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

Qualifying DEMO Components
Possible failure modes, their failure rate (Fr), and the mean time to repair or replace (mttR) must
be established for every component. in simple terms, the unavailability due to failure is the sum
of individual Fr x mttR. (There is also an unavailability due to components wearing out.) note
that an availability of 50% means there is an unavailability of 50% to allocate to the various power
plant components. For example, scheduled maintenance of nonreactor systems in fission typically
uses up 5% of the unavailability. This corresponds to allocating 2.5 weeks per year for this
activity. The remaining 45% unavailability can be assigned to scheduled maintenance of fusion
components and unscheduled downtime.
maintenance or replacement of fusion’s in-vessel components will generally require cooling down
the system and opening the vacuum vessel. The time to accomplish this and return to an evacuated
state may take weeks, not including the time to repair or replace components. consequently,
the hundreds of in-vessel components (yet to be designed and developed) must be extremely
reliable, with long lifetimes. many of these components will require scheduled replacement after
they have accumulated their designed neutron damage. The question is: how will their lifetime
and reliability be established on a design yet to be finalized? The complexity of a fusion plant is
much greater than that of a fission plant, which makes achieving high availability more difficult.
even when iteR has operated successfully, there will remain gaps in the knowledge and technology
base needed for demo. specifically:
• little reliability or lifetime data that could be extrapolated to demo-relevant components
— notably for the in-vessel components (the iteR superconducting coils system is much
more relevant).
• iteR will provide a valuable test of remote handling techniques, but demo needs are
more demanding due to the higher degree of complexity and high availability needs.
consequently, an extensive component testing program, under demo-like conditions, will be required
to establish the data base of failure rates and maintenance procedures to qualify components
for the demo. numerous nuclear and nonnuclear facilities will be required for this task;
notably, for plasma facing components, blankets, divertors, and the front ends of heating and
current drive systems and diagnostics. as discussed in other parts of this Report, a d-t burning
component test Facility (ctF) will have to play a key role.
We must assume that during the testing phase some failures will occur, leading to an evolution
in component design and in the integrated design itself. This process with a series of incremental
improvements is the time-honored approach in developing airplanes and fission reactors.
in the near-term we should undertake a number of activities:
• determine what we might learn from iteR and other experiments and remotely handled
facilities.
• Perform studies to identify the extent of testing beyond iteR needed to qualify
components for the integrated design, and ultimately be able to allocate unavailability
and evaluate the design for safety.
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