Loss of Cryogen

DOE-STD-6003-96

B.4.6 Loss of Cryogen

Loss of cryogen (either helium or nitrogen) is a potential safety concern because the

pressure that can be developed as a result of the leak can threaten radioactivity confinement

barriers in the fusion machine, and the cryogen can displace oxygen and present a suffocation

potential for personnel. For superconducting magnets, quenching of a superconductor without

electrical discharge could lead to leakage or even local bursting of the superconductor and sub-

sequent release of helium. Faults in the cryoplant can lead to flashing of liquid nitrogen. The

amount of cryogen that can be released is a function of the design details of the cryoplant and of

the superconducting magnets (if used).

B.4.7 Tritium Plant Events

The tritium processing and fueling/pumping systems contain inventories of tritium that can

be released in the event of an accident that could breach the tritium confinement barrier system.

Generally, tritium system design standards call for double or triple containment for components

or systems that contain tritium that would tend to reduce the frequency of large releases. In

addition, the potential for hydrogen explosions must be considered. Dispersion and oxidation

characteristics will influence the severity of the hazard.

B.4.8 Auxiliary System Accidents

Fusion machines may use a number of auxiliary systems associated with plasma heating,

current drive, machine bakeout, and fueling. In general, accidents with these systems may

include toxic materials and gram-quantities of tritium that may reside on individual components.

B.4.8.1 Neutral Beams

Neutral beam injectors may be used as a means of providing heating to the plasma

during startup and operation. Operation of the beam without a plasma or misalignment in the

chamber can lead to ablation and/or melting of material from the surface where the beam lands

and potential release of radioactivity. Circuitry control interlocks and protective armor in the

torus are usually employed to preclude this scenario from being credible.

B.4.8.2 RF Heating

Some fusion designs call for the use of RF heating to assist in startup and operation.

Safety concerns related to the high power levels are adequately addressed in traditional

electrical safety standards.

B.4.8.3 Fuel System

Pellet injectors are one method of fueling the core of the plasma. These injectors drive

solid pellets (T, D, Li, etc.) into the plasma at high velocity (several km/s). The kinetic energy

imparted by the injector can be large enough to warrant preventive safety measures, such as

backstops.

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