DPA and Material Damage

Displacements per atom (dpa) measures neutron damage; 14 MeV neutrons also produce helium, and the He/dpa ratio is what makes fusion damage distinct.

Two numbers that describe damage

Neutron damage to a solid is quantified primarily by displacements per atom (dpa): the average number of times each atom is knocked from its lattice site over an irradiation. A high-dpa material has been thoroughly rearranged at the atomic scale, which drives hardening, swelling, and loss of ductility.

But dpa alone does not capture fusion damage. The 14 MeV spectrum also produces helium inside the material through (n,α) reactions, and hydrogen through (n,p). The ratio of helium produced to displacements — the He/dpa ratio — is far higher for fusion neutrons than for fission neutrons, and it is this transmuted helium that most aggressively embrittles and swells fusion structures.

Why the ratio matters

• Same dpa, different He content → different real damage
• Fission testing under-produces helium, understating embrittlement
• Helium collects at grain boundaries and voids, causing swelling
• Accurate lifetime prediction needs the correct He/dpa ratio

What Hyperion provides

Because Hyperion delivers real 14 MeV neutrons, materials irradiated in it accumulate dpa and helium in the correct proportion for fusion service. That makes the resulting data directly usable for qualifying first-wall and blanket materials, where the center post and first wall themselves face limited lifetime — the center post is noted at roughly 0.01 fpy. Damage prediction remains a design and validation subject validated by irradiation data at FOAK and beyond.

Construction begins Q2 2027; first-of-a-kind (FOAK) first tritium is targeted near 2030. No net-gain claim is made before FOAK.