Impact of neutronics analyses on the lower port configuration of 2024 Volumetric Neutron Source

The nuclear integration of different systems into a tokamak is a strong driver of the design, with limits for different nuclear responses defined to ensure the integrity of the device over its operational lifetime. The European Volumetric Neutron Source (VNS) is a tokamak-based facility currently being developed through the EUROfusion consortium which is designed to test and qualify essential components for the development of fusion, such as the breeding blanket under fusion reactor conditions. The primary objective is to reduce the risk associated with the construction of the demonstration power plant EU-DEMO. In recent years, the feasibility of this facility has been studied and integration of key systems investigated. In VNS, the design of the lower port presents an integration challenge due to the competing requirements to provide neutron and gamma ray shielding and at the same time a sufficiently large opening to allow both vacuum pumping and divertor extraction. This leads to increased nuclear loads in superconducting toroidal field coils. In addition, the divertor maintenance also requires the divertor cooling pipes to be cut and rewelded, which necessitates a sufficient protection of these pipes to ensure that He production is kept below the limit of reweldability. The neutronics analysis of the 2024 design of the VNS divertor and the lower port is presented. Various configurations were tested to determine configurations that provide sufficient shielding of the divertor cooling pipes, vacuum vessel and toroidal field coils. We have not identified any “show stoppers” and have either demonstrated that nuclear loads are within design limits, i.e. nuclear loads in vacuum vessel and toroidal field coils, or showed ways to improve shielding towards reaching this goal, i.e. for He production in divertor cooling pipes and nuclear loads in poloidal field coils.