Transmutation and activation analysis for divertor materials in a fusion power reactor

Full text of publication follows: Within the European Power Plant conceptual study (PPCS), a helium gas cooled divertor with challenging material performance has been suggested. As a heat load of 10 MW/m2 and more needs to be removed, the design is presently based on tiles made of W (∼2000 deg. C), as well as on structural oxide dispersion strengthened refractory alloys (∼700- 1300 deg. C) and RAF-ODS steel (∼650 deg. C) as backbone structure. In order to overcome the substantial gap between already available refractory alloys and the demanding needs of fusion power reactors, a material science program needs to be initiated with the targeted development of high temperature resistant alloys, possibly on the basis of nano-scaled, homogeneously distributed ODS particles. In order to select and optimize the elemental alloy compositions with respect to maintenance, nuclear inventory, decommissioning and waste management issues, the activation and transmutation characteristics of the considered divertor materials must be known for the irradiation in a fusion power reactor (FPR). This necessitates a dedicated computational approach which couples three-dimensional neutron transport and activation calculations for the divertor in a typical FPR configuration such as one of the PPCS models. A corresponding programme system linking the Monte Carlo transport code MCNP and the fusion inventory code FISPACT through an appropriate (automated) interface system, has been applied to this end. Use was made of a suitable PPCS reactor model with integrated helium cooled divertor developed previously in the frame of the PPCS study for MCNP calculations. Various structural materials and brazing alloys were considered that are presently under discussion for helium cooled FPR divertors. For any of these materials, the neutron flux spectra and the nuclide inventories were calculated at different locations in the divertor cassette. Special attention was given to the proper treatment of the resonance shielding of tungsten and tantalum by using reaction rates provided directly by MCNP on the basis of continuous energy activation cross-section data. Following this approach, the transmutation rates, the activity inventories, and other nuclear responses such as the nuclear heating and the contact gamma dose rates were calculated for the considered divertor materials. (authors)