Limiting Airborne and Liquid Releases to the Environment

DOE-STD-6003-96

possibility of an increase in the plasma temperature would be if the auxiliary power is increased.

Finally, it is possible to operate at or near the thermally unstable point, if active feedback

mechanisms are employed. This issue is primarily an operational one; the main task for the

in-vessel component designer is to design for a conservative, but credible, value of fusion

power taking into account all credible plasma transients.

b. Disruptions

Any magnetic confinement geometry has the consequence of both thermal and magnetic

stored energy in the plasma. If the confinement scheme is known to have the possibility of sud-

denly losing this stored energy, the in-vessel device hardware that receives these loads should

be designed to accommodate these events. For example, the tokamak configuration is known to

"disrupt" due to magnetohydrodynamic instabilities. In this event, the stored thermal energy in

the plasma is rapidly lost to the plasma-facing components, introducing large thermal loads.

Loss of the magnetic energy associated with fields generated by current flowing in the plasma

can induce large currents in the surrounding first wall, breeding blanket, and vacuum vessel,

which results in large mechanical loads. Disruptions can also generate high-energy runaway

electrons that impact the first wall (with currents of the order of the plasma current). All of these

disruption-related issues impose special design requirements on the affected fusion island

hardware components.

The fusion island hardware should be designed to withstand credible disruptions. Care

has to be taken during the safety analysis process to conservatively identify credible disruptions/

VDEs and resultant loadings to components; some insight can be gained from the disruption

data base of contemporary large tokamaks. It is also prudent to operate in a parameter regime

where these events can be minimized. For instance, in tokamaks, disruptions can be initiated

by (1) exceeding a plasma density limit, (2) operating at too low an edge safety factor, or

(3) operating at too high a plasma beta. It is especially important to avoid this latter type disrup-

tion during plasma startup and shutdown. In practice, these disruption causes can be identified

and avoided, greatly reducing the probability of a disruption. Also, it may be possible to identify

the onset of disruption and use active means to subsequently control it.

Another known concern for sudden loss of plasma control is related to position control.

For highly shaped tokamak plasmas, active vertical position control is required to maintain the

vertical position. Loss of the position control due to noise in the feedback system or power sup-

ply saturation is known as a VDE. If the main plasma contacts the plasma-facing components,

the currents in the plasma can rapidly disappear, leading to a disruption.

6.3.4 Limiting Airborne and Liquid Releases to the Environment

The facility design should include means to control the release of radioactive materials in

gaseous and liquid effluent and to handle radioactive solid wastes produced during normal

operation. Furthermore, the design should aim at minimizing the generation of radioactive and

hazardous wastes in all forms. Suggested means to accomplish this and also minimize worker

exposures are discussed in Section 6.2.2. A specific long-term goal for fusion facilities is the

elimination of all materials in the fusion island whose activation would require disposal by deep

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