Spontaneous fission sources of 252Cf, lightly encapsulated and with neutron source strengths approaching 1010 n/s, have been developed especially for integral cross-section measurements and neutron reaction rate calibrations. An irradiation facility at the National Bureau of Standards makes use of these sources in two well-investigated geometries. A free-field neutron flux in the range of 107 n/(cm2 s) (105 n/mm2 · s) and fluences of up to 1013 n/cm2 (1011 n/mm2) are established at the facility based only on a distance measurement and the absolute source strength of the national standard Ra-Be photoneutron source. The error in the 252Cf source strength (±1.1%) dominates the total free-field flux uncertainty of ±1.4% (1σ). Neutron scattering effects in the source capsule and support structures and neutron return from concrete and earth boundaries have been calculated and investigated experimentally. In the worst case, they contribute ±0.7% to the total flux response uncertainty for all observed neutron reaction rates, including those with sensitivity to low-energy neutrons.