Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

Oxidation kinetics and oxygen diffusion in low-tin Zircaloy-4 up to 1523 K

This paper deals with the study of oxidation kinetics and the identification of oxygen diffusion coefficients of low-tin Zy-4 alloy at intermediate (973 K ? T ? 1123 K) and high temperatures (T ? 1373 K). Two different cases were considered: dissolution of a pre-existing oxide layer in the temperature range 973 K ? T ? 1123 K and oxidation at T ? 1373 K. The results are the following ones: in the temperature range 973-1123 K, the oxygen diffusion coefficient in α_Zr phase can be expressed as D_α = 6.798 exp(−217.99 kJ/RT) cm²/s. In the temperature range 1373-1523 K, the oxygen diffusion coefficients in α_Zr, β_Zr and ZrO₂, were determined using an ‘inverse identification method’ from experimental high temperature oxidation data (i.e., ZrO₂, and α_Zr(O) layer thickness measurements); they can be expressed as follows: D_α = 1.543 exp(−201.55 kJ/ RT) cm²/s, D_β = 0.0068 exp(−102.62 kJ/ RT) cm²/s and D_ZrO2=0.115exp(−143.64kJ/RT)cm²/s. Finally an oxygen diffusion coefficient in α_Zr in the temperature range 973 K ? T ? 1523 K was determined, by combining the whole set of results: D_α = 4.604exp(−214.44 kJ/RT) cm²/s. In order to check these calculated diffusion coefficients, oxygen concentration profiles were determined by Electron Probe MicroAnalysis (EPMA) in pre-oxidized low-tin Zy4 alloys annealed under vacuum at three different temperatures 973, 1073 and 1123 K for different times, and compared to the calculated profiles. At last, in the framework of this study, it appeared also necessary to reassess the Zr-O binary phase diagram in order to take into account the existence of a composition range in the two zirconia phases, α_ZrO2 and β_ZrO2.     

Radiation damage in ZnO ion implanted at 15 K

Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 × 10¹¹ to 7 × 10¹⁶ cm⁻². The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms.Four stages of the damage evolution can be identified. At low ion fluences up to about 2 × 10¹³ cm⁻² the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E_d(Zn) = 65 eV and E_d(O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 × 10¹⁵ cm⁻² a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10¹⁶ cm⁻² the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.     

Ion-induced electron emission monitoring of the structure and morphology evolution in HOPG

The temperature dependences of the ion-induced electron emission yield γ of highly-oriented pyrolytic graphite (HOPG) under high-fluence (10¹⁸-10¹⁹ ions/cm²) 30 keV Ar⁺ ion irradiation at ion incidence angles from θ = 0^o (normal incidence) to 80^o have been measured to trace both the structure and morphology changes in the basal oriented samples. The target temperature has been varied during continuous irradiation from T = −180 to 400 ^oC. The surface analysis has been performed by the RHEED and SEM techniques. The surface microgeometry was studied using laser goniophotometry (LGF). The dependences of γ(T) were found to be strongly non-monotonic and essentially different from the ones for Ar⁺ and N₂⁺ ion irradiation of the polygranular graphites. A sharp peak at irradiation temperature T_p ≈ 150 ^oC was found. A strong influence of electron transport anisotropy has been observed, and ion-induced microgeometry is discussed.     

Formation of silicon hydride using hyperthermal negative hydrogen ions (H) extracted from an argon-seeded hydrogen sheet plasma source

An E × B probe (a modified Wien filter) is constructed to function both as a mass spectrometer and ion implanter. The device, given the acronym EXBII selects negative hydrogen ions (H⁻) from a premixed 10% argon-seeded hydrogen sheet plasma. With a vacuum background of 1.0 × 10⁻⁶ Torr, H⁻ extraction ensues at a total gas feed of 1.8 mTorr, 0.5 A plasma discharge. The EXBII is positioned 3 cm distance from the sheet core as this is the region densely populated by cold electrons (T_e ∼ 2 eV, N_e ∼ 3.4 × 10¹¹ cm⁻³) best suited for H⁻ formation. The extracted H⁻ ions of flux density ∼0.26 A/m² are segregated, accelerated to hyperthermal range (<100 eV) and subsequently deposited into a palladium-coated 1.1 × 1.1 cm², n-type Si (1 0 0) substrate held at the rear end of the EXBII, placed in lieu of its Faraday cup. The palladium membrane plays the role of a catalyst initiating the reaction between Si atoms and H⁻ ions simultaneously capping the sample from oxidation and other undesirable adsorbents. AFM and FTIR characterization tests confirm the formation of SiH₂. Absorbance peaks between 900-970 cm⁻¹ (bending modes) and 2050-2260 cm⁻¹ (stretching modes) are observed in the FTIR spectra of the processed samples. It is found that varying hydrogen exposure time results in the shifting of wavenumbers which may be interpreted as changes in the frequencies of vibration for SiH₂. These are manifestations of chemical changes accompanying alterations in the force constant of the molecule. The sample with longer exposure time exhibits an additional peak at 2036 cm⁻¹ which are hydrides of nano-crystalline silicon.     

Oxygen potential and thermal conductivity of (U, Pu) mixed oxides

(U, Pu) mixed oxides, (U_1−yPu_y)O_2−x, with y = 0.21 and 0.28 are being considered as fuels for the Prototype Fast Breeder Reactor (PFBR) in India. The use of urania-plutonia solid solutions in PFBR calls for accurate measurement of physicochemical properties of these materials. Hence, in the present study, oxygen potentials of (U_1−yPu_y)O_2−x, with y = 0.21 and 0.28 were measured over the temperature range 1073-1473 K covering an oxygen potential range of −550 to −300 kJ mol⁻¹ (O/M ratio from 1.96 to 2.000) by employing a H₂/H₂O gas equilibration technique followed by solid electrolyte EMFmeasurement. (U_1−yPu_y)O_2−x, with y = 0.40 is being used in the Fast Breeder Test Reactor (FBTR) in India to test the behaviour of fuels with high plutonium content. However, data on the oxygen potential as well as thermal conductivity of the mixed oxides with high plutonium content are scanty. Hence, the thermal diffusivity of (U_1−yPu_y)O₂, with y = 0.21, 0.28 and 0.40 was measured and the results of the measurements are reported.     

Determination of plasma parameters in rf glow-discharge plasma

The Langmuir probe measurements were carried out in a planar rf discharge of N₂ and Ar gases. The dc characteristic I-U curves are calculated from the measured rf characteristic I-U curves for frequencies 10-60 MHz. The measured parameters such as electron temperature and electron density are compared with the simulated results. At gas pressures 10-40 Pa, the sheath thickness at the powered electrode was proportional to f^−0.5 for simulation and f^−2/3 by using the electrical parameters of the probe measurements and to p^−0.5 for both cases.     

Corrosion behaviour of steels in lead-bismuth at 823 K

The corrosion behaviour of the martensitic T91 steel and the austenitic AISI 316L steel was analysed. The steels were immersed in stagnant molten Pb-55.2wt%Bi alloy at 823 K for different exposure times (t = 550-2000 h). The corrosion tests were carried out both under Ar and under Ar-5%H₂ mixture. Under the oxidising conditions (P_O2 = 6 × 10⁻³ Pa), the formation of oxide layers was observed which prevent the penetration of the liquid alloy into the matrix, while under the Ar-5%H₂ mixture (P_O2 = 3.2 × 10⁻²³ Pa), two phenomena occurred: a ‘reactive penetration’ at the liquid alloy/steel interface and the competition between oxidation and penetration.     

Thermal conductivity of (U,Pu,Np)O2 solid solutions

The thermal conductivities of (U_,Pu_,Np)O₂ solid solutions were studied at temperatures from 900 to 1770 K. Thermal conductivities were obtained from the thermal diffusivity measured by the laser flash method. The thermal conductivities obtained below 1400 K were analyzed with the data of (U,Pu,Am)O₂ obtained previously, assuming that the B-value was constant, and could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(z₁, z₂) = 3.583 × 10⁻¹ × z₁ + 6.317 × 10⁻² × z₂ + 1.595 × 10⁻² (m K/W) and B = 2.493 × 10⁻⁴ (m/W), where z₁ and z₂ are the contents of Am- and Np-oxides. It was found that the A-values increased linearly with increasing Np- and Am-oxide contents slightly, and the effect of Np-oxide content on A-values was smaller than that of Am-oxide content. The results obtained from the theoretical calculation based on the classical phonon transport model showed good agreement with the experimental results.     

Determination of bulk etch rate for CR-39 nuclear track detectors using an X-ray fluorescence method

The thickness of a CR-39 detector is determined using an energy dispersive X-ray fluorescence (EDXRF) method of analysis. The method is based on exciting a suitable target and measuring the intensity of its fluorescence X-ray lines passing through the CR-39 sample in a fixed geometry. By properly selecting the target material, the method succeeds in assessing the thickness change of CR-39 detectors etched for different time intervals. The bulk etch rate (V_b) may thus be obtained, which is an important parameter for any solid state nuclear track detector. Application of the EDXRF method yielded a value of V_b = (2.01 ± 0.04) μm h⁻¹ for etching in a 6 N NaOH solution at 75 °C. This value agrees with the bulk etch rate of (1.90 ± 0.03) μm h⁻¹, obtained by the conventional mass-change method.     

Effect of temperature on studtite stability: Thermogravimetry and differential scanning calorimetry investigations

The main objective of this work is the study of the influence of temperature on the stability of the uranyl peroxide tetrahydrate (UO₂O₂ · 4H₂O) studtite, which may form on the spent nuclear fuel surface as a secondary solid phase. Preliminary results on the synthesis of studtite in the laboratory at different temperatures have shown that the solid phases formed when mixing hydrogen peroxide and uranyl nitrate depends on temperature. Studtite is obtained at 298 K, meta-studtite (UO₂O₂ · 2H₂O) at 373 K, and meta-schoepite (UO₃ · nH₂O, with n < 2) at 423 K. Because of the temperature effect on the stability of uranyl peroxides, a thermogravimetric (TG) study of studtite has been performed. The main results obtained are that three transformations occur depending on temperature. At 403 K, studtite transforms to meta-studtite, at 504 K, meta-studtite transforms to meta-schoepite, and, finally, at 840 K, meta-schoepite transforms to U₃O₈. By means of the differential scanning calorimetry the molar enthalpies of the transformations occurring at 403 and 504 K have been determined to be −42 ± 10 and −46 ± 2 kJ mol⁻¹, respectively.     

Secondary ion emission from Cu(1 0 0) surfaces with atomic adsorbates (N, O, Cl, S and Br)

Several targets that consist of atomic species X (X = N, O, Cl, S, Br) adsorbed at hollow sites on the Cu(1 0 0) surface have been examined with low-fluence secondary ion mass spectrometry (SIMS). The positive and negative secondary ion (SI) abundance distributions, which show a range of characteristics, have been discussed with the aid of thermochemical data derived from ab initio calculations. In positive SIMS, CuX⁺ is never observed, while the only heteronuclear (mixed-atom) SI that is observed for all five systems is Cu₂X⁺. In negative SIMS, the dominant heteronuclear species for all systems is , except for N/Cu(1 0 0), which produces no , ions. Cu⁻ emission is observed only for O/Cu(1 0 0). By analogy with results from laser ablation studies of O/Cu targets, it is conjectured that Cu⁻ is a daughter product of the gas-phase dissociation of polyatomic Cu-O anion clusters.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.     

Swelling and optical properties of Si3N4 films irradiated in the electronic regime

Silicon nitride layers of 140 nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD) and irradiated at GANIL with Pb ions of 110 MeV up to a maximum fluence of 4 × 10¹³ cm⁻². As shown in a previous work these irradiation conditions, characterized by a predominant electronic slowing-down (S_e = 19.3 keV nm⁻¹), lead to damage creation and formation of etchable tracks in Si₃N₄. In the present study we investigated other radiation-induced effects like out of plane swelling and refractive index decrease. From profilometry, step heights as large as 50 nm were measured for samples irradiated at the highest fluences (>10¹³ cm⁻²). From optical spectroscopy, the minimum reflectivity of the target is shifted towards the high wavelengths at increasing fluences. These results evidence a concomitant decrease of density and refractive index in irradiated Si₃N₄. Additional measurements, performed by ellipsometry, are in full agreement with this interpretation.     

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).     

Photostimulated luminescence studies of γ-irradiated KBr1−xIx:TlI mixed crystals

Results on photoluminescence and photostimulated luminescence studies of KBr_1−xI_x:TlI (0.01 and 0.05 mol%) mixed crystals grown in vacuum and air are presented. Photoluminescence spectra of the mixed crystals excited in the A-absorption band of KBr:Tl⁺ exhibited the characteristic emission bands of Tl⁺ ions in KBr:Tl⁺. When excited in the low energy tail of A-band absorption, additional emission bands were observed. Compared with earlier reports, excitation bands observed around 4.8, 4.6 and 4.4 eV are attributed to complex thallium centres of the form TlBr_6−nI_n (n = 0, 1, 2, 3). Photostimulated luminescence of γ-irradiated KBr_1−xI_x:TlI mixed crystals showed the presence of emission bands similar to the characteristic photoluminescence of Tl⁺ ions. The photostimulated luminescence slightly shifted towards the low energy side with increasing iodide composition x. The mechanism of emission in these mixed crystals is discussed. Dose response and storage stability (fading characteristics) in these mixed crystals are reported.     

Synchrotron X-ray induced damage in polymer (PS) thin films

Thin polystyrene (PS) films (M_w = 234,000) are spin coated on silicon substrates with a Chromium (Cr) layer as a sandwiched metallic layer that produces photoelectrons (by synchrotron X-rays). Earlier studies on synchrotron radiation damage in PS films, without metallic layer, have shown a decrease in interfacial roughness and a slight increase in thickness, at temperatures below T_g [A.G. Richter, R. Guico, K. Shull, J. Wang, Macromolecules 39 (2006) 1545]. Similar trend is observed in the presence of a thin layer of Cr film (∼2.5 nm). For the sample with a thick Cr layer the opposite effect was observed for X-ray radiation damage. For the 50 nm thick Cr film system thickness of the polystyrene film decreased by ≈4.4% which amount to a loss of about 0.021 nm³ per incident photon in the fluence range studied (6.8 × 10⁹ photons mm⁻² to 1 × 10¹⁴ photons mm⁻²). Interfacial roughness also increased from about 1.0 nm to 2.1 nm in the process. These effects are explained by invoking the presence of more number of X-ray induced photoelectrons and secondary electrons for 50 nm thick Cr film case compared to 2.5 nm thin film case.     

Existence states of deuterium irradiated into LiAlO2

The existence states of deuterium in LiAlO₂ were analyzed by in situ IR absorption spectroscopy during irradiation with 3 keV at room temperature. Multiple IR absorption peaks that were related to O-D stretching vibrations were observed, mainly at 2650 cm⁻¹ (O-D_α), 2600 cm⁻¹ (O-D_β), and 2500 cm⁻¹ (O-D_γ). The O-D_α was assigned to the surface O-D. The O-D_β and O-D_γ were interpreted as two distinct O-D states for three candidates: O-D of substitutional D⁺ for Li⁺; O-D of substitutional D⁺ for Al³⁺; and O-D of interstitial D⁺. O-D_β was the dominant O-D state for deuterium irradiated into LiAlO₂, and had higher stability than O-D_γ. Heating after ion irradiation led to the desorption of D₂ and an increase in the intensity of O-D_β, which implies that some of the deuterium irradiated into LiAlO₂ exists in non-O-D states, such as D⁻ captured by F centers.     

Oxygen potential measurements of Am0.5Pu0.5O2−x by EMF method

The dependence of the oxygen potentials on oxygen non-stoichiometry and temperature of Am_0.5Pu_0.5O_2−x has been obtained by the electromotive force (EMF) method with the cell: (Pt) air |Zr(Ca)O_2−x| Am_0.5Pu_0.5O_2−x (Pt). The x value of Am_0.5Pu_0.5O_2−x was changed at 1333 K over 0.02 < x ? 0.25 by the coulomb titration method. The temperature dependence of the oxygen potential was also measured over the range of 1173-1333 K. It was found that the oxygen potential decreased from −80 to −360 kJ mol⁻¹ with increasing x from 0.021 to 0.22 at 1333 K and that it remained almost constant at −360 kJmol⁻¹ around x = 0.23. It was concluded that Am_0.5Pu_0.5O_2−x should be composed of the single fluorite-type phase over 0.02 < x ? 0.22 and the mixed phases of fluorite-type and (Am, Pu)₉O₁₆ at around x = 0.23.     

Angular dependence of electronic sputtering from HOPG

We have studied the angular distribution of 120 MeV Au ion beam induced sputtering yield for three cases: from crystalline highly oriented pyrolytic graphite (HOPG) for (A) normal and (B) 70° incidence and from (C) amorphous carbon sample for normal incidence. An anisotropic distribution of sputtering is observed for HOPG samples studied with a distribution Y = Acosⁿθ + Bexp[−(θ − μ)²σ²]. Though the over-cosine function dependence is observed for all the cases, the anomalous peak observed at 53° for normal incidence for HOPG sample is found to shift to 73° when the sample is tilted by 20°. No peak is observed in the amorphous carbon sample which further confirms that the anisotropy observed is due to the crystal structure and formation of a pressure pulse. The high exponent of over-cosine distribution of sputtering yield (n = 3.2-3.8) signifies formation of intense pressure pulse induced jet like sputtering.