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

number of complementary facilities to fill in the technology gaps and evaluate component reliability
and lifetimes.
DEMO Requirements
There are three elements involved in achieving the required availability:
• developing a tokamak design that facilitates reliable operation and efficient maintenance
and inspectability.
• Qualifying components to meet their reliability requirements.
• Providing an effective remote handling system and facility design.
Designing for availability
The main design challenges for availability are the problems caused by 14 mev neutrons, which
damage the materials that surround the plasma and induce radioactivity. after relatively little irradiation,
metals can no longer be welded because of helium buildup, and bolts swell. These effects
have a profound influence on design because neutrons stream down gaps and can scatter around
corners. a rough rule for protecting sensitive areas — such as cooling pipe welds, bolts, in-vessel
diagnostics and control components — at the demo neutron fluence is that neutrons would have
to scatter through a right angle three or more times to reach the area.
over the years, numerous approaches to designing for high maintainability and availability have
been studied: for example, constructing the toroidal system in wedges, and having large ports to
allow large components to be removed radially and/or vertically; standardizing components; using
redundancy; and having in-service monitoring. nevertheless, many designs begin to resemble
interlocking block, wooden puzzles.
high-level design choices must take into account reliability and maintainability impacts. For instance,
liquid metal divertors may be needed to survive plasma transients and provide adequate
lifetime. advanced fuel cycles, e.g., a catalyzed d-d fuel cycle (with tritium sequestration), which
avoid the need for complex tritium breeding blankets and have reduced neutron yield promoting
long component lifetimes, may be required for power reactor applications. Plasma elongation may
be limited by robust plasma position control with a simple, survivable complement of diagnostics.
external transform may be required for robust disruption and elm avoidance.
it is essential:
• to analyze and test approaches for segmented neutron capture and shielding, and remote
handling.
• to develop improved integrated designs that satisfy availability and safety requirements
by applying lessons learned from iteR, other experiments and remotely handled facilities;
and making design choices that promote reliability and maintainability.
161