TRASCO-ADS is a national funded program in which INFN, ENEA, and Italian industries work on the design of an accelerator driven subcritical system for nuclear waste transmutation. TRASCO is the Italian acronym for Transmutation (TRAsmutazione) of Waste (SCOrie). One of the most critical aspects in the design of an Accelerator Driven System is related to the interface region, which is the part of the beamline located between the accelerator, operating under UHV conditions, and the pressurized reactor vessel, consisting of a contained plenum of Pb-Bi eutectic (LBE). A so-called window could separate these two environments, but thermomechanical considerations and radioprotection issues point out that this component could be critical. In the windowless interface, no window is located between the linac and the spallation target. Only a suitable pumping and trapping system, for the gases and the vapors outcoming from LBE, divides the UHV accelerator and the spallation target vacuum. Vacuum gas dynamics theoretical considerations and calculations are presented in this article. The need for a validation of the theoretical models gave the motivation for an experimental work, whose results are also discussed. Scale-up of the experimental setup to the full system needs accurate analyses for a proper dimensioning of the system in the interface region. (C) 2004 American Vacuum Society.