Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

e exposed to simultaneously high-heat fluxes (up to 15 mW/m 2 ), high-neutron fluence (up to
20 mW-y/m 2 ), high concentrations of transmutation produced gases (~2000 appm helium and
~8000 appm hydrogen), and high time-varying thermal and mechanical stresses. The effects of
helium on microstructural evolution have been investigated to near fusion-relevant levels in a
limited set of materials systems in time-accelerated ion irradiation experiments, but these simulation
experiments were not able to provide the bulk mechanical and physical property information
needed by fusion power system designers. a unique aspect of the d-t fusion environment is
the simultaneous production of displacement damage and very large quantities of gaseous transmutation
products such as helium and hydrogen. most of the hydrogen may diffuse out, but some
may be trapped at bulk defects and interfaces, enhancing such phenomena as void nucleation and
hydrogen-embrittlement. due to its low solubility, helium precipitates into clusters or gas bubbles.
at high temperatures, grain boundary helium bubbles grow and coalesce under stress, resulting
in severe degradation of creep and fatigue properties. at lower temperatures, there is growing
evidence that high helium synergistically interacts with displacement damage induced hardening,
resulting in severe degradation of fracture toughness and intergranular fracture. at intermediate
temperatures, he bubbles may serve as nucleation sites for growing voids, potentially
leading to swelling and enhanced creep rates. a critical need is the capability to investigate the effects
of neutron irradiation on bulk material properties while producing fusion-relevant levels of
transmutation gases. The greatest need is for structural materials (e.g., ferritic steel for the blanket
and W-alloys for the divertor), but also critical are other materials systems such as high-heat
flux plasma facing components, breeding blanket materials, and a wide variety of functional materials
essential for successful operation of a fusion power system.
gap: Development of high-performance alloys and ceramics, including large-scale fabrication and joining
technologies, is needed.
We anticipate that the current generation of reduced-activation structural materials may not provide
adequate performance for a safe, environmentally sound and economically attractive fusion
power system. There is a compelling need to evolve the next generation of candidate materials to
increased levels of performance. We must seek breakthroughs in materials science and technology
to discover a revolutionary class of high-performance materials for a fusion power system
that fulfills safety, economic and environmental attractiveness goals. This can only be accomplished
by implementing a full-scale alloy and ceramics development program. This includes the
need to research large-scale fabrication and joining technologies for complex structures, including
investigation of near-net-shape fabrication technologies that may provide substantially lower
delivered costs for high-performance components. The most appropriate fabrication and joining
techniques significantly depend on material selection, component geometry and expected service
conditions. The traditional approach to developing the needed technologies relies heavily on
trial- and-error approaches. a science-based program is needed to determine the applicability of
recently developed technologies (for example, friction stir welding) for joining materials such as
nanostructured ferritic alloys.
153