Material Compatibility

DOE-HDBK-1129-99

solidified on clay or on molecular sieve material, regardless of the quantity, is stable and non-

corrosive and may be stored for many years in the container.

5.1 Material Compatibility

Proper materials selection and rigorous design have led to tritium-handling systems that are

extremely safe for long periods of time. Materials exposed to tritium under certain conditions can

be susceptible to hydrogen embrittlement. The chances of embrittlement are significantly reduced

by proper material selection. No additional thickness of components, such as the "corrosion

allowance" used to mitigate uniform corrosion, is added to components to reduce or eliminate the

chance of hydrogen-induced cracking and subsequent failure as material degradation effects are

not uniform. In the past, tritium systems were located in hood systems having a single-pass air

flow to protect workers should a component fail and tritium be released; the released tritium would

be removed from the facility into the atmosphere by the hood exhaust system. Current system

design normally consists of a closed glovebox, with a stripper or getter system to trap the released

tritium. This arrangement protects workers and prevents releases to the environment.

Tritium can permeate vessel barriers, especially in components operating at elevated temperature.

Currently available tritium permeation data is normally sufficient to estimate the order of magnitude

of tritium permeation through barriers. These estimates can be used during system design or to

determine whether additional purging and stripping systems are required to "clean up" permeated

tritium or whether other design changes (such as wall thickness, material, or coating) are required

to reduce permeation. Tritium permeation can lead not only to contamination outside the barrier,

but it can also result in significant quantities of tritium being dissolved in parts, which can lead to

hydrogen embrittlement. Over time, this tritium decays to 3He, which has been found to

accentuate hydrogen embrittlement and to cause weld cracking (see below).

5.1.1 System Design

Tritium primary containers must have an exceptionally low leak rate (10-6 to 10-7 cm3 4He/second)

and probability of failure or leaking. The consequences of tritium leaks can include personnel

uptake, release to the environment, ignition (if mixed with oxygen), and violation of operating

permits. Pressure and vacuum vessels used in tritium systems are generally designed and

constructed using codes and standards applicable to boiler and pressure vessels. In the United

States, use of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel

Code is recommended but not required. Pressure and vacuum vessels constructed using this

code have an accepted rigor in design, construction, and inspection that facilitates approval and

acceptance by regulators. The ASME Boiler and Pressure Vessel Code primarily covers the

design of vessels, and does not cover all of the aspects of a tritium system design. Other design

standards, such as resistance to seismic events, also must be followed, depending on the location

and regulators of the facility. See Sections 4.4 and 5.7 for more discussion of seismic design.

5.1.1.a Leak Testing

After fabrication, thoroughly leak testing tritium systems is extremely important. Normally, leak

testing employs commercial helium-mass-spectrometer-based systems and is performed after any

other required proof, pressure, or vacuum performance tests. Other leak detection methods, such

as rate-of-rise, can also be employed in addition to helium mass spectrometry. A dilute solution of

tritium in an inert gas can also be used to detect small leaks. Tritium is a highly effective leak