Design Considerations - hdbk60040059

DOE-HDBK-6004-99

Design Considerations

General

The tritium primary confinement is the pressure (or vacuum) boundary, wetted routinely by tritium,

outside the fusion machine' vacuum vessel and associated vacuum system. Gages, stubs or other

pressure-containing hardware attached to a safety-class primary confinement subsystem are safety-

class components and design should have them serve as confinement barriers for all operational and

accident modes of the tritium primary confinement.

Radiation Shielding

Radiation shielding is not a design consideration for tritium systems. Tritium decays to a stable

element, helium (3He) by emission of low energy beta radiation, maximum 0.0185 MeV and average

0.0057 MeV. The maximum range (i.e., density thickness) of beta particles, about 0.6 mg/cm2, is less

than the generally accepted 7 mg/cm2 thickness of the epidermis of the skin. The beta radiation is

easily and completely shielded by a relatively thin layer of almost any material, including the materials

of the tritium confinement system. Thus, if the primary confinement system is leak tight, tritium poses

no radiological hazard to operating personnel.

Confinement Barriers

Tritium primary confinement is a major design consideration because tritium is difficult to contain.

As noted above, tritium is not an external radiation hazard. However, when tritium is oxidized and

ingested it produces a significant internal dose. Regardless of the care taken to assure physical

integrity and leak tightness of the confinement, small quantities of tritium will escape at the various

process connections during normal operations. Additionally, an increased level of loss will occur

during maintenance operations which usually breach the primary barrier. By escaping to unwanted

areas and reacting with normally present materials, tritium can create significant biological hazards.

For example, tritiated water (tritium oxide) is a water molecule in which one or both of the hydrogen

atoms is a tritium atom rather than the normal protium, e.g., T2O, HTO, DTO. Tritiated water is on

the order of 104 times more hazardous to humans than elemental tritium. The human radiation dose

hazard is through inhalation, ingestion or absorption through the skin.

Because tritium is very mobile and can create a biological hazard, tritium systems must have barriers

to protect personnel and the environment from tritium and its compounds.

As a minimum, a tritium system should have primary and secondary confinements. If the safety

analysis determines that tritium systems, or certain components, require tertiary or higher order

confinements, the design should provide for these confinements in a similar manner as secondary

confinement as discussed below.

The primary confinement system should consist of piping, tubing, valves, fittings, equipment and

instrumentation components that define the pressure boundary of the tritium systems. The primary

confinement system is normally a closed system in direct contact with tritium and containing it for

conditions ranging from vacuum to full system pressure. Physical integrity is assured by compliance

with the applicable ASME Code for boiler and pressure vessels, or equivalent Codes. If the safety

analysis determines a portion or all of tritium primary confinement to be a safety-class system, the