MANAGING TRITIUM | SPECIAL REPORT
Above left: ITER plans to experiment with tritium production within the vacuum vessel in two equatorial ports Above right: Hydrogen and tritium handling infrastructure typically uses Type 300-series austenitic stainless steels, due to their resistance to hydrogen isotope embrittlement
fittings. But the authors say stainless steels are not immune to embrittlement and exposure to tritium and its decay product (helium) reduces toughness. They say, “Tritium embrittlement is complicated by the combined effects of the hydrogen isotope and the aging effect of tritium’s helium decay product. Because helium is less soluble in the atomic lattice than hydrogen isotopes, mobile atomic helium ultimately becomes trapped as high-pressure nanoscale helium bubbles which are hypothesised to impede dislocation motion and thus alter plastic deformation”. They cite losses in fracture toughness of up to 95% for specimens with high helium buildup, while thermal history associated with welding tends to create microstructures and internal stresses that can be more sensitive to hydrogen-isotope embrittlement. Mechanisms of embrittlement in tritium environments are more complicated than for hydrogen, but “similar relationships have been established comparing composition and processing effects to mechanical performance in tritium environments”. The US DOE’s Standard on Tritium Handling and Safe
Storage, updated in 2015, includes experience with polymers, which all degrade when exposed to radiation. The standard says, “Both tritium and tritiated water permeate all polymers, and permeated tritium deposits the beta decay energy throughout the polymer bulk. Radiation- induced changes in polymer properties include softening or hardening, ductility loss, change in colour or dimensions, and gas evolution. It says, “Because of these effects, polymers should only be used in tritium systems where no metal alternatives exist”. In addition to polymer breakdown itself, products of degradation can form corrosive liquids or acids such as HF and HCl. As a result, “The system should be designed to expose
any polymers to as little tritium as possible” and the Standard notes that polymers are often used in gas- handling systems including in gaskets, O-rings, electrical cable insulation and valve parts, including seats, stem tips, and packing. There are some flexibilities. Polymers that harden when
exposed to radiation can be used, if they are replaced before they begin to deteriorate, but in this case all polymer parts have to be easily replaceable and regularly
inspected and a replacement regime should be established. Protecting polymers from oxygen or air will help lengthen the lifetime of polymers exposed to tritium because many effects of radiation on polymers are accentuated by oxygen, and adding antioxidants may help. The temperature of any polymer parts should also be kept as low as possible as temperatures above about 120°C accelerate radiation effects. Finally, inert additives such as glass or graphite may enhance polymers’ resistance to radiation. The US DOE Standard lists the effects on some common
plastics. ● Ultra-high molecular weight polyethylene (UHMWPE) and
high density polyethylene (HDPE) have been used for valve stem tips but “their performance has not reached the desired level”.
● Low-density polyethylene is “very permeable by tritium and tritiated water and should not be considered for use in tritium systems”.
● Teflon® (polytetrafluoroethylene) degrades and
decomposes in tritium, forming hydrofluoric acid (HF). In humid air, both hydrochloric acid and HF are formed, which are both highly corrosive.
● Common chlorofluorocarbon polymers are incompatible with tritium. They, like Teflon® should not be used.
● Elstomers with radiation damage harden and lose their sealing ability and, the Standard says, “tritium readily permeates into and diffuses through elastomeric materials and, depending upon thickness, begins appearing on the outside of the elastomeric seal within hours after exposure”.
The DOE Standard refers to decommissioning and dismantling of the Old Tritium Extraction Facility at Savannah River between 1995 and 1997 and says: “The damage done to organic materials by the presence of tritium in the internal structure of the material is not limited to the more obvious radiation damage effects”. Tritium, particularly in the form of T+, “has the insidious ability to leach impurities (and non-impurities) out of the body of the parent material”. It says, “In many cases, particularly where halogens are involved, the damage done by secondary effects such as leaching can be more
www.neimagazine.com | September 2025 | 37 , degrade in tritium gas and
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