Recombination of molecules in channeling conditions

The transmission of molecules through thin gold foils in channeling conditions is studied, both theoretically and experimentally, in the low energy range (E < 10 keV/u). For this purpose we have performed semiclassical computer simulations of the fragment trajectories inside the target and we are able to determine if they can reconstruct at the exit surface via the recombination model. Our calculated transmission yield shows interesting oscillations as a function of the dwell time, and also the calculated exiting number of with respect to H⁺ shows a very good agreement with our experimental measurements. We observed transmission yields almost three orders of magnitude larger than in the case of a random target.     

Luminescence from low-energy impact of and ions (n = 1,2,3) on a Tore Supra carbon tile

The hydrogen Balmer series was observed when beams of H⁺, , and D⁺, , ions impinged at 100–1000 eV on a carbon fiber composite tile (Tore Supra). The excitation functions of the H_β (D_β) lines demonstrate the validity of the independent particle model. The logarithmic plot of the H_β (D_β) line intensity versus the reciprocal projectile ion velocity can be well reproduced by a model in which the hydrogen ion projectiles are neutralized in the entrance phase and subsequently electronically excited in collisions with carbon atoms. Finally, the excited hydrogen atoms leave the solid and emit the observed photons, unless they are quenched in non-radiative processes.     

Fluence dependence of the surface roughness of InP after bombardment

InP(1 0 0) surfaces were sputtered under ultrahigh vacuum conditions by 5 keV ions at an angle of incidence of 41° to the sample normal. The fluence, , used in this study, varied from 1 × 10¹⁴ to 5 × 10¹⁸ cm⁻². The surface topography was investigated using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). At the lower fluences ( 5 × 10¹⁶ cm⁻²) only conelike features appeared, similar in shape as was found for noble gas ion bombardment of InP. At the higher fluences, ripples also appeared on the surface. The bombardment-induced topography was quantified using the rms roughness. This parameter showed a linear relationship with the logarithm of the fluence. A model is presented to explain this relationship. The ripple wavelength was also determined using a Fourier transform method. These measurements as a function of fluence do not agree with the predictions of the Bradley–Harper theory.     

Electron emission from carbon induced by the ions C, O, CO and

The secondary electron yield from carbon induced by the ions C⁺, O⁺, CO⁺ and has been measured as a function of ion energy in the range of 2 to 20 keV. It has been observed that electron yield from carbon increases with projectile energy. By comparing electron yields induced by equally fast atomic and molecular projectiles, a molecular effect as a yield reduction has been observed. The measured molecular effect was stronger than the predictions of sweeping-out-electron model.     

Determination of the electronic energy loss of light ions in a silicon lattice by using the transmission ion channeling method

Charged particle activation analysis (CPAA) is able to analyze light elements such as carbon and oxygen at trace levels in semiconductor materials. This technique requires the knowledge of the stopping powers of these materials for channeled ions. The electronic energy loss for ions entering the crystal lattice in a random direction is well established. The electronic energy losses for protons, deuterons, ³He⁺ and ⁴He⁺ ions entering a 3.6 μm thick silicon single crystal along the 1 0 0 direction were measured by using the transmission of particles technique. Data obtained were compared with those obtained by other authors using theoretical and experimental methods.     

Application of Leak-Before-Break Concept in VVER 440-230: Results and consequences for TACIS-program 91/1.2

The project TACIS 91/1.2 `Primary Circuit Integrity, Application of Leak-Before-Break (LBB) Concept' was performed within the scope of the TACIS-program (Technical Assistance for the Community of Independent States) of the European Community. The application of the LBB concept was carried out for the primary piping (main coolant lines (MCL) and surgelines (SL)) of the first generation of the Russian VVER NPP's of the type VVER 440/230 of the nuclear power plants Kola 1 and 2 (K1, K2) and Novovoronesh 3 and 4 (NV3, NV4). The work was performed by the Consultant (Siemens, EdF, Framatome) and Russian subcontractors (headed by Vniiaes). The paper describes the comparison of the relevant safety concept, the scope of the LBB-program, the applied LBB-procedures (German and NUREG 1061 procedure), the results obtained and the recommendations (analysis of seismicity, if necessary LBB analysis taking into account additional optimized seismic supports and their installation for NV3 and 4 adaption of inservice inspection (ISI) and leak monitoring systems (LMS)). Proposals have been made for future work to improve the safety of these plants. This project was financed by the European Community's TACIS Programme, which provides grant finance for know-how to foster the development of marked economies and democratic societies in the New Independent States and Mongolia.     

Application of fracture mechanics in the design and operation of PWRs in France

Fracture mechanics techniques are used to evaluate the risks associated with material aging, the occurrence of fabrication defects or stress corrosion cracks, and to determine the margins with respect to fast fracture resulting from the design and manufacturing practices. Important programs are underway to improve the accuracy and domain of validity of the fracture mechanics methods.     

Defect production considerations for gamma ray irradiation of reactor pressure vessel steels

Motivated by the recent interest in gamma ray embrittlement of nuclear reactor pressure vessels (RPVs), calculations were performed to evaluate aspects of defect production by gammas in iron and steel. In addition to determining displacement damage cross-sections, the atomic recoil energy dependence of gamma-induced defect production was described by integral recoil damage spectra, W(T), and their associated median recoil damage energies, T_1/2. These latter characterizations, should be particularly useful in evaluating the contribution of gamma ray generated defects to microstructural changes causing radiation embrittlement. The results for monoenergetic gammas, as well as for gamma rays with a spectrum of energies characteristic of a RPV, reveal T_1/2 values of <100 eV, about three orders of magnitude smaller than for fast-neutrons, the radiation of primary concern in previous embrittlement studies. The relative contributions of various gamma interactions to defect production, as well as the role of light alloy element-induced secondary displacement mechanisms, are also considered.     

Experimental assessment of neutron fluence for the Kozloduy Unit 1 pressure vessel by scraps and cavity dosimetry

In accordance with the Safety Improvement Program the neutron exposure onto the Kozloduy Unit 1 pressure vessel has been evaluated. The conservative fluence values have been used for the reembrittlement assessment of the pressure vessel. The verification of the calculated neutron fluence has been provided by comparing the calculated and experientially determined induced activities of the scraps taken out from the weld four and of the copper, iron, and niobium detectors irradiated in the cavity behind the vessel.     

Alternative methods for postweld treatment of austenitic pipe welds to increase the operational safety of BWR plants

Experience has shown that austenitic piping is susceptible to IGSCC in the weld root area under BWR service conditions. Besides non-optimized materials, the residual stresses which are an inherent result of conventional welding processes are also responsible for this susceptibility. In the past, mechanical, thermal and welding post-treatment processes were developed and used with the objective of reducing tensile stresses in the root area. This paper discusses past experience and more recent developments, in particular the latest results with pipe welds treated by means of welding processes (last pass heat sink welding). These measures are suitable for producing compressive stresses in the medium-swept ID-HAZ of austenitic welds, or to at least significantly reduce the tensile stresses and thus practically eliminate the risk of IGSCC.     

Long-term installations of the DC-potential drop method in four nuclear power plants and the accuracies thereby obtained for monitoring of crack initiation and crack growth

After a total monitored operational timescale of almost five years on long-term installations, both in the laboratory and in four nuclear power plants, evidence can be put forward that the DC-potential drop method is now, at its current stage of development, suitable for inspecting and monitoring material regions such as, e.g. weld seams in pipework, for crack initiation and crack growth at power plant temperatures. This function can be performed with reliability and high sensitivity. The inspection and monitoring of cracks on the internal surface of the pipework can also be carried out from the external surface. The studies have shown that the method is basically able to monitor the growth of cracks found at discontinuous intervals using permanently installed potential probes, i.e. from plant inspection to plant inspection, while a transition to continuous monitoring is possible at any time. Thus a measure of redundancy can be provided for conventional ultrasonic and radiographic inspection, in particular for difficult to check austenitic weld seams. The method can also be seen as an alternative to the conventional techniques. When necessary, the cracks found can be measured more accurately than was previously possible with conventional ultrasonic and radiographic inspections. The total exposure to radiation can be reduced in comparison to other methods of inspection.     

A generalized model for radiation-induced amorphization and crystallization of U3Si and U3Si2 and recrystallization of UO2

A rate-theory model of radiation-induced amorphization and crystallization of U₃Si during ion irradiation has been generalized to include U₃Si₂ and UO₂. The generalized model has been applied to ion-irradiation and in-reactor experiments on U₃Si and U₃Si₂ and provides an interpretation for the amorphization curve (dose required to amorphize the material as a function of temperature), for the ion-radiation-induced nanoscale polycrystallization of these materials at temperatures above the critical temperature for amorphization, as well as for the role of the small crystallites in retarding amorphization. An alternative mechanism for the evolution of recrystallization nuclei is described for a model of irradiation-induced recrystallization of UO₂ wherein the stored energy in the UO₂ is concentrated in a network of sinklike nuclei that diminish with dose due to interaction with radiation-produced defects. The sinklike nuclei are identified as cellular dislocation structures that evolve relatively early in the irradiation period. The complicated kinetics involved in the formation of a cellular dislocation network are approximated by the formation and growth of subgrains due to the interaction of shock waves produced by fission-induced damage to the UO₂.     

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal.: Experiments and electronic structure calculations

In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb-Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.