Rapid Plasma Shutdown System

DOE-STD-6003-96

In addition to the general design guidance in Section 6.1, the following system-specific

design guidance is provided.

A complete loss of all in-vessel cooling is an off-normal event and should be evaluated to

provide an upper bound on the importance of this safety concern. In most off-normal event sce-

narios, there will be at least some active cooling. The vacuum vessel may have a safety function

in afterheat removal. Cooling system diversity (i.e., multiple, independent cooling loops) for

in-vessel components may provide a measure of defense-in-depth for this safety concern even

though active cooling of individual in-vessel components may not be reliable because of the

severe plasma-facing operational environment and their experimental nature.

If an active afterheat removal system is required, there should be specific reliability

requirements for a given duration after shutdown. For example, up to one day after shutdown is

important because the afterheat may decrease by up to an order of magnitude. Another

2 months is required for an additional order of magnitude in austinetic stainless steel compo-

nents. Lower activation materials will decay more quickly. The required heat removal capacity

should be evaluated based on actual materials specified in the design and the rated thermal

operating power.

6.3.2 Rapid Plasma Shutdown System

A means of rapid plasma shutdown should be provided for fusion facilities, if required to

ensure that evaluation guidelines are met. An example of a rapid plasma shutdown system

would be a system that reliably injects a fast high-Z impurity pellet into a plasma for rapid termi-

nation in response to a precursor signal for a large plasma disruption. The level of required reli-

ability, redundancy, and diversity of such a system, its effectiveness, and speed of action should

be such that safety functions required to meet evaluation guidelines are ensured. Consideration

should be given to heat, particle, magnetic, and mechanical loads on confinement barriers

resulting from worst-case credible transient overpower events, VDEs, or disruptions in assess-

ing the need for a rapid plasma shutdown.

An off-normal fusion power rampdown system will act on a time scale of the order of a

few tens of seconds and might be sufficient to cover loss-of-flow events in the plasma-facing

components if sufficient pump inertia is installed. In case of the unlikely event of coolant flow

channel blockage, an off-normal fusion power shutdown system acting on the few seconds time

scale is needed. Possible mechanisms are impurity injection by gas puffing/pellets or controlled

equilibrium disturbance. A design constraint is fast termination without otherwise undesirable

consequences. If all above mentioned active fusion power shutdown actions fail during an off-

normal event, the plasma would always be shutdown by passive means due to overheating of

plasma-facing components and consequent impurity influx.

6.3.3 Control of Potential Energy Sources

Five energy sources could drive fusion facility off-normal events including design-basis

events: coolant energy (6.3.3.1), chemical reactions (6.3.3.2), magnets (6.3.3.3) and plasma

(6.3.3.4) as well as afterheat (6.3.1), discussed previously.

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