Loss-of-Vacuum Event

DOE-STD-6003-96

B.4.3 Loss-of-Vacuum Event

A loss-of-vacuum event (LVE) occurs when the vacuum inside the plasma chamber is

lost. An LVE can occur as a result of a failure of a diagnostic window, port, or other seal due to

either incipient flaws, wearout, radiation, embrittlement, or overpressurization of the plasma

chamber due to an in-vessel LCE. The LVE can then provide a pathway for release of tokamak

dust and any tritium gas from the vacuum vessel. The ingressed air can also react with hot PFC

surfaces and generate additional chemical energy that could volatilize radioactivity from the PFC

surface. The ultimate impact of such releases is a function of both in-vessel and ex-vessel

features of the design.

B.4.4 Plasma Transients

The two classes of plasma transients that are potentially important to safety are transient

overpower events and plasma disruptions. A fusion overpower event can occur in an ignited

plasma when a balance is not maintained between fusion generation and loss. The result is an

increase in plasma temperature (and thereby thermal energy) until either a power balance is

reestablished or a beta limit is exceeded. Exceeding a beta limit would trigger a disruption and

shutdown the plasma. Plasma disruptions cover a range of transient events in which confine-

ment of the plasma is lost and the plasma energy is transferred to the surrounding structure very

quickly. The rapid energy transfer can cause armor tile ablation and/or melting. In addition, the

plasma current will rapidly quench (time scale is 1 ms to 1 s) and generate magnetically induced

forces in the structures that must be accounted for in the design. There are numerous initiators

for plasma disruptions including thermal plasma excursions, impurities injected into the plasma,

loss of plasma position control, and vertical displacement events. Many of these disruptions are

considered to be anticipated operational occurrences and hence would need to be covered by

the design. In addition, certain plasma disruptions will generate high-energy electrons, termed

"runaway" electrons. These electrons can damage PFCs and be an initiator for a common mode

failure of blanket and divertor cooling systems.

B.4.5 Magnet Transients

The major concern about magnet transients is the potential for propagating faults to other

components of the fusion machine. The magnet faults of concern from an accident propagation

viewpoint are off-normal forces that would produce large coil displacements, break off magnet

pieces, and pull in ferrous missiles from other areas or arcs that could produce melting and

volatilization in other components. In ITER, these events could have the potential to damage the

vacuum vessel, ducts and piping from the vacuum vessel, and the cryostat and could potentially

result in radioactivity release. Off-normal forces could arise from shorts in coils, faults in the dis-

charge system, or power supply faults. Arcs between coils, arcs to ground, and arcs at open

leads could lead to melting and/or volatilization. Arcs could arise from insulation faults, gas

ingress, overvoltage, or other causes.

170