The meaning of “low activation” is quantified with regard to accident safety for naturally occurring elements from hydrogen through bismuth. The activation inventory from a 5 MW/m2 neutron flux for a 4-yr exposure at the first wall to a deuterium-tritium plasma was used for four figures of merit (FOM):

  1. prompt dose rating, based on the release fraction required to avoid any off-site acute radiation fatalities, relevant to inherent safety
  2. latent dose rating, based on the allowable release fraction to minimize off-site latent statistical cancers, relevant to risk-based goals
  3. time before damage, based on the initial temperature rise rate from decay heat and the time available before damage (a 300°C temperature rise)
  4. maximum temperature rise, based on the maximum temperature rise.
The first two FOM use the radioactive inventory and off-site dose calculations using the FUSCRAC3 computer code. The second two FOM are based on decay heat and simple generic blanket thermal models. The comparisons provide quantitative guidance for desired material compositions for those materials receiving high neutron fluence. Thus, these safety FOM should be used as the waste disposal rating to help tailor materials from the waste management standpoint. The lowest activation coolants and breeders are helium, water, lithium, Flibe (LiF-BeF2), and lithium oxide. The lowest activation neutron multiplier is beryllium. The lowest activation materials for reflectors and plasma-facing tiles are carbon, beryllium, and boron. Among metallic alloys, the lowest activation options are clean vanadium alloys and ferritic steels. “Clean” means that the alloying elements do not worsen the activation levels compared with pure vanadium or pure iron, respectively. Worrisome alloying elements include bismuth and tantalum. Some ceramics are lower in activation than metal alloys, especially silicon carbide, a true low-activation material.