Finite size effects near the onset of the oscillatory instability

A system of two complex Ginzburg - Landau equations is considered that applies at the onset of the oscillatory instability in spatial domains whose size is large (but finite) in one direction; the dependent variables are the slowly modulated complex amplitudes of two counterpropagating wavetrains. In order to obtain a well posed problem, four boundary conditions must be imposed at the boundaries. Two of them were already known, and the other two are first derived in this paper. In the generic case when the group velocity is of order unity, the resulting problem has terms that are not of the same order of magnitude. This fact allows us to consider two distinguished limits and to derive two associated (simpler) sub-models, that are briefly discussed. Our results predict quite a rich variety of complex dynamics that is due to both the modulational instability and finite size effects

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

Interaction of caesium hydroxide vapour with 304 stainless steel and Inconel-600

SIGLEAvailable from British Library Document Supply Centre- DSC:9091.9(AEA-RS--5164) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Irradiation-induced heterogeneous nucleation in uranium dioxide

Using classical molecular dynamics simulations, we have studied the first stages of defect cluster formation resulting from 10 keV displacement cascades in uranium dioxide. Nanometre size cavities and dislocation loops are shown to appear as a result of the irradiation process. A specifically designed TEM experiment involving He implanted thin foils have also been carried out to support this modelling work. These results, in conjunction with several other observations taken from the literature of ion implanted or neutron irradiated uranium dioxide, suggest a radiation damage controlled heterogeneous mechanism for insoluble fission product segregation in UO(2). (C) 2010 Elsevier B.V. All rights reserved

In situ TEM study of temperature-induced fission product precipitation in UO2

International audienceTransmission electron microscopy (TEM) experiments were performed, on a set Of UO2 thin foils following the implantation of 390 keV Xe3+ and 300 keV Cs2+ ions. TEM results reveal the nature and quantity of defects formed as a function of Xe and Cs ion fluences. These do not appear to be dependent upon the nature of the implanted ions. In addition to dislocation and dislocation loop formation, Xe and Cs aggregates were observed. At several Xe and Cs concentrations, the threshold temperatures for aggregate formation were determined. At concentrations of 0.4 at.%, Xe precipitation occurred above 870 K. The threshold temperature for Xe precipitation decreased to 670 K for the higher concentration samples (2 at.%) and to 770 K for Cs implanted samples at this concentration. No significant differences in the bubble size and densities were observed between the high concentration Xe or Cs samples. The aggregates formed were in the nanometer size range. (c) 2008 Elsevier B.V. All rights reserved

Assessment of the performance of the JET Exhaust Detritiation System

The Exhaust Detritiation System (EDS) is one of the key safety related systems which allows operation of the JET machine. Its role is to detritiate the exhaust gases in case of an accident and to minimise the tritium release to the environment. EDS has also been used on JET during the last three years for maintenance purposes. During the main phase of the deuterium-tritium experiment (DTE1), the EDS was the final stage for detritiation of gases coming from the Torus vessel. There is a regulatory requirement to prepare an ''Engineering Substantation'' of key safety related systems. Although this requirement applies to EDS when dealing with accidental tritium releases, it has been possible to investigate the efficiency of the system by analysing the experimental data obtained during maintenance. In 2000, during the shutdown, campaign measurements on EDS have been performed to determine its efficiency on a daily basis. Detritiation factors range between 20 and 80,000. Results are presented and discussed in relation to the requirements of the JET safety case and the factors which cause apparent low detritiation factors. An enhancement of the system for tritium control tracking is proposed. (author)Preprint of a paper to be submitted for publication in the proceedings of the IAEA Technical Meeting on Fusion SafetyAvailable from British Library Document Supply Centre-DSC:4672.262(00/10) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Helium behaviour in UO2 through low fluence ion implantation studies

In this work we focus on experiments involving implantation of 500 keV 3He ions in sintered polycrystalline material. Samples are implanted at low fluences (∼2 ×1013 ions/cm2) and subsequently isothermally annealed in a highly sensitive thermal desorption spectrometry (TDS) device PIAGARA (Plateforme Interdisciplinaire pour l'Analyse des GAz Rares en Aquitaine). The helium fluencies studied are two to three orders of magnitude lower than previous Nuclear Reaction Analysis (NRA) experiments carried out on identical samples implanted at identical energies. The fractional release of helium obtained in the TDS experiments is interpreted using a three-dimensional axisymmetric diffusion model which enables results to be quantitatively compared to previous NRA data. The analysis shows that helium behaviour is qualitatively independent of ion fluency over three orders of magnitude: helium diffusion appears to be strongly inhibited below 1273 K within the centre of the grains presumably as a result of helium bubble precipitation. The scenario involving diffusion at grain boundaries and in regions adjacent to them observed at higher fluencies is quantitatively confirmed at much lower doses. The main difference lies in the average width of the region in which uninhibited diffusion occurs

In-situ TEM observation of nano-void formation in UO2 under irradiation

International audienceTransmission electron microscopy (TEM) observations of UO2 polycrystals irradiated in situ with 4 MeV Au ions were performed at room temperature (RT) to better understand the mechanisms of cavity and ultimately fission products nucleation in UO2. Experiments were carried out at the JANNuS Orsay facility that enables in situ ion irradiations inside the microscope to be carried out. The majority of 4 MeV gold ions were transmitted through the thin foil, and the induced radiation defects were investigated by TEM. Observations showed that nano-void formation occurs at ambient temperature in UO2 thin foils irradiated with energetic heavy ions under an essentially nuclear energy loss regime. The diameter and density of nano-objects were measured as a function of the gold irradiation dose at RT. A previous paper has also revealed a similar nano-object population after a Xe implantation performed at 390 keV at 870 K. The nano-object density was modelled using simple concepts derived from Classical Molecular Dynamics simulations. The results are in good agreement, which suggests a mechanism of heterogeneous nucleation induced by energetic cascade overlaps. This indicates that nano-void formation mechanism is controlled by radiation damage. Such nanovoids are likely to act as sinks for mobile fission products during reactor operation