Transient diffuser flow in an HF/DF laser is analyzed via a simplified model. A radial laser configuration is assumed where the diffuser is idealized as a ring-symmetric channel of uniform width. The outward radial flow following an abrupt start-up of diffuser inflow is computed numerically using a GRP-type code for integrating the Euler equations, which include an ongoing reaction of hydrogen recombination. A new kinetic scattering effect is suggested, resulting from the recoil of an HF or DF molecule assisting as third body in the hydrogen recombination reaction. The main conclusion is that transient diffuser plume flow effects do not drastically aggravate the potential for contaminating backflow (HF + DF) caused by either thermal or kinetic backscattering. Keywords: Laser exhaust; Spacecraft contamination; Chemical laser; Exhaust plume, Hydrogen recombination; Transient flow.