An essential element of any fusion or fission reactor materials development effort is the availability of irradiation facilities for conducting radiation effects experiments. A Radiation Effects Facility (REF) was provided for such studies at the Los Alamos Meson Physics Facility. Neutron spectra at the REF can be tailored to approximate those in either a fusion or fission reactor, while providing flux levels of ∼1.4 × 1018 m−2 s−1 at design maximum beam currents. An intranuclear-cascade/evaporation model was used for computing neutron production. Detailed Monte Carlo neutron transport calculations were performed, some of which were experimentally verified in a foil dosimetry program. Such calculations provide the radiation effects experimentalist with information on spatial-spectral variations of the neutron flux over much of the easily accessible experimental volume (∼19 000 cm3), which includes irradiation specimen capsule locations and a rabbit tube. From these data, radiation damage indices such as ratios of parts per million helium to displacements per atom can be calculated and compared to those anticipated in fusion reactor blankets or fast fission reactor cores.