By altering the collision mechanism in the numerical transport calculations, and by constructing an analytical model based on age-diffusion theory, the outstanding feature in the life history of deuterium-deuterium (D-D) fusion neutrons penetrating deeply into ordinary concrete is shown to be the transport in the 2.3-MeV oxygen antiresonance. This result is used to assess the impact of the cross-section uncertainties and the uncertainties due to variations in the D-D fusion spectrum and temperature.