Activation of silicon carbide composites in fusion systems

The potential of silicon carbide(SiC) composites as a low-activation structural material for fusion applications will depend both on transmutations from base materials as well as material impurities. In this study, the impact of both transmutation and impurities on the neutron induced activation of SiC is examined. A sample of Nicalon (SiC) fiber reinforced, silicon carbide matrix composite was irradiated in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory to detect the impurity level present as well as their associated activities. Impurity concentrations were calculated with the aid of the HFIR fission library in the transmutation analysis code REAC3. Using this data, the REAC3 code and fusion neutron library were employed to determine the activation levels for the Nicalon/SiC first wall assumed in the ARIES I fusion reactor design. These results were compared with those for a pure (impurity free) SiC first wall in the same reactor design. The level of impurities in the HFIR irradiated composite were found to be small, typically a few parts per billion, yielding only a modest effect on the associated activation. The only significant effect on the delivered dose was due to the isotopes ⁵⁴Mn and ⁶⁰Co which are the result of iron and cobalt impurities. The effect was apparent in the middle range of the time study, from about 42 days to 11 years. However, the overall activation levels of these materials, either pure or composite, were dominated by the original isotopes, silicon and carbon. The major transmuted isotopes were aluminum, from silicon, ¹⁴C from neutron capture in carbon, and tritium. The effect of the impurities and transmuted isotopes on waste disposal issues will be addressed.