Key Questions 2 – Advances in evolutionary concepts


Solid Plasma Facing Components


Recent plasma experiments on ASDEX-U, JT-60U and DIII-D have shown that detached plasma conditions are possible in relatively closed divertors. The detached divertor plasmas radiate most of the plasma thermal energy to the walls of the divertor channel. This reduces the heat load on the divertor surfaces a factor of 3-5 compared to attached divertor operation. Another method for reducing heat loads is to increase radiation in the outer region of the core plasma. Such operating modes have been demonstrated recently. In addition, the low temperature and high-density plasmas found in the divertor are suitable for the use of high Z refractory metals for the plasma-facing surface. With the exception of disruption heat load effects and ELM’s, a refractory metal detached plasma divertor has the potential of having no sputter erosion of the surfaces. The lifetime of such components will be determined by thermal fatigue and/or neutron irradiation. Higher power density devices will require even more care to make the heat loads on the plasma facing components tolerable.

Results from the ITER plasma facing component development program have shown that changes in the size of the pieces of refractory metals placed on a heat sink greatly improve the lifetime of such components. The use of tungsten rods (3 mm dia.) has successfully been tested in thermal steady-state for up to 1000 thermal cycles at 30 MW/m 2. New methods for joining tungsten rods to a copper alloy substrate have also been developed. New joining methods will be needed for refractory metals to refractory heat sinks.

New molybdenum and tungsten alloys have been developed in the Japanese fusion program. The new alloys are formed by the addition of nano-particles of TiC to either Mo or W. The effect of the addition is to increase the recrystallization temperature and lower the ductile to brittle transition temperature. These alloys are only made in small quantities. Even though there are still many questions about how these materials will behave under irradiation, they have the potential of enabling advances in the capability of refractory metal PFCs. Joining of refractory metals is an area that remains to be investigated.

Since refractory metals prefer to operate at high temperatures, the use of helium gas cooling is a logical choice. Several Department of Energy funded Small Business Innovative Research Grants have focused on development of porous metal heat exchangers for gas cooling. The critical heat flux for gas cooling has improved significantly (up to about 30 MW/m 2) as a result of these efforts. Extension of these results to refractory porous metal heat exchangers would make application of high-temperature refractory-metal components to a fusion device possible.

The issues are:
  1. How effective are the improvements that have been made to refractory metals in increasing the useful fatigue life of such materials for plasma facing materials? Under low neutron fluence in near term devices? Eventually for reactors with high fluence?
  2. What are the best joining materials and methods for attaching refractory rods to a refractory heat sink? How high can the operating temperature be? What are the irradiation effects on such materials? What are the failure modes of the joints?
  3. What are the best techniques for fabrication of refractory porous metals for the heat sink? What are the operating temperature limits?
  4. What purity must be achieved in the He gas to prevent damage to the refractory metal heat sink or the porous metal? What is the generation rate of impurities in the coolant? Is tritium an issue for the coolant?
  5. What is the optimum physical arrangement for the coolant passages in a divertor application? How can flow instabilities be avoided under non-ideal conditions?
  6. How can components like these be applied to existing tokamaks or innovative concepts?
  7. What is the trade-off in safety between higher temperature operation versus decay heat of the refractory materials?

First Wall/Blanket Area

An attractive fusion power system should incorporate high power conversion efficiency, the ability to accommodate high power densities, a low failure rate, the capability for faster maintenance and extended component lifetime, adequate tritium breeding, as well as exhibiting favorable safety and environmental features. In order to meet these goals, it will be necessary to push the candidate design concepts and materials to the limits of performance in terms of temperature, mechanical properties, compatibility, and radiation damage. The overall objective of this topic is to address the key issues associated with improving the attractiveness of conventional first wall/blanket systems.

Possible ways of improving performance include reducing the mechanical constraints on the FW/Blanket to reduce thermal strains, simplifying the design to improve reliability and maintenance, moving radiation sensitive joints to the rear of the blanket, inserting insulator breaks to reduce EM forces during disruptions, enhancing the heat transfer coefficient to the coolant to achieve higher power densities.

Optimization of system performance requires compromises among the key areas. For example, higher operating temperatures are possible if the operating stresses of the structure can be reduced. High power density and thermal efficiency may be achievable with the use of refractory metal alloys, like tungsten, but these benefits must be traded-off against a greater difficulty of fabrication and joining for these alloys which can affect system reliability.

Factors to be considered for materials performance include temperature limits of structural materials, operating temperature windows for breeding materials and coolants, radiation lifetime limits, mechanical property limits. Possible ways of improving material performance include increasing thermal conductivity, improving the mechanical properties, improving the radiation damage resistance, improving joint properties, etc.

The issues are:
  1. What are the limiting factors to achieving the desired performance goals?
  2. Are there ways of improving performance through design modification?
  3. Are there ways of improving performance through improved materials both sturctural and non-sturctural?
  4. Are there ways of improving performance through less restrictive design criteria?
Is a code based on existing codes like the ASME B&PV code too restrictive? Where can the code be relaxed? How can design margin be increased?
  1. What R&D is required to achieve improved performance?
  2. What improvements can be made in the power limits of solid breeder materials?