Energy Sources

DOE-STD-6003-96

Standards exist in other industries for dealing with these hazards to provide adequate protection

for workers.

B.3 Energy Sources

In fusion a number of distributed energy sources could potentially induce accidents that

can result in release of radioactivity or toxic materials. The amount of energy, the time scales for

its release, and the potential consequences are a function of the specific fusion design. The var-

ious energy sources are discussed below.

B.3.1 Plasma Energy

The fusion plasma generally contains very little stored energy (e.g., on the order of 1 GJ

for ITER). However, because the fusion reaction is a reaction that takes place in the plasma, a

complex control system may be needed to provide for control of the plasma during the reaction.

This is known as plasma burn control. The control system contains a fueling system, a magnetic

confinement and plasma position control system, a current drive system, an auxiliary heating

system, an impurity control system, and a vacuum system. Failure in any of these systems

would result in extinguishing the plasma, which may be accompanied by a plasma disruption.

The plasma can disrupt very quickly and the energy contained in the plasma can be imparted to

the plasma-facing materials very quickly (~ms), which can cause significant PFC armor tile abla-

tion and/or melting. In addition, the plasma current will rapidly quench (time scale is ~ms to 1 s)

and produce magnetically induced forces in the structures that must be accounted for in the

design.

B.3.2 Magnetic Energy

The energy stored in the superconducting magnets of a fusion device can be very large.

For ITER, the magnets will contain on the order of 100 GJ that can be released on the order of

seconds to minutes as the result of arcing, shorts, or a quench with magnet discharge (loss of

cryogen). Fusion designs must contain provisions for control and potential dissipation of this

stored energy source without causing propagating faults in other systems. The most important

aspect of magnet design from a safety viewpoint is to ensure that the magnet structural integrity

and geometry are maintained for credible accident conditions so that magnet structural failure

cannot result in the release of radioactive or toxic materials.

B.3.3 Decay Heat

The activation products produced during operation of a fusion device will generate decay

heat. The level of decay heat may be on the order of 2 to 3% of the steady state operating

power but is a function of the structural materials used and the accumulated neutron fluence.

For smaller fusion devices, decay heat may not be a significant energy source because of the

low power level and fluence expected. For ITER, operating at 1500 MW, the decay heat would

be about 30 to 40 MW. Removal of this energy is needed during normal operation between

pulses, during maintenance and bakeout, and during decommissioning to prevent overheating

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