broadening. For the first time, three different mechanisms for helium and neon release from graphite and tungsten limiters could be distinguished quantitatively in a tokamak: thermal desorption, ion-induced desorption and particle reflection. Under the assumption that the thermal desorption follows a Maxwellian velocity distribution, the ion-induced desorption can be expressed by a Thompson velocity distribution. Calculating the particle reflection by the Monte-Carlo code TRIM, these processes could be separated in the measured velocity distribution.     

Heats of formation of (U,Mo)Al3 and U(Al,Si)3

The heats of formation of (U,Mo)Al₃ intermetallic compounds were obtained by measuring the reaction heats of U-Mo/Al dispersion samples by differential scanning calorimetry. Based on literature data for the reaction heats of U₃Si/Al and U₃Si₂/Al dispersion samples, the heats of formation of U(Al,Si)₃ as a function of the Si content were calculated. The heat of formation of (U,Mo)Al₃ becomes less negative as the Mo content increases. Conversely, the heat of formation of U(Al,Si)₃ becomes more negative with increasing Si content.     

The standard molar enthalpy of formation of CdMoO4

The molar enthalpies of solution of CdMoO₄(s), CdO(s), Na₂ MoO₄(s) and NaF(s) in (10 mol HF(aq) + 4.41 mol H₂O₂(aq)) dm⁻³ have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of CdMoO₄(s) has been calculated to be Δ_fH°(298.15 K) = −(1034.3 ± 5.7) kJ mol⁻¹. This value of enthalpy of formation of CdMoO₄(s) agrees well with the estimated enthalpy of formation of this compound. There is no other report on the thermodynamic property measurements on this compound.     

Experimental investigation of the Zr corner of the ternary Zr-Nb-Fe phase diagram

Intermediate phases in the Zr-rich region of the Zr-Nb-Fe system have been investigated by X-ray diffraction, optical and electron microscopy and electron microprobe analysis. The chemical composition ranges covered by the alloys studied here are: (41-97) at.% Zr, (32-0.9) at.% Nb and (0.6-38) at.% Fe. The phases found in this region were: the solid solutions α-Zr and β-Zr, the intermetallic Zr₃Fe with less than 0.2 at.% Nb in solution, two new ternary intermetallic compounds (Zr+Nb)₂Fe `λ<sub>1</sub>' with a cubic Ti<sub>2</sub>Ni-type structure in the composition range (2.4-13) at.% Nb and (31-33) at.% Fe, and (Fe+Nb)<sub>2</sub>Zr `λ₂' indexed as hexagonal Laves phase MgZn₂ type (C14) with a wide range of compositions close to (35-37) at.% Zr, (12-31) at.% Nb and (32-53) at.% Fe.     

Channelling studies on the lattice location of B atoms in graphite

In order to obtain information on the lattice location of B atoms in graphite, channelling experiments have been performed at room temperature with a proton beam of an energy of 0.65-0.77 MeV for the 〈0 0 0 1〉 axial channel in highly oriented pyrolytic graphite (HOPG) crystals doped with 0.32 at.% B. The B atoms are detected by measuring α-particles which are emitted as a result of a nuclear reaction ¹¹B(p,α)αα. It is clearly demonstrated that most of B atoms are shadowed behind the 〈0 0 0 1〉 C atomic rows. Taking account of the already reported experimental results on a change of lattice parameters by B-doping, it is concluded that most of B atoms are located at substitutional sites. It is also observed that B-doping introduces lattice strain on the c-plane. In addition, the presence of a small portion of interstitial B atoms is suggested.     

Dissolution of magnetite using an environmentally friendly chelant: an electrochemical impedance spectroscopy study

Ethylenediaminedisuccinic acid (EDDS), a biodegradable substitute for EDTA, was used as a chelant for dissolving magnetite and magnetite formed on iron metal surface. Dissolution was found to increase in presence of ferrous ions and depend on pH of solution, concentration of ferrous ion, EDDS concentration, applied cathodic potential and temperature. The impedance spectrum for the dissolution of magnetite film formed on iron exhibited two time constants. The first at high frequency range is ascribed to the reductive dissolution of magnetite. It is very crucial to carry out the dissolution process at the appropriate temperature to insure complete removal of oxide layer.     

Steam oxidation of Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr and Zircaloy-4 at 900-1200 °C

The steam oxidation characteristics for the Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr (HANA-4) and Zircaloy-4 claddings were elucidated at LOCA temperatures of 900-1200 °C by using a modified thermo-gravimetric analyzer. After the oxidation tests, the oxidation behaviors, oxidation rates, surface appearances, and microstructures of the as-received, as-oxidized, and burn-up simulated claddings were evaluated in this study. The high-temperature oxidation resistance of the as-received HANA-4 cladding was superior to that of the Zircaloy-4. The superior oxidation resistance of the HANA-4 cladding could be attributed to the higher Nb and the lower Sn within its cladding. The pre-oxidized layer formed at the low temperatures below 500 °C could retard the oxidation rate at the high temperatures above 900 °C. And the soundness of the pre-oxidized layer formed at a lower temperature could influence the oxidation kinetics and the rate constants during a steam oxidation at LOCA temperatures from 900 to 1200 °C.     

Comparison of binding energy to the sorption enthalpy of radionuclides with trap materials used in LMFBRs

In liquid metal fast breeder reactors (LMFBR), traps are provided in the primary coolant circuit to reduce the contamination due to the deposition of long lived γ-emitting nuclides. The binding energies of the radionuclides with iron and nickel were estimated using Pauling’s electronegativity. The results are comparable to the sorption enthalpies derived from the experimental isotherms.     

Thermodynamic considerations and prediction of the primary coolant activity of Tc

A physical model has been developed to describe the coolant activity behaviour of ⁹⁹Tc, during constant and reactor shutdown operations. This analysis accounts for the fission production of technetium and molybdenum, in which their chemical form and volatility is determined by a thermodynamic treatment using Gibbs-energy minimization. The release kinetics are calculated according to the rate-controlling step of diffusional transport in the fuel matrix and vaporization from the fuel-grain surface. Based on several in-reactor tests with defective fuel elements, and as supported by the thermodynamic analysis, the model accounts for the washout of molybdenum from the defective fuel on reactor shutdown. The model also considers the recoil release of both ⁹⁹Mo and ⁹⁹Tc from uranium contamination, as well as a corrosion source due to activation of ⁹⁸Mo. The model has provided an estimate of the activity ratio ⁹⁹Tc/¹³⁷Cs in the ion-exchange columns of the Darlington Nuclear Generating Station, i.e., 6 × 10⁻⁶ (following ∼200 days of steady reactor operation) and 4 × 10⁻⁶ (with reactor shutdown). These results are consistent with that measured by the Battelle Pacific Northwest Laboratories with a mixed-bed resin-sampling device installed in a number of Pressurized Water Reactor and Boiling Water Reactor plants.     

Experimental study on the reaction between nuclear graphite IG-110 and carbon dioxide

With nuclear graphite IG-110, we measured various kinetic parameters and reaction rates of the C/CO₂ reaction. As a result, its activation energy is 295 ± 8 kJ/mol and the order of reaction is 0.9. It turns out that the rate of C/CO₂ reaction is much smaller than the rate of the C/O₂ reaction which is dominant in HTGR air-ingress below 1400 °C. Finally, we propose the following rate equation for the C/CO₂ reaction of IG-110: