The so-called ''threat spectra'' of an inertial fusion energy (IFE) high gain target (neutron, x-ray, and ion energy fraction and particle spectra) are the usual starting point for IFE reactor conceptual design. The threat spectra are typically computed using the same radiation hydrodynamics and thermonuclear burn computer simulation codes used to compute implosion, ignition and burn. We analyze the validity of this model for simulating the expansion of the direct drive IFE target plasma and for computing threat spectra. Particular attention is paid to the collisionality of the expanding plasma.