No abstract available

[en] The Fuel Rod Analysis Program (FRAP-S3) is a fairly comprehensive computer code that is developed for the analysis of light water reactor fuel elements during steady-state operation. However, the code predicts an increase in the fuel radial temperature distribution with an increase in the fuel density, which is contrary to experiments. A simple modification of the code was used where the thermal conductivity is treated as porosity independent in the inner iteration loops of the program. The resulting temperature profile is corrected for the effects of porosity after it has converged. The modified code shows good agreement with the IFA-11 series of experiments using the Halden Boiling Water Reactor in Sweden. 7 refs

[en] Tin-lithium (Sn-Li) has been identified as a candidate liquid metal coolant for fusion power reactors. Sn-Li coolants offer a number of advantages compared with pure lithium. The vapor pressure of Sn-25Li (0.25 Li mol fraction) is a factor of ∼1000 lower than that of pure Li, which allows an increase in coolant temperatures by as much as 450 K. Experimental data of the stability of ceramic materials in Sn-Li is scarce. The thermodynamic stability of various oxides, carbides, and nitrides in Sn-Li is estimated as a function of lithium composition and temperature at saturated solute concentrations by evaluating the Gibbs free energy of reaction, (Δ_rG). At 773 K most of the studied nitrides, carbides, and some oxides were found to be stable (Δ_rG>0). However, oxides of Fe-based alloys, such as Cr₂O₃ and Fe₂O₃ were found to be unstable (Δ_rG<0) for all lithium compositions

[en] A model which can treat the growth of voids under pulsed irradiation conditions is presented. The model is applied to point defect concentration and void radius change in stainless steel 316. (Auth.)

[en] The symposium, Multiscale Modeling of Materials, was held at the 1998 MRS Fall Meeting in Boston, Massachusetts, November 30 to December 3. Though multiple scale models are not new the topic has recently taken on a new sense of urgency. This is in large part due to the recognition that brute force computational approaches often fall short of allowing for direct simulation of both the characteristic structures and temporal processes found in real materials. As a result, a number of hybrid approaches are now finding favor in which ideas borrowed from distinct disciplines or modeling paradigms are unified to produce more powerful techniques. Topics included are modeling dislocation properties and behavior, defect dynamics and microstructural evolution, crystal defects and interfaces, novel methods for materials modeling, and non-crystalline and nanocrystalline materials. Eighty papers have been processed separately for inclusion on the data base

[en] Ceramic foam and cellular materials are being used in a wide variety of industries and are finding ever growing number of applications. Over the past decade advances in manufacturing of cellular materials have resulted in ceramics with highly uniform interconnected porosities ranging in size from a few μm to several mm. These relatively new ceramic foam materials have a unique set of thermo-mechanical properties, such as excellent thermal shock resistance and high surface to volume ratios. Based on new advances in processing ceramic foams, we suggest the development of ceramic foams or cellular ceramics for solid breeders in fusion reactor blankets. A cellular breeder material has a number of thermo-mechanical advantages over pebble beds, which can enhance blanket performance, improve operational stability, and reduce overall blanket costs

[en] The sensitivity of lifetime predictions for fusion reactor blanket structures is investigated by applying the Monte Carlo numerical technique. A structural computer code, STAIRE, which has been developed for the analysis of mirror fusion blankets is used for the prediction of structural lifetime. Uncertainties in material variables are treated as probabilistic input to the STAIRE code. Irradiation Creep rates are shown to be sufficient for relaxation of swelling stresses under the majority of conditions. An engineering swelling limit, which is dependent on blanket geometry, is shown to be an important failure criterion. In the case of HT-9, it appears that the blanket structure lifetime could be several hundred displacements per atom. (orig.)

[en] In this paper, we present a theoretical analysis of the stability of cavities in SiC under fusion irradiation conditions. Nucleation and growth of helium-filled cavities is modeled by dynamical equations. The growth/shrinkage of a cavity is given in terms of two equations representing the rates of change of the net number of vacancies and helium atoms inside the cavity. The equations describe the drift motion of cavities in a two-dimensional phase under the influence of point defect and helium absorption. First the linear stability of these equations is performed to identify the nature of singular points in phase space. The directions of trajectories around the saddle point are also found. The effects of dislocation density and void density on the nature of the singular points are considered. The effect of helium atom generation rate is also investigated. It is found that in the case of high helium generation, as in SiC under fusion conditions, the critical cavity size is relatively small (about 30 A), and one saddle point is found between two stable critical points. This is shown to correspond to a bimodal cavity distribution. (orig.)

[en] The Molecular Dynamics (MD) method is used to calculate defect energetics in β-silicon carbide. Many-body interaction effects in this covalent material are accounted for by using a hybrid of two-body and three-body potentials. Calculated bulk properties of β-SiC based on this potential are in agreement with experimental data to within 17%. A micro-crystal is constructed to represent the computational cell and external forces are applied to the micro-crystal so that it behaves as a part of an infinite medium. The potential energy for the unperturbed computational cell is first calculated. The cell is then set at a defect configuration and relaxed, and the potential energy of the relaxed cell is calculated. The difference between the potential energy of the unperturbed cell and that of the defect-containing cell is used to calculate the formation and binding energies of point defects, defect clusters and helium-vacancy clusters in SiC

[en] Calculations of neutron displacement damage cross sections for SiC are presented. We use Biersack and Haggmark's empirical formula in constructing the electronic stopping power, which combines Lindhard's model at low PKA energies and Bethe-Bloch's model at high PKA energies. The electronic stopping power for polyatomic materials is computed on the basis of Bragg's Additivity Rule. A continuous form of the inverse power law potential is used for nuclear scattering. Coupled integro-differential equations for the number of displaced atoms j, caused by PKA i, are then derived. The procedure outlined above gives partial displacement cross sections, displacement cross sections for each specie of the lattice, and for each PKA type. The corresponding damage rates for several fusion and fission neutron spectra are calculated. The stoichiometry of the irradiated material is investigated by finding the ratio of displacements among various atomic species. The role of each specie in displacing atoms is also investigated by calculating the fraction of displacements caused by each PKA type. The study shows that neutron displacement damage rates of SiC in typical magnetic fusion reactor first walls will be ∝10-15 dpa MW^-1 m²; in typical lead-protected inertial confinement fusion reactor first walls they will be ∝15-20 dpa MW^-1 m². For fission spectra, we find that the neutron displacement damage rate of SiC is ∝74 dpa per 10²⁷ n/m² in FFTF, ∝39 dpa per 10²⁷ n/m² in HFIR, and 25 dpa per 10²⁷ n/m² in NRU. Approximately 80% of displacement atoms are shown to be of the carbon-type. (orig.)