Thomas J. Dolan

Electrostatic‐Inertial Plasma Confinement

Electrostatic‐inertial plasma confinement consists of trapping charged particles in potential wells of the electric field, which are created by ions or electrons injected radially inwards into a hollow sphere or cylinder. Theoretical expressions are derived for the potential and particle densities as functions of radius, grid voltage, and current. A neon plasma is produced in cylindrical geometry, using a grid 4 cm in diameter and 16 cm long. Using the laser heterodyne technique at 0.6401 and 0.6328 μ, the density of neon atoms in the 1s5 metastable state is measured (109−1012 cm−3) as a function of radial position, time, grid current (20 ‐ μsec pulses of up to 4 A), grid voltage (0.2–3.0 kV), gas pressure (0.001–0.01 Torr), and grid mesh spacing, and compared with theoretical predictions. The peak electron density is 1011 cm−3. When the spacing between grid wires is larger than 1 mm, a decrease in metastable density, attributed to the presence of a plasma sheath around the grid wires, is observed. The ra...

Preclude failure: A philosophy for materials selection and simulated service testing

Failure of products in service under foreseeable conditions often subject manufacturers to legal liability when personal injuries occur; hence, the care used (or the philosophy employed) in material selection, design, analyses, fabrication and maintenance must be sufficient to preclude failure. Failure analysis requires careful sorting of a wide variety of information to determine how and why a metal part failed in service or in testing and to determine what can be done to prevent a recurrence. Valuable knowledge is available in the literature from documentation of prior failures that may be used to develop logical approaches to the design and development of new components. A philosophy of design and prototype evaluation based on the prevention of failure is more sound and workable than the stereo-typed application of empiricisms, codes, specifications and factors of safety now commonly used. Reliance on design for static loadings and for factors of safety based on tensile strength as a criterion are frequently erroneous and dangerous. If a part does not fulfill its intended function satisfactorily, it “fails” by: (a) excessive deformation, (b) fracture, (c) surface disintegration, and (d) deterioration of properties. A variety of failures will be discussed to emphasize the factors that must be considered in selection of an optimum material and in prescribing an effective method of simulated-service testing. Considerable latitude in use and misuse of equipment must be foreseen in order to predict possible modes of failure. A broad consideration of service environment is necessary for correcting faulty design and selecting proper materials which will withstand modifications due to processing, fabrication, maintenance or repair operations that lead to failure.

Safety and Environment

Tritium and tokamak dust are the main radioactive hazards of fusion reactors. Tritium emits a low-energy beta ray with a half-life of 12.3 years. It is hazardous if inhaled or ingested, but cannot penetrate the skin. The tritium inventory in the fuel system and walls should be well contained, minimized, and closely monitored, to keep the source term low in case of an accident. Neutron absorption will make reactor internal components radioactive, so their radioactivities will be minimized by design, with a goal of clearance or recycling most materials after a cooling period of 50–100 years. If many superconducting cables and coils are used in industry and in fusion reactors, shortages of materials such as He and Nb may occur. The ITER safety team is analyzing dozens of potential accident scenarios to prevent them or to mitigate their consequences, so that public safety will be assured without the need for an evacuation plan.

“Models” of the fatigue process

Some of the instabilities in behavior of material and the structural readjustments that occur to alter the relationships between load and peak stress are discussed by visualizing each level of observation as a type of idealized “model” representing the structural behavior. Nonlinearities and chance effects occur in making predictions from models on the atomistic or crystalline scale to those of specimens, structural components, or full-scale structures. These nonlinear effects, together with the statistical aspects of material behavior, load history, and manufacturing variables, are emphasized to illustrate the difficulty in present-day practice of making any quantitative prediction of fatigue life or fatigue strength of the final product.

Power Plant Designs

The electrical power companies have specified what features are desired for attractive power plants, with regard to economics, regulatory simplicity, and public acceptance. The plants should achieve high availability, and should have maintenance procedures that can be performed in a few months, which is very difficult for large fusion reactors. A company must borrow money to build a power plant long before it starts earning revenue, so short construction times are important, and the total capital cost is often about twice as high as the direct capital cost. Large fusion power plants (3 GWe vs. 1 GWe) have an economy of scale that reduces the cost of electricity by about 20–30 %, but grid perturbation during shutdown would be a problem. Fusion power plant design studies in Europe, Japan, China, and the USA have estimated the cost of fusion-power electricity to be higher than from fission power and fossil fuels, but fusion could become competitive under several possible scenarios.

Risk and Failure Analysis for Improved Performance and Reliability

The care and philosophy employed in material selection, design analyses, fabrication, and maintenance must be sufficient to limit the risk of failure. Failure analysis requires careful sorting of a variety of information to determine how and why a metal part failed, and to prevent a recurrence. To improve safety and reliability, a philosophy of design and prototype evaluation based on the risk of failure is more sound than the stereotyped application of empiricisms, codes, specifications and factors of safety commonly used. Designers must document all conceivable failures in a system, determine by analyses the effect on system operation, and rank the risk of each potential failure according to its combined influence of severity and probability of occurrence. Design codes based upon handbook values for properties of materials are often misleading. A probability of failure exists due to the many uncertainties or variability of the basic structural reactions of a metal; significant changes in mechanical behavior occur due to processing operations, field repairs, adverse or unforeseen loadings and environment, or deterioration with time, temperature, or operating conditions. Consideration must be given to man-machine interactions to prevent accidents in complex systems. Considerable latitude in use and misuse of equipment must be foreseen in order to predict and evaluate the resistance to each possible mode of failure. Careful consideration of the complete life cycle is necessary for selecting optimum materials that will withstand the modifications due to processing and service history, yet provide minimum risk of failure with improved safety and reliability.

2 – Electricity production

This chapter examines the basic options available to converting thermal energy from high temperature molten salt into mechanical shaft work from a turbine and the resultant electrical energy from the generator coupled to the turbine shaft.

Issues and conclusions

Abstract The USA demonstrated the feasibility of molten salt reactors (MSRs) with the Aircraft Reactor Experiment (1954) and the Molten Salt Reactor Experiment (1965–69). Liquid fuel MSRs can avoid many of the problems of light-water reactors (LWRs): fuel manufacture, fuel lifetime, refueling shutdowns, core melt (TMI accident), steam explosion (Chernobyl accident), hydrogen explosion (Fukushima Daichi accident), and long-lived radioactive (actinide) waste disposal. Thorium fuel can be converted into U-233 fuel, instead of using U-235 from enrichment of natural uranium. One ton of ThO 2 can generate as much energy as 293 tons of U 3 O 8 , and thorium is four times as abundant in the Earth’s crust as uranium. Solid fuel MSRs could be similar to LWRs with molten salt coolant instead of water, so they could be developed quickly, but would lack the advantages of liquid fuel, which include no manufacture of fuel pellets, no fuel melt hazard, fuel burnup not limited by radiation damage, continuous refueling, actinide recycling, and fission product removal. The fuel processing plant must be developed to separate uranium, thorium, actinides, and fission products in a highly radioactive environment. Actinides generated by LWRs could be burned in MSRs, instead of being treated as radioactive waste requiring geological disposal. Research on MSRs and thorium energy is underway in 23 countries, and reactor designs from several companies are described in this book.

Plasma Heating and Current Drive

We must heat the plasma to about 10 keV to initiate fusion reactions, and, in a tokamak, provide a means to sustain the plasma current. Heating is done by the plasma current (ohmic heating) and with auxiliary heating using radio waves, microwaves, and high-energy neutral beam injection. Fusion product alpha particles also heat the plasma as they slow down. If alpha heating is powerful enough, the external auxiliary sources may be turned off (except if required for current drive in tokamaks). This chapter describes these heating and current drive systems, the hardware that they require, the current achievements, and plans for ITER.

First Wall, Blanket, and Shield

The first wall, blanket, and shield must withstand a high heat flux, remove hundreds of MW of heat, minimize impurity flow into the plasma, survive neutron and gamma radiation damage, breed tritium fuel, maintain a low tritium inventory, sustain large temperature differences, support high stress levels, avoid safety hazards, shield the coils and external environment, avoid neutron streaming through ducts, be reliable, and be maintainable. High heat flux components, such as helium-cooled tungsten armor tiles, are being developed. Reduced activation ferritic/martensitic (RAFM) steel is the preferred structural material, and silicon carbide may be developed in the future for higher temperature operation. To achieve a satisfactory tritium breeding ratio it may be necessary to use enriched 6Li or a beryllium neutron multiplier. Monte Carlo computer codes can simulate 3D models of neutron and gamma transport, tritium breeding, shielding and radioisotope production. Several blanket configurations with cooling by helium, by PbLi liquid metal, or by molten salt, are being developed, and some of them will be tested in ITER tritium breeding modules. Rankine or Brayton cycle heat engines may be used to generate electricity with efficiency ≳40 %, and fusion energy may also be used for other applications, such as hydrogen production.