S. Sharafat

On the exploration of innovative concepts for fusion chamber technology

Abstract This study, called APEX, is exploring novel concepts for fusion chamber technology that can substantially improve the attractiveness of fusion energy systems. The emphasis of the study is on fundamental understanding and advancing the underlying engineering sciences, integration of the physics and engineering requirements, and enhancing innovation for the chamber technology components surrounding the plasma. The chamber technology goals in APEX include: (1) high power density capability with neutron wall load >10 MW/m 2 and surface heat flux >2 MW/m 2 , (2) high power conversion efficiency (>40%), (3) high availability, and (4) simple technological and material constraints. Two classes of innovative concepts have emerged that offer great promise and deserve further research and development. The first class seeks to eliminate the solid “bare” first wall by flowing liquids facing the plasma. This liquid wall idea evolved during the APEX study into a number of concepts based on: (a) using liquid metals (Li or Sn–Li) or a molten salt (Flibe) as the working liquid, (b) utilizing electromagnetic, inertial and/or other types of forces to restrain the liquid against a backing wall and control the hydrodynamic flow configurations, and (c) employing a thin (∼2 cm) or thick (∼40 cm) liquid layer to remove the surface heat flux and attenuate the neutrons. These liquid wall concepts have some common features but also have widely different issues and merits. Some of the attractive features of liquid walls include the potential for: (1) high power density capability; (2) higher plasma β and stable physics regimes if liquid metals are used; (3) increased disruption survivability; (4) reduced volume of radioactive waste; (5) reduced radiation damage in structural materials; and (6) higher availability. Analyses show that not all of these potential advantages may be realized simultaneously in a single concept. However, the realization of only a subset of these advantages will result in remarkable progress toward attractive fusion energy systems. Of the many scientific and engineering issues for liquid walls, the most important are: (1) plasma–liquid interactions including both plasma–liquid surface and liquid wall–bulk plasma interactions; (2) hydrodynamic flow configuration control in complex geometries including penetrations; and (3) heat transfer at free surface and temperature control. The second class of concepts focuses on ideas for extending the capabilities, particularly the power density and operating temperature limits, of solid first walls. The most promising idea, called EVOLVE, is based on the use of a high-temperature refractory alloy (e.g. W–5% Re) with an innovative cooling scheme based on the use of the heat of vaporization of lithium. Calculations show that an evaporative system with Li at ∼1 200°C can remove the goal heat loads and result in a high power conversion efficiency. The vapor operating pressure is low, resulting in a very low operating stress in the structure. In addition, the lithium flow rate is about a factor of ten lower than that required for traditional self-cooled first wall/blanket concepts. Therefore, insulator coatings are not required. Key issues for EVOLVE include: (1) two-phase heat transfer and transport including MHD effects; (2) feasibility of fabricating entire blanket segments of W alloys; and (3) the effect of neutron irradiation on W.

Theory of helium transport and clustering in materials under irradiation

A theoretical model is developed to describe helium transport and clustering during irradiation. Diffusional reactions of helium with vacancies and vacancy clusters, with extended sinks for helium absorption, and direct reactions with displacement producing particles are included. The full description developed is employed in numerical computations. A simpler description is also developed in the limits where certain reactions are unimportant. Analytical expressions for the effective diffusion coeficient of helium are derived. Regimes of the parameter space of dose rate, temperature, helium generation rate, sink strength and other important conditions, where the effective diffusion coefficient is dictated by three different physical processes, are defined. The result is determined by the dominant release mechanism of helium bound in vacancies-thermal detrapping, replacement by the self-interstitial or direct displacement. Results from the full computations and the analytical expressions are compared.

The ARIES-I Tokamak Reactor Study †

The ARIES research program is a multi-institutional effort to develop several visions of tokamak reactors with enhanced economic, safety, and environmental features. Three ARIES visions are currently planned for the ARIES program. The ARIES-I design is a DT-burning reactor based on modest extrapolation from the present tokamak physics data base; ARIES-II is a DT-burning reactor which will employ potential advances in physics; and ARIES-III is a conceptual D-3He reactor. The first design to be completed is ARIES-I, a 1000 MWe power reactor. The key features of ARIES-I are: (1) a passively safe and low environmental impact design because of choice of low activation material throughout the fusion power core, (2) an acceptable cost of electricity, (3) a plasma with performance as close as possible to present-day experimental achievements, (4) a high performance, low activation, SiC composite blanket cooled by He, and (5) an advanced Rankine power cycle as planned for near term coal-fired plants. The ARIES-I research has also identified key physics and technology areas with the highest leverage for achieving attractive fusion power system.

Results of an International Study on a High-Volume Plasma-Based Neutron Source for Fusion Blanket Development

AbstractAn international study conducted by technical experts from Europe, Japan, Russia, and the United States has evaluated the technical issues and the required testing facilities for the develo...

A NUMERICAL SOLUTION TO THE FOKKER-PLANCK EQUATION DESCRIBING THE EVOLUTION OF THE INTERSTITIAL LOOP MICROSTRUCTURE DURING IRRADIATION

Abstract A new calculational method has been developed for the numerical solution of the Fokker-Planck equation describing voids and interstitial loops. Small-size interstitial clusters were studied using a detailed rate theory approach, while largesize loops were simulated by descretizing a transformed Fokker-Planck equation. Interstitial loops containing up to millions of atoms were investigated using this hybrid technique. The numerical results of the model compare reasonably well with previous detailed rate theory calculations, as well as with experimental findings on heavy ion irradiated 316 stainless steel.

Plasma spraying of micro-composite thermal barrier coatings

The thermal barrier coatings (TBCs) by gas tunnel-type plasma spraying exhibited ceramic-composite features consisting of a host oxide matrix ceramic with an embedded second phase material.The densities of the composite TBC were found to be higher than those sprayed with 100 wt% ZrO2 or Al2O3.In the coatings produced with powder mixtures of 50 wt%, the embedded splats are found to have a relatively uniform thickness between 1 and 10mm and they exhibited clear and pore-free interfaces with the host material.The micro-composite coatings also exhibited thicknessdependent functionally gradient Vickers hardness values by the hardness measurements across the coating thickness.A one-dimensional series heat transfer model was developed to estimate upper and lower bounds of the transverse thermal resistance as a function of alumina–zirconia weight ratio.The model shows that the addition of higher thermally conducting Al2O3 can result in an increase in the transverse thermal resistivity of YSZ. r 2002 Elsevier Science Ltd. All rights reserved.

ARIES-I fusion-power-core engineering

Abstract The ARIES research program is a multi-institutional project, the goal of which is to determine the economic, safety, and environmental potential of tokamak fusion reactors. The ARIES-I steady-state tokamak reactor is a conceptual, DT-burning, 1000 MWe reactor with a major radius of 6.75 m, a minor radius of 1.5 m, and an average neutron wall loading of 2.5 MW/m2. The ARIES-I plasma operates in the first MHD stability regime with a toroidal beta of 1.9%. The choice to operate in the first stability regime, with a high aspect ratio and with a low plasma current, leads to the need for high magnetic field to achieve adequate fusion power density (β2B4). The toroidal field at the plasma center is 11 T and the maximum field at the coil is 21 T. Nonetheless, it is found that the maximum stress in the structural material of these magnets is ∼ 700 MPa and industrially available alloys can be used. The impurity-control and particle-exhaust system is based on a high recycling double-null divertor system. The low-activation silicon-carbide (SiC) composite is used as structural material. The breeder material, Li2ZrO3, and the multiplier material, Be, are both sphere-packed between poloidally nested SiC-composite shells. The divertor plates consist of SiC-composite tube shells protected with 2 mm-thick tungsten armor. The first wall, blanket, shield, and divertor are all helium cooled with an inlet coolant temperature of 350°C at a pressure of 10 MPa. The high helium-outlet temperature of 650°C ensures a relatively high gross thermal efficiency of 49%. The ARIES-I design has demonstrated that tokamak reactors have the potential to achieve a high level of safety coupled with a Class-C waste-disposal rating.

The Science and Technologies for Fusion Energy With Lasers and Direct-Drive Targets

We are carrying out a multidisciplinary multi-institutional program to develop the scientific and technical basis for inertial fusion energy (IFE) based on laser drivers and direct-drive targets. The key components are developed as an integrated system, linking the science, technology, and final application of a 1000-MWe pure-fusion power plant. The science and technologies developed here are flexible enough to be applied to other size systems. The scientific justification for this work is a family of target designs (simulations) that show that direct drive has the potential to provide the high gains needed for a pure-fusion power plant. Two competing lasers are under development: the diode-pumped solid-state laser (DPPSL) and the electron-beam-pumped krypton fluoride (KrF) gas laser. This paper will present the current state of the art in the target designs and lasers, as well as the other IFE technologies required for energy, including final optics (grazing incidence and dielectrics), chambers, and target fabrication, injection, and tracking technologies. All of these are applicable to both laser systems and to other laser IFE-based concepts. However, in some of the higher performance target designs, the DPPSL will require more energy to reach the same yield as with the KrF laser.

Development Status of a SiC-Foam Based Flow Channel Insert for a U.S.-ITER DCLL TBM

Abstract The U.S.-ITER DCLL (Dual Coolant Liquid Lead) TBM (Test Blanket Module) uses a Flow Channel Insert (FCI), to test the feasibility of high temperature DCLL concepts for future power reactors. The FCI serves a dual function of electrical insulation, to mitigate MHD effects, and thermal insulation to keep steel-PbLi interface temperatures below allowable limits. As a non-structural component, the key performance requirements of the FCI structure are compatibility with PbLi, long-term radiation damage resistance, maintaining insulating properties over the lifetime, adequate insulation even in case of localized failures, and manufacturability. The main loads on the FCI are thermally induced due to through the thickness temperature gradients and due to non-uniform PbLi temperatures along the flow channel (∼1.6 m). A number of SiC-based materials are being developed for FCI applications, including SiC/SiC composites and porous SiC bonded between CVD SiC face sheets. Here, we report on an FCI design based on open-cell SiC-foam material. Thermo-mechanical analysis of this FCI concept indicate that a SiC-foam FCI structure is capable of withstanding anticipated primary and secondary stresses during operation in an ITER TBM environment. A complete 30 cm long prototypical segment of the FCI structure was designed and is being fabricated, demonstrating the SiC-foam based FCI structure to be very low-cost and viability candidate for an ITER TBM FCI structure.

Development of composite thermal barrier coatings with anisotropic microstructure

A novel ZrO 2 }Al 2 O 3 thermal barrier composite coating was produced using a gas-tunnel-type plasma spraying torch. To enhance visualization of the microstructure features, image enhancement techniques were used during the microphotograph digitization processes. The unique microstructure features of this composite coating include a relatively even distribution of embedded thin ZrO 2 splats in a Al 2 O 3 matrix, parallel alignment of the ZrO 2 splats relative to the substrate surface, absence of porosity between the ZrO 2 splats and the Al 2 O 3 matrix, etc. This anisotropic composite coating combined with the large di!erence in thermal conductivity between ZrO 2 and Al 2 O 3 will alter the thermal behavior of the coating. ( 2000 Elsevier Science Ltd. All rights reserved.