The EUROfusion Consortium is conducting a preconceptual design and feasibility study of a volumetric neutron source (VNS) facility to address perceived needs in the development of integrated breeding blanket and fuel cycle testing and qualification, with aim to mitigate risks stemming from the current low reliability and technical maturity of present design concepts for a DEMO fusion reactor. The main requirements driving the selection of key physics and technical concepts have been identified to cope with a construction and operation schedule consistent with the DEMO design activities.

The VNS needs to provide a steady-state plasma operation based on reliable beam-target fusion plasmas with a fusion power at 30 MW, generating a peak neutron wall load of at least 0.5 MW/m2 up to neutron fluences of 30 to 50 displacements per atom. Neutronics assessments are requested to provide essential nuclear loads and shielding performances of this device. To support the technical feasibility of the VNS tokamak, a series of analyses were conducted in support of the architecture and system design concept. The preconceptual design of the VNS was based on ITER-like shielding structures at the inboard side (ca. 70 cm radial depth) and enhanced outboard side shielding (about a 120-cm radial depth).

According to the assumed port configuration and toroidal segmentation of the VNS tokamak, a torus sector model of 60 deg has been generated from available computer-aided design models and converted to MCNP geometry descriptions. The primary objectives of the initial nuclear analyses and scoping assessments were the radial build on the inboard side to protect, specifically, the toroidal field coils and the shielding environment around the neutral beam injector (NBI) port duct, as well the divertor and lower ports, which are critical areas for ex-vessel shielding objectives. It could be demonstrated that the protection from a radial build adopting tungsten-based shields is adequate, whereas additional measures on shielding and integration of NBI ports and lower ports are required.