Computational approaches for nuclear design analyses of the stellarator power reactor HELIAS

The Helical-Axis Advanced Stellarator (HELIAS) is a conceptual design of a stel-larator-type fusion power reactor proposed by Max-Planck-Institute for Plasma Physics (IPP) Greifswald with a thermal fusion power of 3000 MW. The shape of the stellarator is defined by the non-planar shaped magnetic field coils, which generate a twisted plasma confining magnetic field. For the HELIAS 5-B reactor design, a comprehensive Monte Carlo based neutronic design analysis with the Monte Carlo N-Particle (MCNP) transport code is developed to provide important nuclear parameters like the neutron wall loading (NWL), the neutron flux distribution, the tritium breeding ratio (TBR) and the shielding performance. The complex toroidal twisted HELIAS stellarator geometry is created fully by higher-order surfaces following the shape of the magnetic field from a CAD model into an MCNP representation. Three different geometry transfer options have been investigated and the faceted geometry formed by a triangular surface mesh on the CAD geometry in the Direct Accelerated Geometry Monte Carlo (DAGMC) approach is considered as the most promising option because it is capable of using the complex geometry without any restrictions. For the neutronic characterisation of HELIAS, the original CAD envelope model is separated radially into different functional layers filled with homogenised material mixtures to represent the breeding blanket based on the Helium Cooled Pebble Bed (HCPB) concept, the back supporting structure, the vacuum vessel and the shielding. The NWL distribution shows a variation of the load across the first wall with a peak load of 1.93 MW/m${}^{-2}$ and an average load of 0.95 MW/m${}^{-2}$. The neutron flux distribution is obtained with a peak flux inside the plasma of 4 × 10${}^{14}$ n/cm${}^2$/s, which is attenuated through the blanket and vacuum vessel to 1 × 10${}^{10}$ n/cm${}^2$/s. The TBR in this configuration is ~1.38, a rather optimistic value due to the applied homogenised material mixtures. The reactor components need to provide sufficient shielding for the radiation protection of sensitive equipment. The attenuation of the plasma neutron flux by four orders of magnitude through the bulk shield leads to an increase of the relative statistical error which requires the application of variance reduction methods to reduce the statistical error in these domains. The shielding calculations conducted with application of the mesh based weight window approach for variance reduction have been assessed in an area with a NWL of ~1.35 MW/m${}^{-2}$ and less radial thickness of ~1.0 m. The requirements specified for the EU DEMO tokamak fusion reactor cannot be met by HELIAS and the need for enhancing the shielding capabilities is indicated by increasing the shielding layer and/or applying advanced shielding materials. Here, for the first time a comprehensive neutronic investigation of HELIAS is conducted, which is based on a verified computational radiation transport methodology. This approach is applicable to future nuclear engineering design analyses for this reactor type, targeting at optimised breeding blanket concepts and advanced nuclear shielding indispensable for a fusion reactor.