A Technical Note on D-T Fusion Reactor Afterheat

The afterheat in a D-T fusion reactor of conventional design is shown to be a serious consideration for loss-of-flow or loss-of-coolant accidents. Some previous estimates of the radioactivation of a niobium vacuum wall and structure are shown to be grossly low. The major additional contributions to the afterheat are from ^94mNb, ^92mNb, ⁹⁵Nb, ^95mNb, and a long-lived metastable state of⁹³Nb. By considering some of these additional decay sources, the afterheat from a 5-GW(th) D-T reactor of typical design is computed to be at least 30 MW. Early in the reactor lifetime, the decay of the after-heat power should be characterized by the 10.1-day half-life of ^92mNb. A qualitative review of the activation cross sections of molybdenum, an alternate possibility as a structural material, does not give expectation of decay powers significantly lower than niobium.