M. Caro

Implications of ab initio energetics on the thermodynamics of Fe–Cr alloys

The authors analyze the implications of the recently reported results of ab initio calculations of formation energies of the Fe–Cr alloy. The formation energies show a change in sign from negative to positive as Cr composition increases above ∼10%. By developing a classic potential to evaluate the thermodynamic properties, they determine the location of the solubility limit and compare it with earlier results. A significant difference appears in a region of temperature and composition that is relevant for the nuclear applications of this alloy. Experimental results seem to confirm the validity of the location of the new solvus line.

Corrosion of Ferritic Steels in High Temperature Molten Salt Coolants for Nuclear Applications

Corrosion of ferritic steels, including oxide dispersion strengthened (ODS) variants, in high temperature molten fluoride salts may limit the life of advanced reactors, including some hybrid systems that are now under consideration. In some cases, the steel may be protected through galvanic coupling with other less noble materials with special neutronic properties such as beryllium. This paper reports the development of a model for predicting corrosion rates for various ferritic steels, with and without oxide dispersion strengthening, in FLiBe (Li 2 BeF 4 ) and FLiNaK (Li-Na-K-F) coolants at temperatures up to 800 °C. Mixed potential theory is used to account for the protection of steel by beryllium, Tafel kinetics are used to predict rates of dissolution as a function of temperature and potential, and the thinning of the mass-transfer boundary layer with increasing Reynolds number is accounted for with dimensionless correlations. The model also accounts for the deceleration of corrosion as the coolants become saturated with dissolved chromium and iron. Electrochemical impedance spectroscopy has been used for the initial in situ study of an ODS ferritic steel in high-temperature molten fluoride salt environments, with the complex impedance spectra obtained at its open circuit corrosion potential (OCP) interpreted in terms of the basic components of the equivalent circuit, which include the electrolyte conductivity, the interfacial charge transfer resistance, and the interfacial capacitance. Such in situ measurement techniques may provide valuable insight into the degradation of materials under realistic conditions.

TRISO Particle and Beryllium Pebble Thermo-Mechanical Response in a Fusion/Fission Engine for Incineration of Weapons Grade Plutonium

Among the laser inertial fusion-fission energy (LIFE) engine concepts being considered at Lawrence Livermore National Laboratory (LLNL), weapons-grade plutonium (WGPu) LIFE is of particular interest because it is designed to burn excess WGPu material and achieve over 99% fraction of initial metal atoms (FIMAs). At the center of the LIFE concept lies a point source of 14MeV neutrons produced by inertial-confinement fusion (ICF) which drives a sub-critical fuel blanket located behind a neutron multiplier. Current design envisions tristructural isotropic (TRISO) particles embedded in a graphite matrix as fuel and Be as multiplier, both in pebble bed form and flowing in Flibe molten salt coolant. In previous work, neutron lifetime modeling and design of Be pebbles was discussed [10]. Constitutive equations were derived and a design criteria were developed for spherical Be pebbles on the basis of their thermo-mechanical behaviour under continued neutron exposure in the neutron multiplier for the LIFE engine. Utilizing the available material property data, Be pebbles lifetime could be estimated to be a minimum of 6 years. Here, we investigate the thermo-mechanical response of TRISO particles used for incineration of WUPu under LIFE operating conditions of high temperature and high neutron fast fluence. To this purpose, we make use of the thermo-mechanical fuel performance code HUPPCO, which is currently under development. The model accounts for spatial and time dependence of the material elastic properties, temperature, and irradiation swelling and creep mechanisms. Preliminary results show that the lifetime of WGPu TRISO particles is affected by changes in the fuel materials properties in time. At high fuel burnup, retention of fission products relies on the SiC containment boundary behavior as a minute pressure vessel. The discussion underlines the need to develop high-fidelity models of the performance of these new fuel designs, especially in the absence of a fast neutron source to test these fuels under relevant conditions.© 2010 ASME

Thermo-mechanical and neutron lifetime modelling and design of Be pebbles in the neutron multiplier for the LIFE engine

Concept designs for the laser inertial fusion/fission energy (LIFE) engine include a neutron multiplication blanket containing Be pebbles flowing in a molten salt coolant. These pebbles must be designed to withstand the extreme irradiation and temperature conditions in the blanket to enable a reliable and cost-effective operation of LIFE. In this work, we develop design criteria for spherical Be pebbles on the basis of their thermo-mechanical behaviour under continued neutron exposure. We consider the effects of high fluence and fast fluxes on the elastic, thermal and mechanical properties of nuclear-grade Be. Our results suggest a maximum pebble diameter of 30 mm to avoid tensile failure, coated with an anti-corrosive, high-strength metallic shell to avoid failure by pebble contact. Moreover, we find that the operation temperature must always be kept above 450 °C to enable creep to relax the stresses induced by swelling. Under these circumstances, we estimate the pebble lifetime to be at least 16 months if uncoated, and up to six years when coated. We identify the sources of uncertainty on the properties used and discuss the advantages of new intermetallic beryllides and their use in LIFEs neutron multiplier. To establish Be-pebble lifetimes with improved confidence, reliable experiments to measure irradiation creep must be performed.

Nanoscale Compositional Changes Along Fast Ion Tracks in Equilibrium Solid Solutions: A Computer Simulation of Ultra-Fast Solidification and Thermomigration

Abstract : Starting from two equilibrium solid solutions in the Au-Ni system, we analyze the change in composition due to a 400 eV/A fast ion track simulated by molecular dynamics in the Embedded Atom approximation. We aim at determining the influence of the thermodynamic forces derived from the large thermal gradients and the rapid solidification across the solidus and liquidus on the motion of solute atoms. One dimensional gradients as well as analytic models are used to quantitatively determine the domains of influence of these forces. Evidence shows that the liquidus and solidus equilibrium solidification predicted by the phase diagram is not reached during the track. The solute concentration is mainly determined by the combined diffusion and thermomigration mechanisms in the liquid stage.