Magnetic Energy

DOE-STD-6003-96

b. Explosions

Another related chemical energy source term is from explosions. Examples include H2,

metal/carbon dust, and cryogenic ozone. The lower flammability limit for H2 is about 4% volume

in air; the lower explosive limit for H2 in air depends on geometry and is about 15% by volume

(deflagrations are possible with lower concentrations of H2). The lowest H2 concentration shown

experimentally to detonate in air is 13.5%.

Where explosion hazards theoretically exist, the design must do one or more of the

following:

1. keep oxidizers (e.g., air) out preventing an explosive mixture (only applicable if oxi-

dizer is required);

2. contain the explosion;

3. show consequences are acceptable in terms of public and plant personnel safety.

An explosion hazard exists related to the use of liquid nitrogen in the thermal shield,

specifically irradiation-induced ozone production (Brereton 1989). Explosions in liquid nitrogen

systems in a radiation environment have been reported over the years. These explosions are

thought to be caused by the production of ozone (O3) by the action of radiation on the intrinsic

oxygen impurity. Ozone can spontaneously decompose back into oxygen releasing 144 kJ/

mole. Production rates for large thermal shields could be of order several moles of O3/day.

Ozone is even less volatile than oxygen and may accumulate in the shield. Seven moles of O3

represents an explosion hazard with a potential energy release of 1 MJ (250 g TNT). This much

energy represents a significant hazard and seems to indicate the necessity of operating with

very pure nitrogen or replacing liquid nitrogen with cold helium gas or a passively cooled

structure.

6.3.3.3 Magnetic Energy

The magnet system (for a tokamak device) consists of the toroidal field (TF) coils, the

poloidal field (PF) coils, and the central solenoid. TF coils are normally superconducting cables

cooled with liquid helium and are wound into D shapes. The PF coils are also typically super-

conducting cables cooled with liquid helium and wound into horizontal rings, which are located

above and below the vacuum vessel with typically some coil sets inside and outside the TF

coils. The TF and PF coils provide the basic magnetic field geometry for plasma confinement

and position control. The central solenoid cables are typically superconducting cables wound

horizontally and situated at the center of the vacuum vessel torus supported by, for example, a

bucking cylinder. The central solenoid set provides the transient field to induce all or part of the

plasma current.

Fusion magnets contain significant stored energy that can cause materials, either in the

magnet itself or in adjacent structures, to become volatile. Such faults could release missiles

which could then cause damage that would release cryogenic liquids whose overpressure could

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