Robert W. Conn

First wall and divertor plate material selection in fusion reactors

Abstract The criteria for selecting first wall materials in magnetic and inertial confinement fusion reactors are discussed. These criteria include radiation damage, compatibility, thermomechanical properties, fabricability, joining, industrial capability, the existing data base, cost, induced radioactivity, and resource availability. At present, stainless steel remains the primary choice for a structural material because of the large existing data base and industrial capability. Titanium and vanadium alloys are the primary backup materials. The influence of surface heating on the allowable neutron wall loading is described in detail for magnetic and ICF reactors. The use of divertors may permit reactors to operate with higher neutron wall loadings but heat transfer and excessive wall erosion by sputtering of the collector plates are fundamental problems requiring resolution before divectors are employed in reactors.

Analytic fits to several atom–diatom abinitio potential hypersurfaces

Some recent available ab initio atom–diatom potential hypersurfaces are investigated. It is found that the behavior of these interaction potentials with respect to the atom–molecule distance and molecular bond distance can be rather accurately described by a simple functional form involving two exponential terms. The description appears to apply equally well to SCF or Gordon–Kim electron gas results. Fit parameters at various angles are presented for the systems Ar–HF, He–HF, He–CO, Li+–CO, Li+–N2, and Li+–H2. These potential fits are particularly well adapted to use as input to treatments of vibrationally inelastic collision processes.

Interaction potentials for He--HF and Ar--HF using the Gordon--Kim method

Interaction potentials computed by the method of Gordon and Kim are presented for the He–HF and Ar–HF systems. Results are given at five angles and five HF distances for 12 He–HF separations and 14 Ar–HF separations, spanning both the shallow attractive well and part of the repulsive core in each case. Agreement with variationally determined results for He–HF reported previously is very good and similar accuracy is expected for the Ar–HF results. These potentials should therefore be suitable for scattering calculations.

Space-dependent effects on the Lawson and ignition conditions and thermal equilibria in tokamaks

The Lawson and ignition criteria are sensitive to the plasma density and temperature profiles and it is found that peaked profiles lead to a considerable easing of these criteria, as well as to a significant increase in the average fusion power density. Space and time-dependent reactor plasma calculations show that these profiles may indeed be sharply peaked at the centre owing to alpha heating and to rapid diffusion near the edge. The thermal equilibria are sensitive to the size of the device, to the conditions imposed at the plasma boundary and to the fuelling profile. A comparison between tokamak plasma equilibrium parameters predicted by often used global energy balance models and by space-dependent calculations indicates that the global-model results can differ sharply from the space-dependent calculations with regard to mean temperature, mean density, particle confinement time and fractional burn-up. In particular, the fractional burn-up predicted from global calculations can be too high and therefore strongly affect the anticipated tritium inventory and re-fuelling problems in tokamak reactors.

Lower Activation Materials and Magnetic Fusion Reactors

Radioactivity in fusion reactors can be effectively controlled by materials selection. The detailed relationship between the use of a material for construction of a magnetic fusion reactor and the materials characteristics important to waste disposal, safety, and system maintainability has been studied. The quantitative levels of radioactivation are presented for many materials and alloys, including the role of impurities, and for various design alternatives. A major outcome has been the development of quantitative definitions to characterize materials based on their radioactivation properties. Another key result is a four-level classification scheme to categorize fusion reactors based on quantitative criteria for waste management, system maintenance, and safety. A recommended minimum goal for fusion reactor development is a reference reactor that (a) meets the requirements for Class C shallow land burial of waste materials, (b) permits limited hands-on maintenance outside the magnets shield within 2 days of a shutdown, and (c) meets all requirements for engineered safety. The achievement of a fusion reactor with at least the characteristics of the reference reactor is a realistic goal. Therefore, in making design choices or in developing particular materials or alloys for fusion reactor applications, consideration must be given to both the activation characteristics of amorexa0» material and its engineering practicality for a given application.«xa0less

Plasma modeling and first wall interaction phenomena in Tokamaks

Abstract Fluxes of neutral hydrogenic particles, alpha particles and electromagnetic radiation to the liner or first wall of both two component tokamaks and large ignited tokamak fusion reactors are estimated using space and time dependent models of tokamak plasmas. Data requirements and the effects of uncertainties in the areas of first wall interaction phenomenon and plasma transport coefficients are described.

Characteristic vibrational coupling behavior of intermolecular potentials

Some recently available intermolecular potentials have been investigated and cast into a form suggestive of similar vibrational coupling behavior. The underlying physical factors behind these similarities are discussed. It is argued that a typical potential will exhibit an intermolecular distance(s) at which the vibrational coupling vanishes. It is expected that knowledge of this behavior will be particularly useful in model calculations of vibrationally inelastic collision processes.

The potential of driven tokamaks as thermonuclear reactors

Unignited, and therefore externally driven, tokamak plasmas can be the core for economic tokamak power reactors over a significant range of machine sizes where the plasma amplification factor Q exceeds 15. In such driven systems, high-Z impurity levels from 2 to 4 times that in ignited plasmas can be tolerated. Large driven tokamaks with plasma currents greater than 4 MA achieve the highest Q-value by operating with a 50-50 D-T plasma using the minimum amount of injected power to maintain the plasma in energy equilibrium. Driving large tokamaks with injection can also be used to extend the reactor burn time should impurities accumulate. For smaller tokamaks, the optimum Q is obtained by injecting deuterium neutral beams at maximum power onto a pure tritium plasma and operating the device in the two-energy-component mode described by Dawson, Furth and Tenney. A natural connection between these two operating modes is thus obtained. The potential benefits of low-Z liners in tokamaks are also studied and it is found that the tolerable impurity level of a contaminant like carbon can be an order of magnitude larger than the tolerable level of impurities like Fe or Mo.

Space-dependent thermal stability of reacting tokamak plasmas

A technique is presented for the analysis of thermal stability in reacting tokamak plasmas using a one-dimensional, time-dependent fluid transport model. Application is made to the analysis of density-related thermal instabilities in a neutral-beam-driven, two-component plasma (TETR) and a conceptual reactor-size ignited plasma (UWMAK-III). A density-driven thermal instability can exist when the particle confinement varies as τp ∝ n. This condition is satisfied by the trapped-ion-mode diffusion model and an empirical model. A time delay in the heating due to finite alpha thermalization does not significantly alter the character of the instability at normal plasma densities. A linear feedback response for the particle source is found to provide a stabilized equilibrium in all cases. Strong radial variation of the transport and physical properties of the plasma is not found to introduce radial-dependent feedback requirements. Feedback on the average density is sufficient for stabilization with moderate response times.

Tokamak reactors and structural materials

Abstract A summary is given of the status of the tokamak as a reactor concept and the relationship between first wall and blanket materials behavior and reactor design.