As part of the work of the International Partnership for Nuclear Disarmament Verification (IPNDV), an international exercise was held at the site of the Belgian Nuclear Research Centre (SCK CEN) in Mol, Belgium. Unirradiated plutonium-uranium mixed oxide (MOX) fuel in different amounts and various isotopic vectors in combination with different absorber materials were provided. Various detector types were used. Their potential for verifying the presence or absence of fissile material before and after dismantlement of a nuclear warhead. In addition to the measurements, simulations were performed using the Monte Carlo codes KENO, MCNP and Geant4 to simulate neutron signatures of the different configurations. In this paper results are presented for the effective neutron multiplication factor (keff), neutron energy spectra and spatial variability of emitted neutron flux densities obtained with the three codes. Spatial variations of neutron interaction processes within the various configurations are discussed. The significant effect of the building structures is demonstrated. The potential of the results to be used for benchmarking nuclear disarmament verification simulations is addressed.