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A first neutronics analysis of the Helical-Axis Advanced Stellarator (HELIAS) power reactor is conducted in this work. It is based on Monte Carlo (MC) particle transport simulations with the Direct Accelerated Geometry Monte Carlo (DAGMC) approach which enables particle tracking directly on the CAD geometry. A suitable geometry model of the HELIAS reactor is developed, including a rough model of a breeder blanket based on the Helium Cooled Pebble Bed (HCPB) breeder blanket concept. The resulting model allows to perform first neutronic calculations providing a 2D map of the neutron wall loading, a 3D distribution of the neutron flux, and a rough assessment of the tritium breeding capability. It is concluded that the applied methodology, making use of MC particle transport simulations based on the DAGMC approach, is suitable for performing nuclear analyses for the HELIAS power reactor.

As a possible long-term alternative to a tokamak fusion power plant, the stellarator concept offers salient physics features (no external current drive, no risk of plasma disruptions and low recirculating power, among others) that could be offset by the more complex configurations and challenging maintenance schemes. Very ffew conceptual design studies have been performed compared to those in tokamaks, and the enhancement of engineering aspects should follow. With the recent start of operation of Wendelstein 7-X, the helical-axis advanced stellarator (HELIAS) line has again raised interest in the scientific and technologic EUROfusion programme. The main aim at present is showing that stellarators (particularly helical-axis stellarators) are viable as potential fusion reactors. For the conceptualisation of a mature HELIAS power reactor, different engineering and technological aspects must be studied, improved and solved. To this end, starting from a very preliminary reactor design called HELIAS 5-B (5-field-period), with a fusion power of 3000 MW, a neutronic model has been developed and analysed introducing in the baseline the relevant components of breeding blankets (BB). The vast experience achieved at CIEMAT in BB designs for the DEMO tokamak has been exploited, adapting the dual-coolant lithium-lead BB design elaborated in the frame of the WPBB programme of EUROfusion/PPPT to the HELIAS configuration. Preliminary neutronic assessments have been performed focussing on tritium production, power density distributions and damage/shielding responses such as nuclear heating, neutron fluence, dpa and helium production. Particle transport calculations have been performed with MCNP5v1.6 Monte Carlo code.

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