R.F. Mattas

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.

Comparison of lithium and the eutectic lead-lithium alloy, two candidate liquid metal breeder materials for self-cooled blankets

Abstract Liquid metal blankets are attractive candidates for both near-term and long-term fusion applications. The subjects of this comparison are the differences between the two candidate liquid metal breeder materials Li and LiPb for use in breeding blankets in the areas of neutronics, magnetohydrodynamics, tritium control, compatibility with structural materials, heat extraction system, safety, and required research and development program. Both candidates appear to be promising for use in self-cooled breeding blankets which have inherent simplicity with the liquid metal serving as both breeder and coolant. Each liquid metal breeder has advantages and concerns associated with it, and further development is needed to resolve these concerns. The remaining feasibility question for both breeder materials is the electrical insulation between the liquid metal and the duct walls. Different ceramic coatings are required for the two breeders, and their crucial issues, namely self-healing of insulator cracks and tolerance to radiation-induced electrical degradation, have not yet been demonstrated.

ALPS–advanced limiter-divertor plasma-facing systems

The advanced limiter-divertor plasma-facing systems (ALPS) program was initiated in order to evaluate the potential for improved performance and lifetime for plasma-facing systems. The main goal of the program is to demonstrate the advantages of advanced limiter:divertor systems over conventional systems in terms of power density capability, component lifetime, and power conversion efficiency, while providing for safe operation and minimizing impurity concerns for the plasma. Most of the work to date has been applied to free surface liquids. A multi-disciplinary team from several institutions has been organized to address the key issues associated with these systems. The main performance goals for advanced limiters and divertors are a peak heat flux of \ 50 MW:m 2 , elimination of a lifetime limit for erosion, and the ability to extract useful heat at high power conversion efficiency (40%). The evaluation of various options is being conducted through a combination of laboratory experiments, www.elsevier.com:locate:fusengdes

Crucial issues on liquid metal blanket design

Abstract Typical design concepts of liquid metal breeder blankets for power reactors are explained and characterized. The major problems of these concepts are described for both water-cooled blankets and self-cooled blankets. Three crucial issues of liquid metal breeder blankets are investigated. They are in the fields of magnetohydrodynamics, tritium control and safety. The influence of the magnetic field on liquid metal flow is of special interest for self-cooled blankets. The main problems in this field and the status of the related R&D-work are described. Tritium permeation losses to the cooling water is a crucial issue for water-cooled blankets. Methodes for its reduction are discussed. An inherent problem of all liquid breeder blankets is the potential release of activated products in the case of chemical reactions between the breeder material and water or reactive gases. The most important issues in this field are described.

Development of electrically insulating coatings on vanadium alloys for lithium-cooled blankets

The self-cooled lithium blanket concept with a vanadium structure offers a potential for high performance with attractive safety and environmental features. Based on blanket design studies, it became apparent that electrically insulating duct walls would be required to reduce the magnetohydrodynamic (MHD) pressure drop for liquid metal-cooled blankets for high magnetic field fusion devices. As a result, development of insulator coatings was recommended as the most appropriate approach for resolving this issue. Oxides such as CaO, Y 2 O 3 . BeO. MgO, MgAl 2 O 4 , and Y 3 Al 2 O 12 and nitrides such as A1N, BN and Si 3 N 2 were initially considered potential candidate coating materials. Based on results of scoping studies, CaO and A1N have been selected as primary candidates for further development. Progress on the development of CaO and A1N coatings, including in-situ formation and electrical properties measurements, are summarized in this paper.

Liquid lithium self-cooled breeding blanket design for ITER

Abstract To meet the technical objectives of the ITER extended performance phase (EPP) an advanced tritium breeding lithium/vanadium (Li/V) blanket was developed by two home teams (US and RF). The design is based on the use of liquid Li as coolant and breeder and vanadium alloy (V-Cr-Ti) as structural material. The first wall is coated with a beryllium protection layer. Beryllium is also integrated in the blanket for neutron multiplication and improved shielding. The use of tungsten carbide in the primary shield and in vacuum vessel provides adequate protection for toroidal field coils. A self-healing electrical insulator in the form of CaO or AlN coating layer is utilized to reduce MHD pressure drop in the system. To have a self-consistent ITER design, liquid metal cooling of the divertor and vacuum vessel is considered as well.

Materials integration issues for high performance fusion power systems

One of the primary requirements for the development of fusion as an energy source is the qualification of materials for the frost wall/blanket system that will provide high performance and exhibit favorable safety and environmental features. Both economic competitiveness and the environmental attractiveness of fusion will be strongly influenced by the materials constraints. A key aspect is the development of a compatible combination of materials for the various functions of structure, tritium breeding, coolant, neutron multiplication and other special requirements for a specific system. This paper presents an overview of key materials integration issues for high performance fusion power systems. Issues such as: chemical compatibility of structure and coolant, hydrogen/tritium interactions with the plasma facing/structure/breeder materials, thermomechanical constraints associated with coolant/structure, thermal-hydraulic requirements, and safety/environmental considerations from a systems viewpoint are presented. The major materials interactions for leading blanket concepts are discussed.

Experimental results for Phase II of the JAERI/USDOE collaborative program on fusion blanket neutronics

As the first period of the Phase II series of the JAERI/USDOE collaborative program, neutronic parameters have been measured for a simulated Li 2 O/Be breeder blanket in closed geometry. The experimental system consists of a lithium-oxide test zone and a lithium carbonate enclosure containing a DT neutron source at the Fusion Neutronics Source (FNS) facility at JAERI. Tested blankets were of three 5 cm thick configurations of beryllium neutron multiplier zone. The experiments were performed to examine spatial distributions of reaction rates and the neutron spectrum in the source cavity, and relative profiles of the tritium production rate (TPR), reaction rates, and neutron spectra between the beryllium configurations. A zonal TPR measuring technique, suitable especially for direct comparison with a Monte Carlo method, was applied to a steep gradient distribution. The experimental results of TPRs showed that the beryllium sandwiched system provided the most effective TBR gain (integrated TPR) of about 20% compared with the non-multiplier system. The reaction rate distributions and neutron energy spectra were also provided to test a calculational code system for nuclear design.

Results of R&D for lithium/vanadium breeding blanket design

Abstract The self-cooled lithium/vanadium blanket concept has several attractive features for fusion power systems, including reduced activation, resistance to radiation damage, accommodation of high heat loads and operating to temperatures of 650–700°C. The primary issue associated with the lithium/vanadium concept is the potentially high MHD pressure drop experienced by the lithium as it flows through the high magnetic field of the tokamak. The solution to this issue is to apply a thin insulating coating to the inside of the vanadium alloy to prevent the generation of eddy currents within the structures that are responsible for the high MHD forces and pressure drop. This paper presents the progress in the development of an insulator coating that is capable of operating in the severe fusion environment, progress in the fabrication development of vanadium alloys, and a summary of MHD testing. A large number of small scale tests of vanadium alloy specimens coated with CaO and A1N have been conducted in liquid lithium to determine the resistivity and stability of the coating. In-situ measurements in lithium have determined that CaO coatings, ∼5 μm thick, have resistivity times thickness values ( p * t ) exceeding 10 6 Ω cm 2 . These results have been used to identify fabrication procedures for coating a large vanadium alloy (V–4Cr–4Ti) test section that was tested in the ALEX (Argonne Liquid metal Experiment) facility. Similar test sections have been produced in both Russia and the USA.

Materials for impurity control

Abstract The materials data base for candidate impurity control materials is reviewed. The plasma side materials considered are Be, C, SiC, TiC, and W, and the heat sink materials considered are copper alloys and vanadium alloys. The properties which are evaluated are physical and chemical sputtering, bulk properties, and irradiation properties. Design implications on design are also discussed.