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

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number of complementary facilities to fill in the technology gaps and evaluate component reliability<br />

and lifetimes.<br />

DEMO Requirements<br />

There are three elements involved in achieving the required availability:<br />

• developing a tokamak design that facilitates reliable operation and efficient maintenance<br />

and inspectability.<br />

• Qualifying components to meet their reliability requirements.<br />

• Providing an effective remote handling system and facility design.<br />

Designing <strong>for</strong> availability<br />

The main design challenges <strong>for</strong> availability are the problems caused by 14 mev neutrons, which<br />

damage the materials that surround the plasma and induce radioactivity. after relatively little irradiation,<br />

metals can no longer be welded because of helium buildup, and bolts swell. These effects<br />

have a profound influence on design because neutrons stream down gaps and can scatter around<br />

corners. a rough rule <strong>for</strong> protecting sensitive areas — such as cooling pipe welds, bolts, in-vessel<br />

diagnostics and control components — at the demo neutron fluence is that neutrons would have<br />

to scatter through a right angle three or more times to reach the area.<br />

over the years, numerous approaches to designing <strong>for</strong> high maintainability and availability have<br />

been studied: <strong>for</strong> example, constructing the toroidal system in wedges, and having large ports to<br />

allow large components to be removed radially and/or vertically; standardizing components; using<br />

redundancy; and having in-service monitoring. nevertheless, many designs begin to resemble<br />

interlocking block, wooden puzzles.<br />

high-level design choices must take into account reliability and maintainability impacts. For instance,<br />

liquid metal divertors may be needed to survive plasma transients and provide adequate<br />

lifetime. advanced fuel cycles, e.g., a catalyzed d-d fuel cycle (with tritium sequestration), which<br />

avoid the need <strong>for</strong> complex tritium breeding blankets and have reduced neutron yield promoting<br />

long component lifetimes, may be required <strong>for</strong> power reactor applications. Plasma elongation may<br />

be limited by robust plasma position control with a simple, survivable complement of diagnostics.<br />

external trans<strong>for</strong>m may be required <strong>for</strong> robust disruption and elm avoidance.<br />

it is essential:<br />

• to analyze and test approaches <strong>for</strong> segmented neutron capture and shielding, and remote<br />

handling.<br />

• to develop improved integrated designs that satisfy availability and safety requirements<br />

by applying lessons learned from iteR, other experiments and remotely handled facilities;<br />

and making design choices that promote reliability and maintainability.<br />

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