The diffusivities of fission-created or thermally-doped tritium in UO2

The diffusion behavior of tritium in UO₂ was studied. Two methods were adopted for the introduction of tntium into UO₂: one via ternary fission of ²³⁵U and the other via thermal doping. In the former, the diffusion constants decreased with increase in sample weight. The diffusion constants obtained from the pellet with the same specification (9 mm in diameter, 5 mm high) were D″_bulk = 3.03 × 10⁻³(^+0.369_−0.003) exp[−163±43(kJ/mol)/RT](cm²/s) for fission-created tritium and D′_bulk = 0.15(^{+ 0.94}_−0.13) exp[−76±13 (kJ/mol)/RT](cm²/s) for thermally-doped tritium. The difference of the diffusion constants between two systems was discussed in terms of the effects associated with the recoil processes of energetic tritium.     

Release of tritium from boron carbide irradiated with reactor neutrons

The release of tritium from irradiated boron carbide in a pure Ar atmosphere was investigated between 500 and 900°C. The sintered B₄C samples with densities between 75 and 95% of the theoretical density were irradiated with reactor neutrons with total neutron doses up to 5 × 10²⁰/cm². Effective diffusion coefficients, D_eff, were derived from the release data using the model “diffusion out of a sphere”. D_eff decreases by about 3 orders of magnitude with increasing total neutron dose, levels off at about 10¹⁸n/cm² and increases at very high doses ( > 10²⁰ n/cm²). The decrease in the tritium mobility is attributed to the radiation defects formed in the B₄C. The activation energy of 210 ± 30 kJ/mol for the tritium diffusion in the irradiated B₄C is much higher than the value found for unirradiated material. D_eff depends also very strongly on the density of the sintered material.     

Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.     

A TEM study of D -implanted Cu following Ar-ion milling: Microstructure and epitaxial Cu2O formation

In examining the microstructure of TEM specimens prepared from D⁺-implanted Cu for the presence of bubbles it was found that cuprous oxide (Cu₂O) layers had formed over large areas of the specimen surfaces. The Cu was irradiated at normal incidence with 200 keV D⁺ ions at a temperature of 120 K to a dose of ˜2 × 10²¹ D⁺/m². Ar⁺ ion milling at 330 K was used to erode irradiated surfaces to various depths prior to chemical back-thinning in a jet electropolishing bath. There was no evidence for the formation in the Cu of bubbles of either deuterium or argon, but dislocations at high density and planar defects were evident. Lattice fringes from {110}, {111} and {200} planes in Cu₂O and moiré patterns formed by double diffraction in the Cu and overlaid Cu₂O film were obvious features in bright-field micrographs. The moiré patterns include examples of magnified images of lattice defects.     

Effects on channeling of radiation damage due to 28 GeV protons

For an irradiation of 2.9 × 10¹⁹ protons at a beam energy of 28 GeV, the channeling minimum yield in a silicon single crystal increased from 2.3% to 4.1%. The radiation damage occurred with a proton fluence of (4.1 ± 1.4) × 10²⁰/cm². The degradation was measured with MeV-range He ions using Rutherford backscattering. The relevance to bent crystal extraction of TeV beams is discussed.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Neutron irradiation effects in uranium monosulfide

Uranium monosulfide (US) was irradiated to investigate the effects of fission damage. Post-irradiation examinations were done by measuring the electrical resistivity, and partly the magnetic properties, at low temperature. The lattice parameter and the electrical resistivity measured at room temperature just after the irradiations showed an increase starting at a fission dose of 1 × 10¹⁶ fissions/cm³ and attaining a maximum at 3 × 10¹⁶ fissions/cm³. After that, a saturation of both increases persiste until 3 × 10¹⁷ fissions/cm³. The low-temperature electrical resistivity in the magnetic ordered state (ferromagnetic transition, T_c, at about 180 K) increased remarkably, while decreasing drastically in the magnetization, with increasing fission dose, apparently corresponding to the lattice expansion. In addition, the Curie point (T_c) shifted to lower temperatures with accumulating fission damage.     

Effect of tin on the irradiation growth of polycrystalline zirconium

Measurements of irradiation growth of polycrystalline Zr-1.5% Sn and Zr-0.1% Sn alloys at 353 K and 553 K have been made following fast neutron irradiation with fluences up to 3.1 × 10²⁵ n/m². At 353 K, growth of Zr-1.5% Sn virtually saturated at a strain of 4.5 × 10⁻⁴ after a fluence of ˜10²⁴ n/m². At this temperature, Zr-0.1% Sn continued to grów until ˜ 2 × 10²⁵ n/m², when the strain levelled off at ˜ 1.2×10⁻³. At 553 K, Zr-1.5% Sn initially grew about twice as fast as the 0.1% Sn alloy, but both eventually reached the same steady state rate of ˜ 2.4 × 10⁻²⁹ m²/n. Comparison of the data for the 1.5% Sn material with those for Zircaloy-2 from earlier work reveals that at 353 K, growth is suppressed by the presence of Sn atoms, which may serve as vacancy traps. However, at 553 K, minor additions and impurities in Zircaloy-2 (such as Fe, Ni, Cr and O) play an important role and cannot be neglected. The growth behaviour of Zr-0.1% Sn is similar to that of pure polycrystalline zirconium, especially at 353 K, indicating that the addition of Sn at this concentration does not strongly influence the growth of zirconium.     

Thermal recovery of defects in neutron irradiated UO2

Thermal recovery of both lattice and volume expansions of UO₂ irradiated to a dose of less than 9.97 × 10¹⁷ fissions/cm³ was studied in a temperature range from 200 to 1000°C. Two or three steps were observed in the recovery of lattice expansion. Volume expansion was also recovered in two or three steps. Recovery behavior of volume expansion of the specimen irradiated to 9.97 × 10¹⁷ fissions/cm³ was different from those irradiated to lower doses, and an abrupt volume increase was observed in the temperature range from 500 to 700 °C. This behavior might be related to the bubble swelling due to fission product gases. From the results on the recovery of lattice and volume expansions, the behavior of lattice point defects and defect clusters was discussed.     

Fast neutron irradiation of A-203 steel — recovery studies

A-203 steel has been irradiated by fast neutrons to fluences of 1 × 10²² and 1.25 × 10²² n/m². The isochronal annealing studies have been carried out from 333 to 913 K. The results show a recovery peak at ˜ 385 K, which has been explained to arise due to migration of carbon and carbon-vacancy complexes. After this stage the resistivity instead of decreasing starts increasing. This has been explained on the basis of increase of short range ordering on annealing. The influence of nickel and carbon on the recovery behaviour has also been discussed.     

The release of fission-recoiled xenon from Zr-2.5 wt% Nb alloy

The release of fission-recoiled ¹³³Xe from Zr-2.5 wt% Nb alloy was measured in the temperature range 640–1080 K. In the range 640–880 K, where purely phase exists, a linear relationship between log D versus 1/T is observed and can be represented by the equation: D(640–880 K) = 6.24 × 10⁻⁹exp(−142.7 kJmol/RT)m²/s. The release has been attributed to the non-volume diffusion process.

In the temperature range 930–1080 K where both and β phases coexist, the linearity in the plots of log D versus 1/T is violated.

The present values of the release parameters have been compared with the corresponding values for the release of fission-recoiled ¹³³Xe from Zircaloy-2. Alloying elements seem to have very small effect on the release kinetics. The results have been presented and discussed.     

Photoluminescence of Au formed in 12CaO · 7Al2O3 single crystal by Au-implantation

Au⁺ ion implantation with fluences from 1 × 10¹⁴ to 3 × 10¹⁶ cm⁻² into 12CaO · 7Al₂O₃ (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10¹⁵ cm⁻² exhibited photoluminescence (PL) bands peaking at 3.1 and 2.3 eV at 150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au⁻ ion having the electronic configuration of 6s², judged from their similarities to those reported on Au⁻ ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (2.3 × 10²¹ cm⁻³), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au⁺ implantation if an appropriate fluence is chosen.     

Iron-implanted sintered alumina studied by RBS, CEMS AND SEM techniques

Sintered plates of alumina have been implanted at room temperature with 1.2 × 10^{17 57}Fe⁺/cm² at 110 keV. Rutherford backscattering spectrometry and conversion electron Mössbauer spectrometry have been used to characterize respectively the depth distribution and charge states of iron. A theoretical approach of the as-implanted iron profile has been carried out by using a modified TRIM code which takes into account the fluence, the sputtering effects and the modification of the surface composition during ion bombardment. The iron profile and charge-state evolutions after isochronal air annealings from 200° C to 1600° C have been investigated. Correlations with the surface topography evolution investigated by SEM are proposed.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Low temperature fast-neutron and gamma irradiation of glass fiber-epoxy composite. Part 2: Structure and chemistry

Structure and chemistry of S2-glass fiber-epoxy composite to fast-neutron and gamma irradiation at 4.2 K have been studied. Fast-neutron fluence ranged from 0.9 × 10²² to 1 × 10²³ n/m². At the neutron fluence of 3.1 × 10²² n/m², ordering of the amorphous S2-glass structure intervened. Ordering was followed by partial crystallization at the neutron fluence of 1 × 10²³ n/m². Although the epoxy matrix retained its amorphous structure, reactor irradiation induced crosslink formation and hydrogen evolution. These processes became significant at neutron fluences equal to or higher than 1.8 × 10²² n/m². No evidence has been found for hydrogen bubble or void nucleation at the glass-epoxy interface at the neutron fluence of 3.1 × 10²² n/m². Futhermore, sharp compositional changes measured at the glass-epoxy interface preclude an extended irradiation-induced atom mixing at this neutron fluence.     

The thermal conductivity of UO2 vapor

The thermal conductivity, λ of a saturated vapor over UO_1.96 is calculated in the temperature range 3000–6000 K. The calculation shows that the contribution to λ from the transport of reaction enthalpy dominates all other contributions. All possible reactions of the gaseous species UO₃, UO₂, UO, U, O, and O₂ are included in the calculation. We fit the total thermal conductivity to the empirical equation λ = exp(a+ b/T+cT+dT² + eT³), with λ in cal/(cm s K), T in kelvins, a = 268.90, B = − 3.1919 × 10⁵, C = −8.9673 × 10⁻², d = 1.2861 × 10⁻⁵, and E = −6.7917 × 10⁻¹⁰.     

Defect evolution during ion bombardment of amorphous Si probed by in situ conductivity measurements

In this work we use in-situ conductivity measurements during ion irradiation as a sensitive probe of the defect structure of amorphous Si. Electronic transport in amorphous Si occurs by hopping at the high density ( 10²⁰ cm⁻³ eV⁻¹) of deep lying localized states introduced by the defects in the band gap. In-situ conductivity measurements allow to follow directly the defect generation and annihilation kinetics during and after ion bombardment of the material. Amorphous Si layers, patterned to perform conductivity measurements, were annealed at 500°C in order to reduce the defect density by about a factor of 5. Defects were subsequently reintroduced by high energy ion irradiation at different temperatures (77–300 K). During irradiation the conductivity of the layer increases by several orders of magnitude and eventually saturates. Turning off the beam results in a decrease of the conductivity by a factor of 2 in times as long as a few hours even at 77 K. The effects of different ions (He, C, Si, Cu, and Au) and different ion fluxes (10⁹–10¹² ions/cm² s) on these phenomena have been explored. These data give a hint on the mechanisms of defect production and annihilation and demonstrate a strong correlation between electrical and structural defects in amorphous silicon.     

Helium retention of vanadium alloy after energetic helium ion irradiation

Helium irradiation experiments of V–4Ti alloy were conducted in an ECR ion irradiation apparatus by using helium ions with energy of 5 keV. The ion fluence was in the range from 1 × 10¹⁷ He/cm² to 8 × 10¹⁷ He/cm². After the helium ion irradiation, the helium retention was examined by using a technique of thermal desorption spectroscopy (TDS). After the irradiation, the blisters with a size of about 0.1 μm were observed at the surface, and the blister density increased with the ion fluence. Two desorption peaks were observed at approximately 500 and 1200 K in the thermal desorption spectrum. When the ion fluence was low, the retained helium desorbed mainly at the higher temperature regime. As increase of the ion fluence, the desorption at the lower temperature peak increased and the retained amount of helium saturated. The saturated amount was approximately 2.5 × 10¹⁷ He/cm². This value was comparable with those of the other plasma facing materials such as graphite.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.