Global research programs seeking to achieve a commercially viable model of a fusion power plant are being accelerated at an unprecedented rate. One critical element to the design and licensing is an accurate understanding of the radiation environment throughout the plant lifetime and subsequent decommissioning phase. The radiation field, which results from the nuclear fusion reaction, gives rise to highly complex phenomena such as flux leakage, materials activation, and decay gamma fields. Demonstration of compliance with limits, the integrity of components, and the permissibility of operations are all fundamental to regulatory approval and the overall safety of a nuclear device. As such, neutronics, which is used in the general sense to refer to the mapping of radiation fields in nuclear devices, is a critical design driver. The Applied Radiation Technology group at the United Kingdom Atomic Energy Authority is a world leader in this field, developing new methods and deploying state-of-the-art codes to conduct nuclear analysis. As well as applied neutronics in areas spanning fusion reactors, medical applications, spallation neutron sources, and nuclear fission, there is an extensive parallel experimental program undertaking critical radiation field characterization and conducting measurements using an array of bespoke particle detection systems. This paper highlights recent technical developments made by this group in the context of outstanding challenges in this field, as well as providing an overview of current methods and capabilities for the broader interest of the community.