No abstract available

[en] The criticality safety of final disposal of TRIGA Mark II fuel in specially designed copper-iron canisters has been studied. An MCNP-model for the fuel in the canister has been written and used in the calculations. The fuel rods were modelled in detail as well as the basket that collects them into 3 x 3 bunches resembling a fuel assembly. The copper cylinder and the massive iron insert with steel linings were also described. The insert has 12 positions into which the baskets are placed. Axially the model was one fuel rod high with reflecting boundaries at the end of the guiding tips. Both normal and abnormal conditions were studied. In all studied normal conditions a fully filled canister was well below criticality with the largest value k_eff ≅ 0.72. In the worst accident scenario, where the fuel rods collapsed and formed a 4 x 4 configuration, the water filled canister was close to critical. However, five positions are enough to host the expected number of fuel rods of the whole lifetime of the FiR 1 reactor. When only five positions out of 12 were filled the canister remained well subcritical when the positions were chosen as far from each other as possible. Subcriticality was achieved even with reflective boundary conditions at the edge of the copper canister, and at the top and bottom ends of the fuel column. (authors)

[en] Understanding edge plasma phenomena in the presence of radio-frequency heating is crucial to constructing launchers that can survive long-pulse or steady-state high-power operation. In some experiments, significant fractions of lost power for ion cyclotron range-of-frequency (ICRF) antennas and for lower hybrid (LH) grills have been observed with adverse effects like impurity generation and hot spots. Theoretical aspects of parasitic power absorption in the edge plasma for both high-power ICRF and LH launching are reviewed. (orig.)

No abstract available

[en] Ion Bernstein wave excitation is investigated with the self-consistent two-dimensional particle-in-cell method. The real ion to electron mass ratio is used in simulations in high harmonic frequency bands. The simulation results are compared with linear theory and ray tracing. Successful excitation of the ion Bernstein wave has been demonstrated with the particle-in-cell method. In some cases, the excited wave temporarily propagates in the opposite direction and slows down permanently due to complicated dispersive behavior, which makes it very difficult to use the particle-in-cell method. The excitation is studied as a function of temperature and frequency, i.e., it is determined how the dispersive behavior varies in the parameter space. The simulations indicate that there is a temperature-and-frequency-dependent critical level of coupled energy flux above which excitation fails. Possible effects causing the failure of excitation at high power intensity are identified

[en] Ion Bernstein waves (IBWs) may offer a viable method for heating ions and driving plasma rotation in fusion plasmas. Within present experiments at the FTU tokamak, the IBW at 433 MHz is first launched from waveguides at the plasma edge as a slow wave. In a region of lower hybrid resonance, the slow wave is mode transformed into an ion Bernstein wave by finite Larmor radius effects. In the IBW coupling problem, nonlinear and kinetic effects are important. The particle-in-cell (PIC) method takes them into account, because the electric fields and the particle motion are solved self- consistently. Using the particle-in-cell technique, IBW excitation has so far been studied only for parameters significantly different from those in real experiments. Specifically, only small ion to electron mass ratios and excitation frequencies in the range between the first and the second harmonic frequencies have been used in the simulations. The reason for this is that simulations with real parameters are computationally very heavy, which can easily be understood: Firstly, in order to obtain accurate results, the grid spacing has to be of the order of the Debye length, which decreases with increasing density. Secondly, the time step also has to be decreased with increasing density. Artificially reducing the ion mass and decreasing the excitation frequency leads to lower densities in the simulation region and thus to more manageable simulations. In this work, IBW excitation is for the first time simulated with realistic parameters resembling those of the FTU tokamak. The electrostatic PIC code XPDP2 is used. The electrostatic approximation is justified, because the ion Bernstein wave is essentially an electrostatic wave. A wave launcher model has been added to the 2d3v PIC code. A new energy flux diagnostics based on first principles is presented. The total energy flux launched by the radio frequency (RF) grill is the best indicator of any successful excitation of waves. The necessary resolution and time step for the results to be accurate are determined. Then, edge absorption and coupling are systematically studied as a function of RF power. The temperature effect on the excitation is also investigated. Finally, the results are compared to those obtained in experiments at FTU (author)

[en] Highlights: → Comparison between CASMO-4 and CASMO-4E with standard and extended libraries. → Studied multiplication factor, reactivity coefficients and microscopic cross sections. → Differences in multiplication factor range up to several hundred pcm. → Large differences in void coefficients at low burnup and high void. → Differences in microscopic cross sections range up to slightly over 2%. - Abstract: A detailed comparison between the code CASMO-4 and its extended version CASMO-4E has been made. In addition to the standard library, CASMO-4E calculations have been performed also with its extended libraries. The differences are significant enough to be considered when choosing the library to be used for a particular problem. The differences in the multiplication factor k_∞ range up to several hundred pcm depending on the void history, burnup and other parameters. The differences in fuel temperature or void coefficients are smaller especially at small void fraction and low burnup. At large void and low burnup CASMO-4E with the standard library gives significantly different results than the other combinations. The microscopic cross sections show small differences when calculated with the same library but clear differences due to the extended libraries.

[en] The coupling of lower hybrid waves to the plasma is a crucial issue for efficient current drive in tokamaks. In this work, the coupling problem is attacked with a new tool in this context: an electromagnetic particle-in-cell (PIC) code, XOOPIC (Verboncoeur J P et al 1995 Comp. Phys. Comm. 87 199). A model for a grill with 32 waveguides is constructed using perfectly conducting walls. The wave propagation in the waveguides and the coupling to plasma is followed. The wave-plasma interaction is studied and the evolution of the launched spectrum is resolved in the near-field of the grill. The reflection coefficients in the individual waveguides are determined. (author)

[en] Parasitic absorption of the short wavelength modes of the LH spectrum is a probable reason for the hot spots seen in the grill region of several tokamaks. Experiments suggest that the heat loads on the wall structures depend on the coupled power. In this work, the parasitic absorption of LH power was studied with self-consistent particle-in-cell simulations. The launched spectra were obtained from the SWAN coupling code. The power and temperature dependences of the absorption in the near field of the LH grill were investigated with a series of simulations. The parasitic absorption was found to grow from 0.6 to 1.1% when the coupled power increased from 26 to 67 MW/m². When the edge temperature rose from 12.5 to 100 eV, the absorption increased from 0.4 to 1.7%. The maximum kinetic energies were between 0.6 and 1.8 keV. Estimates for the heat loads and surface temperature of the grill limiter are also obtained. The absorption leads to heat loads between 1.5 and 13 MW/m² and surface temperatures of 510-2390 deg. C. (author)

[en] In lower hybrid (LH) current drive experiments at Tore Supra and Tokamak de Varennes (TdeV), hot spots and the generation of impurities have been observed. Melting of the grill mouth has also occurred in LH current drive experiments. A possible explanation for these observations is parasitic absorption of the short-wavelength part of the LH spectrum close to the grill mouth. In this work, we investigate the parasitic absorption of the LH power in the edge plasma of Tore Supra and JET with particle-in-cell (PIC) simulations. The LH spectra are calculated with the SWAN coupling code and used in the self-consistent electrostatic PIC code XPDP2 which calculates the absorption. The absorption was calculated for different edge densities and density scale lengths when the coupled LH power densities varied between 25 MW m^-2 and 50 MW m^-2. The high-n p_arallel part of the LH spectrum was found to be absorbed near the edge within a few millimetres. The absorbed power density varied from 65 kW m^-2 to 420 kW m^-2 which corresponds to 0.2-0.8% of the coupled power. In an edge plasma having a temperature of 25 eV, the maximum energies of the fast electrons generated by the parasitic absorption were between 0.4 keV and 1.6 keV. (author)