NEREIDA: A computational framework for the design and dosimetric characterization of fast-neutron facilities

Reliable neutron flux characterization and dosimetry are critical to the design, licensing, and safe operation of nuclear facilities. However, obtaining these quantities from as-built geometries usually involves translating computer-aided design (CAD) models into radiation-transport inputs. This is a fragmented process that lacks an end-to-end, auditable pipeline. Widely used tools address transport, but not the seamless integration of CAD-to-dose, and often require restrictive licences. We present NEREIDA: a modular, scalable framework that automates the process of creating high-resolution, three-dimensional neutron dose maps from native CAD models. NEREIDA unifies aspects such as geometry voxelization and activation analysis in common structural materials, producing dose equivalents H ∗ ( 10 ) and H p ( 10 ) across complex facilities. The NEREIDA framework integrates Geant4 with external components to support realistic sources, phantoms, and background fields. Validation against 252 Cf and 241 Am-Be reference spectra demonstrates spectrometric accuracy, and large-scale benchmarks on CAD-derived geometries show near-linear scaling up to 800 0 3 voxels and 15000 particles per run. This scalability enables precise, facility-specific radiation assessments within a single, reproducible toolchain for design iteration, licensing, and routine safety in fast-neutron environments.