Level 1 probabilistic safety assessment to support the design of the CEA 2400 MWth gas-cooled fast reactor

As part of the design of a 4th generation reactor, the integration of safety in the early phase of the concepts is expected. To date, probabilistic insights are increasingly employed in the safety demonstration in combination with the deterministic approach (e.g. to identify the sequences of complex failures, to justify the categorization of situations) and used, even at an early stage of design, to identify the reliability of systems and equipment to handle the safety objectives (expressed in terms of core damage frequency targets). Within this frame, the CEA has undertaken to assess the benefit of developing a probabilistic model to support the design of the 2400 MWth gas-cooled fast reactor.In the building process of this level 1 probabilistic safety assessment, a first phase consisted in making a preliminary model that took only into account families of initiating events that were defined for the design of the decay heat removal dedicated loops, namely the loss of coolant accidents (representative of medium pressure situations) and loss of off-site power/station black-out transients (representative of high pressure situations). Owing to the results obtained with this preliminary L1PSA model, it emerged that an increased reliability of the DHR function in high pressure conditions (i.e. characterized by IEs not associated to the loss of integrity of the helium pressure boundary) is suitable to reduce the overall core damage frequency. The track was therefore chosen to require the use of normal loops as first line of provision of the DHR function, possibly including components or particular operating modes related to the secondary and tertiary circuits. In addition, this final L1PSA model is characterized by success criteria based on transient calculations performed with the CATHARE2 code and to a perimeter extended to all representative internal IEs at full operating power.This paper presents the building process and the main results related to these two successive L1PSA models. Finally, useful insights are translated into GFR design improvements that are leading to an overall CDF at full operating power that satisfies nowadays with the probabilistic target defined for 3rd generation reactors, being at least the objective for 4th generation reactors.     

Development and characterization of the control assembly system for the large 2400 MWth Generation IV gas-cooled fast reactor

The present paper is related to the design and neutronic characterization of the principal control assembly system for the reference large (2400 MWth) Generation IV gas-cooled fast reactor (GFR), which makes use of ceramic–ceramic (CERCER) plate-type fuel-elements with (U–Pu) carbide fuel contained within a SiC inert matrix. For the neutronic calculations, the deterministic code system ERANOS-2.0 has been used, in association with a full core model including a European fast reactor (EFR)-type pattern for the control assemblies as a starting point. More specifically, the core contains a total of 33 control (control system device: CSD) and safety (diverse safety device: DSD) assemblies implemented in three banks. In the design of the new control assembly system, particular attention was given to the heat generation within the assemblies, so that both neutronic and thermal–hydraulic constraints could be appropriately accounted for. The thermal–hydraulic calculations have been performed with the code COPERNIC, significant coolant mass flow rates being found necessary to maintain acceptable cladding temperatures of the absorber pins.     

Temperature dependence of lattice disorder in Ar-irradiated (1 0 0), (1 1 0) and (1 1 1) MgO single crystals

To better appreciate dynamic annealing processes in ion irradiated MgO single crystals of three low-index crystallographic orientations, lattice damage variation with irradiation temperature was investigated. Irradiations were performed with 100 keV Ar ions to a fluence of 1 × 10¹⁵ Ar/cm² in a temperature interval from −150 to 1100 °C. Rutherford backscattering spectroscopy combined with ion channeling was used to analyze lattice damage. Damage recovery with increasing irradiation temperature proceeded via two characteristic stages separated by 200 °C. Strong radiation damage anisotropy was observed at temperatures below 200 °C, with (1 1 0) MgO being the most radiation damage tolerant. Above 200 °C damage recovery was isotropic and almost complete recovery was reached at 1100 °C. We attributed this orientation dependence to a variation of dynamic annealing mechanisms with irradiation temperature.     

Computer simulation of CAICISS spectra of clean Cu(1 0 0) surface and Cu(1 0 0) surface by Pt deposition

The clean Cu(1 0 0) surface and Pt/Cu(1 0 0) surface by Pt deposition at room temperature have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the ACOCT program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3 keV He⁺ ions incident along the 〈1 0 0〉 and 〈1 1 0〉 azimuths of the clean Cu(1 0 0) and Pt/Cu(1 0 0) surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Cu(1 0 0) surface are relatively well reproduced by the ACOCT simulations including the inward relaxation of 1.2% in the first interlayer spacing and the outward relaxation of 1.6% in the second interlayer spacing, and that the ACOCT simulations for the Pt deposition with coverages of 2.35 ML and 2.75 ML on the Cu(1 0 0) surface appear the concentrations of 0.24 ML of Pt sitting 2.3 Å and 0.25 ML of Pt sitting 2.5 Å above the outermost atomic layer, respectively.     

Computer simulation of sputtering at the low index (1 0 0), (1 1 0) and (1 1 1) surfaces of Ni3Al in a STEM

The present study is relevant to the preferential Al sputtering and/or enhancement of the Ni/Al ratio in Ni₃Al observed by the scanning transmission electron microscopy fitted with a field emission gun (FEG STEM). Atomic recoil events at the low index (1 0 0), (1 1 0) and (1 1 1) surfaces of Ni₃Al through elastic collisions between electrons and atoms are simulated using molecular dynamics (MD) methods. The threshold energy for sputtering, E_sp, and adatom creation, E_ad, are determined as a function of recoil direction. Based on the MD determined E_sp, the sputtering cross-sections for Ni and Al atoms in these surfaces are calculated with the previous proposed model. It is found that the sputtering cross-section for Al atoms is about 7-8 times higher than that for Ni, indicating the preferential sputtering of Al in Ni₃Al, in good agreement with experiments. It is also found that the sputtering cross-sections for Ni atoms are almost the same in these three surfaces, suggesting that they are independent of surface orientation. Thus, the sputtering process is almost independent of the surface orientation in Ni₃Al, as it is controlled by the sputtering of Ni atoms with a lower sputtering rate.     

Ionization probability of secondary ions sputtered from Si(1 1 1) and Ge(1 1 1) surfaces

We have measured the energy distributions of the secondary ions sputtered from the Si(1 1 1) and Ge(1 1 1) surfaces and investigated the ionization probabilities of sputtered Si⁺ and Ge⁺ ions for clarifying their ionization mechanisms. The observed ionization probabilities depend on the velocity of Si⁺ and Ge⁺ ions. This velocity dependence can be successfully analyzed by a theoretical expression, which was proposed originally for the metal surfaces. This implies that the ionization mechanism of Si⁺ and Ge⁺ ions is the same as ions sputtered from the metal surface, i.e., the resonant electron transfer in the high velocity regime and the thermal excitation process in the low velocity regime. The difference in the ionization probability between Si⁺ and Ge⁺ ions is well explained by the difference in the band gap energy.     

Energetics and self-diffusion behavior of Zr atomic clusters on a Zr(0 0 0 1) surface

Using a molecular dynamics method and a modified analytic embedded atom potential, the energetic and the self-diffusion dynamics of Zr atomic clusters up to eight atoms on α-Zr(0 0 0 1) surface have been studied. The simulation temperature ranges from 300 to 1100 K and the simulation time varies from 20 to 40 ns. It’s found that the heptamer and trimer are more stable comparing to other neighboring non-compact clusters. The diffusion coefficients of clusters are derived from the mean square displacement of cluster’s mass-center and the present diffusion coefficients for clusters exhibit an Arrhenius behavior. The Arrhenius relation of the single adatom can be divided into two parts in different temperature range because of their different diffusion mechanisms. The migration energies of clusters increase with increasing the number of atoms in cluster. The differences of the prefactors also come from the diverse diffusion mechanisms. On the facet of 60 nm, the heptamer can be the nuclei in the crystal growth below 370 K.     

Wavelengths and transition probabilities for n = 4 → n′ = 4 transitions in heavy Cu-like ions (70 ? Z ? 92)

Wavelengths and transition probabilities have been calculated for the n = 4 → n′ = 4 allowed transitions in the heavy Cu-like ions with Z = 70-92. Fully relativistic multiconfiguration Dirac-Fock (MCDF) calculations were carried out. They take into account the correlations within the n = 4 complex, the core-valence n = 3 → n′ = 4 virtual excitations, and quantum electrodynamics effects. The present results are compared to and agree well with recent electron-beam ion-trap (EBIT) measurements in ytterbium, tungsten, osmium, gold, lead, bismuth, thorium, and uranium.     

Atomic simulation of energetic fluorine interacting with Si(0 0 1)

In this study, Molecular dynamics simulations were performed to investigate F continuously bombarding silicon surfaces at normal incidence and room temperature. The simulated results show that with increasing incident energy and temperature, the etch yield of Si atoms increases, which is in good qualitative agreement with experiments. Accompanying reaching the steady-state F uptake and Si etching, a steady-state SiF_x (x = 1-4) reactive layer is formed whose thickness increases with increasing incident energy. In the reaction layer, SiF species are dominant and SiF₃ species decrease with increasing incident energy.     

Strain-driven (0 0 2) preferred orientation of ZnO nanoparticles in ion-implanted silica

We present an experimental and theoretical study on the structural properties of ZnO nanoparticles embedded in silica. The ZnO-SiO₂ nanocomposite was prepared by ion implanting a Zn⁺ beam in a silica slide and by annealing in oxidizing atmosphere at 800 °C. From an experimental point of view, the structural properties of the ZnO-SiO₂ nanocomposite were studied by using glancing incidence X-ray diffraction. According to the results, zinc crystalline nanoclusters with an average diameter of 13 nm are in the as-implanted sample. The annealing in oxidizing atmosphere promotes the total oxidation of the Zn nanoclusters and increases their size until to an average of 22 nm. Moreover, the formed ZnO nanocrystals have a preferential (0 0 2) crystallographic orientation. From a theoretical point of view, the preferential orientation of the ZnO nanoparticles can be explained satisfactory by the minimization of the strain energy of the nanoparticles placed in proximity of the surface of the matrix.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

Deposition of titanium nitride on Si(1 0 0) wafers using plasma focus

Titanium nitride thin films were deposited on Si(1 0 0) substrates by using a low energy (2.3 KJ) Mather-type plasma focus device. The composition of the deposited films was characterized by X-ray diffraction (XRD). The crystallite size has strong dependence on the numbers of focus shots. The crystallinity of TiN thin films is found to increase with increasing the number of focus shots. The effect of different number of focus shots on micro structural changes of thin films was characterized by Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). SEM results showed net-like structure for film deposited for 15 numbers of shots, which are elongated grains of Si₃N₄ in amorphous form embedded into TiN crystals. The average surface roughness was calculated from AFM images of the thin films. These results indicated that the average surface roughness increased for films deposited with increased number of focus shots. The least crystallite size and roughness are observed for film deposited with 25 focus shots.     

Roughening of silicon (1 0 0) surface during low energy Cs ion bombardment

Nowadays, the use of low energy ion bombardment in secondary ion mass spectrometry (SIMS) is a mandatory step to obtain high depth resolution for the characterization of ultra shallow junctions. However, increment of roughness during ion bombardment leads to the degradation of depth resolution. The evolution of surface roughening and ripple formation on silicon at ambient temperature under 1 keV Cs⁺ ion bombardment with and without sample rotation has been studied by means of atomic force microscopy.Ripple formation with a perpendicular orientation with respect to the Cs⁺ beam direction has been detected, and their wavelength and correlation length have been monitored as a function of the experimental conditions. Roughness and wavelength increased with increasing ion fluence, while variations of ion flux showed little effect. The effect of sample rotation during ion bombardment led to a critical reduction of the surface roughness and disappearance of ripples.     

Anisotropic rearrangement of the substrate atoms during Ar bombardment on Pd(0 0 1) surface

Using a three-dimensional molecular dynamics (MD) simulation, we investigated the atomic scale rearrangement that occurs on a Pd(0 0 1) surface after energetic bombardment by Ar at room temperature. High energy Ar bombardment provoked the significant rearrangement of Pd atoms in a ballistic manner with a fourfold symmetric lateral distribution aligned along the 〈1 1 0〉 direction. The MD simulation of uniform Ar bombardment at normal incidence on a Pd surface reproduced the experimentally observed fourfold symmetric nano-scale surface structure. The present result supports that the ballistic rearrangement of the substrate atoms plays an important role in the ion induced surface structure evolution.     

Wavelengths and transition probabilities in heavy Ge-like ions (70 ? Z ? 92)

Wavelengths and transition probabilities have been calculated for the 4s²4p²-4s4p³ and 4s²4p²-4s²4p4d allowed transitions and for the forbidden (M1 and E2) transitions occurring within the ground configuration (4s²4p²) in the heavy Ge-like ions with Z = 70-92. The fully relativistic multiconfiguration Dirac-Fock method taking into account both the correlations within the n = 4 complex and the QED effects has been used for the calculations. The present results are compared to and agree well with recent electron-beam ion-trap measurements in tungsten, osmium, gold, bismuth, thorium, and uranium.     

Surface roughness of ion-bombarded Si(1 0 0) surfaces: Roughening and smoothing with the same roughness exponent

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the roughness of pristine as well as ion-bombarded Si(1 0 0) surfaces and of ultrathin Ge films deposited on them. One half of a Si(1 0 0) sample (with native oxide layer) was irradiated at room temperature using 45 keV Si⁻ ions at a fluence of 4 × 10¹⁵ ions/cm² while the other half was masked. STM measurements were then carried out on the unirradiated as well as the irradiated half of the sample. Root-mean-square (rms) roughness of both the halves of the sample has been measured as a function of STM scan size. Below a length scale of ∼30 nm we observe surface smoothing and surface roughening is observed for length scales above this value. However, the surface is self-affine up to length scales of ∼200 nm and the observed roughness exponent of 0.46 ± 0.04 is comparable to earlier cases of ion sputtering studies where only roughening [J. Krim, I. Heyvart, D.V. Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57] or only smoothing [D.K. Goswami, B.N. Dev, Phys. Rev. B 68 (2003) 033401] was observed. Preliminary results involving morphology for Ge deposition on clean ion-irradiated and pristine Si(1 0 0) surfaces are presented.     

Classical and quantum mechanical rainbow-scattering of fast He atoms from a KCl(0 0 1) surface

Fast He atoms with energies from 200 eV up to 16 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(0 0 1) surface. For scattering along low-index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical and semiclassical trajectory calculations based on individual Hartree-Fock pair and density functional theory (DFT) potentials, we obtain corresponding rainbow angles for comparison with the experimental data. The calculations were performed taking into account the rumpling of K and Cl in the topmost surface layer. Fair agreement with the experimental data is found for scattering along 〈1 0 0〉 for DFT as well as individual pair potentials calculated from Hartree-Fock wave functions.     

Grazing scattering of 1-2 MeV HeH ions from KCl(0 0 1): Effect of surface track potential

Three dimensional (3D) distributions (energy E, scattering angle θ and azimuth angle φ) of the fragment protons dissociated from HeH⁺ during grazing angle scattering from a KCl(0 0 1) are measured using a magnetic spectrometer in order to study the effect of the surface track potential. The distributions of the fragment protons scattered from a SnTe(0 0 1) are also measured as a reference. Although the observed distributions for KCl(0 0 1) and SnTe(0 0 1) are basically the same, there is small differences, especially in the scattering angle distribution. While the fragment protons are scattered at the specular angle from SnTe(0 0 1), the protons are scattered at slightly larger angles from KCl(0 0 1). The observed angular shift is more pronounced for the trailing protons than the leading protons. It is also found that the angular shift increases with decreasing ion energy. The observed angular shift can be qualitatively explained by the surface track potential induced by the partner He ions using a simple model of the surface track potential.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Study of the useful yield of the Storing Matter technique using Ge(1 0 0) as a sputter target

Storing Matter is a new analytical technique for organic and inorganic materials, which tries to circumvent the well-known matrix effect in SIMS. This technique consists of separating the sputtering of the sample from the subsequent analysis steps. Thus, the sample to be analysed is sputtered with a focused ion beam produced by a floating low-energy ion gun (FLIG) and the particles emitted under the ion impacts are deposited at a sub-monolayer level on a well-known collector. The collector with the deposited material is then analysed in a second step by SIMS (dynamic and static mode). The main advantage of this new technique is to improve the sensitivity and the quantification of the SIMS analysis. All the processes, including all the sample and collector transfers, are performed in UHV conditions. This paper presents preliminary results obtained on the Storing Matter prototype instrument developed in our laboratory. The sputtering of a (1 0 0) Ge wafer by ions was used as a model system to study the influence of using different collector surfaces (W, Ta and Al) on the Storing Matter useful yield.