Ken Natesan

Thermodynamics of carbon in nickel, iron-nickel and iron-chromium-nickel alloys

Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH4-H2 gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.

Development of insulating coatings for liquid metal blankets

Abstract It is shown that self-cooled liquid metal blankets in tokamaks with high magnetic fields are feasible only with electrically insulating coatings at the duct walls. The requirements for the insulation properties are estimated by simple analytical models. Candidate insulator materials are selected, based on their insulating properties and thermodynamic considerations. Different fabrication technologies for insulating coatings are described. The status of the knowledge on the most crucial feasibility issue, i.e. the degradation of the resistivity under irradiation, is reviewed.

Vanadium-base alloys for fusion first-wall/blanket applications

Abstract Vanadium alloys have been identified as a leading candidate material for fusion first-wall/blanket applications. Certain vanadium alloys exhibit favorable safety and environmental characteristics, good fabricability, high temperature and heat load capability, good compatibility with liquid metal coolants and resistance to irradiation damage. The current focus is on vanadium alloys with (3–9 wt%) Cr and (3–10 wt%) Ti with a V–4Cr–4Ti alloy as the reference composition. Substantial progress has been made in the development of vanadium alloys for the fusion first wall/blanket applications including production and welding, characterization of baseline properties, corrosion/compatibility, and effects of irradiation on the properties. This paper presents an overview of the development of vanadium alloys for fusion applications and a summary of key issues requiring further research.

The role of metal nanoparticles and nanonetworks in alloy degradation.

Oxide scale, which is essential to protect structural alloys from high-temperature degradation such as oxidation, carburization and metal dusting, is usually considered to consist simply of oxide phases. Here, we report on a nanobeam X-ray and magnetic force microscopy investigation that reveals that the oxide scale actually consists of a mixture of oxide materials and metal nanoparticles. The metal nanoparticles self-assemble into nanonetworks, forming continuous channels for carbon transport through the oxide scales. To avoid the formation of these metallic particles in the oxide scale, alloys must develop a scale without spinel phase. We have designed a novel alloy that has been tested in a high-carbon-activity environment. Our results show that the incubation time for carbon transport through the oxide scale of the new alloy is more than an order of magnitude longer compared with commercial alloys with similar chromium content.

Oxidation performance of V-Cr-Ti alloys

Vanadium-base alloys are being considered as candidates for the first wall in advanced V-Li blanket concepts in fusion reactor systems. However, a primary deterrent to the use of these alloys at elevated temperatures is their relatively high affinity for interstitial impurities, i.e., O, N, H, and C. The authors conducted a systematic study to determine the effects of time, temperature, and oxygen partial pressure (pO{sub 2}) in the exposure environment on O uptake, scaling kinetics, and scale microstructure in V-(4--5) wt.% Cr-(4--5) wt.% Ti alloys. Oxidation experiments were conducted on the alloys at pO{sub 2} in the range of 5 x 10{sup {minus}6}-760 torr (6.6 x 10{sup {minus}4}-1 x 10{sup 5} Pa) at several temperatures in the range of 350--700 C. Models that describe the oxidation kinetics, oxide type and thickness, alloy grain size, and depth of O diffusion in the substrate of the two alloys were determined and compared. Weight change data were correlated with time by a parabolic relationship. The parabolic rate constant was calculated for various exposure conditions and the temperature dependence of the constant was described by an Arrhenius relationship. The results showed that the activation energy for the oxidation process is fairly constant at pO{sub 2} levels in the range of 5 x 10{sup {minus}6}-0.1 torr. The activation energy calculated from data obtained in the air tests was significantly lower, whereas that obtained in pure-O tests (at 760 torr) was substantially higher than the energy obtained under low-pO{sub 2} conditions. The oxide VO{sub 2} was the predominant phase that formed in both alloys when exposed to pO{sub 2} levels of 6.6 x 10{sup {minus}4} to 0.1 torr. V{sub 2}O{sub 5} was the primary phase in specimens exposed to air and to pure O{sub 2} at 760 torr. The implications of the increased O concentration are increased strength and decreased ductility of the alloy. However, the strength of the alloy was not a strong function of the O concentration of the alloy, but an increase in O concentration did cause a substantial decrease in ductility.

Assessment of alkali metal coolants for the ITER blanket

Abstract The blanket system is one of the most important components of a fusion reactor becuase it has a major impact on both the economics and safety of fusion energy. The primary functions of the blanket in a deuterium-tritium-fueled fusion reactor are to convert the fusion energy into sensible heat and to breed tritium for the fuel cycle. The Blanket Comparison and Selection Study, conducted earlier, described the overall comparative performance of various blanket concepts, including liquid metal, molten salt, water, and helium. This paper discusses the International Thermonuclear Experimental Reactor requirements for a self-cooled blanket that uses liquid Li and for indirectly cooled blankets that use other alkali metals such as NaK. The paper addresses the thermodynamics of interactions between the liquid metals (e.g. Li and NaK) and structural materials (e.g. V-base alloys), together with associated corrosion and compatibility issues. Available experimental data are used to assess the long-term performance of the first wall in a liquid metal environment. Other key issues include development of electrically insulating coatings for the first-wall structural material to minimize magnetohydrodynamic (MHD) pressure drop, and tritium permeation and inventory in self-cooled and indirectly cooled blankets. Acceptable types of coating (based on their chemical compatibility and physical properties) are identified, and surface modifications to achieve these coatings on the first wall are discussed. The assessment examines the extent of our knowledge on the performance of structural materials in liquid metals and identifies needed research and development in several areas to establish performance envelopes for the first wall in a liquid metal environment.

Oxidation kinetic studies of zinc sulfide in a fluidized bed reactor

Kinetic studies of the oxidation of zinc sulfide were carried out in a fluidized bed reactor over a temperature range of 740° to 1000°C with O-N gas mixtures of 20 to 40 pct O2. A mathematical model was developed to describe the overall conversion of the solids. Application of the model to the experimental data indicated that the chemical reaction at the outer boundary of the unreacted sulfide core was the rate-limiting step for the process. The temperature dependence of the kinetic constant corresponded to an activation energy of 40,250 cal per mole. Oxygen starvation in the bed was not limiting in any of the experimental runs, but an increase in the inlet-oxygen mole fraction resulted in a substantial increase in reaction rate.

Procedure development of laser welding of V–4Cr–4Ti alloy

Abstract V–4Cr–4Ti alloy is selected as the structure material for the first wall/blanket in a fusion power reactor. A systematic study was conducted to develop a laser welding procedure for fabrication of vanadium alloy for the first wall/blanket systems. A 1.6 kW pulsed Nd:YAG laser with fiber optic beam delivery was used to carry out the bead-on-plate welding on 4 mm thick V–4Cr–4Ti plates. The process parameters, such as laser schedule power settings, beam travel speed, and welding atmosphere control, and their effects on weld quality, such as weld depth, porosity, and oxygen uptake were studied. Results from metallurgical characterization of the welds are presented. An innovative laser welding procedure has been developed to obtain deep penetration, defect-free, and oxygen contamination-free welds.

Experimental and theoretical MHD performance of a round pipe with an NaK-compatible Al2O3 coating

Abstract A key feasibility issue for the international thermonuclear experimental reactor (ITER) vanadium/lithium breeding blanket is the question of insulator coatings. Design calculations show that an electrically insulating layer is necessary to maintain an acceptably low magnetohydrodynamic (MHD) pressure drop. To begin experimental investigations of the MHD performance of candidate insulator materials and the technology for putting them in place, a new test section was prepared. Aluminum oxide was chosen as the first candidate insulating material because it may be used in combination with NaK in the ITER vacuum vessel and/or the divertor and MHD performance tests could begin early in Argonnes liquid-metal experiment (ALEX) because NaK was already the working fluid in use. Details on the methods used to produce the aluminum oxide layer, as well as the microstructures of the coating and the aluminide sublayer, are presented and discussed. The overall MHD pressure drop, local MHD pressure gradient, local transverse MHD pressure difference and surface voltage distributions in both the circumferential and axial directions are reported and discussed. The overall MHD pressure drop, measured at 30 and 85°C, was higher than the perfectly insulating case, but many times lower than the bare-wall case. It was demonstrated that the increase in MHD pressure drop above the theoretical values is largely due to the presence of instrumentation penetrations in the test section walls, which provide current paths from the fluid to the walls of the pipe, resulting in local areas of near-bare-wall MHD pressure drop.

Sodium Fast Reactor Fuels and Materials: Research Needs.

An expert panel was assembled to identify gaps in fuels and materials research prior to licensing sodium cooled fast reactor (SFR) design. The expert panel considered both metal and oxide fuels, various cladding and duct materials, structural materials, fuel performance codes, fabrication capability and records, and transient behavior of fuel types. A methodology was developed to rate the relative importance of phenomena and properties both as to importance to a regulatory body and the maturity of the technology base. The technology base for fuels and cladding was divided into three regimes: information of high maturity under conservative operating conditions, information of low maturity under more aggressive operating conditions, and future design expectations where meager data exist.