TRASCO ADS is a programme aiming at the design of an accelerator-driven subcritical system in which INFN, ENEA and Italian firms work on the preliminary study and design of the high-intensity proton linac accelerator and on the windowless interface between the accelerator and the reactor. The accelerator UHV and the spallation target vacuum are divided only by a suitable pumping and trapping system for the gases and the vapours emerging from the molten LBE. In order to design a pumping system for the accelerator-reactor interface region, vacuum gas dynamics theoretical considerations and experimental evaluations are needed to understand the behaviour of the vapours and gases in the target region. For the same purpose the literature data and experimental results on the radioactive gases and vapours, produced by the spallation process, must be investigated. From the vacuum gas dynamics point of view, a numerical method based on angular coefficients has been validated to estimate the net flux of the LBE vapours and the gases coming from the spallation process, the oxidation control and the tube outgassing. From the safety point of view, preliminary experiments are under way for an estimation of the net flux of high-activity gases such as mercury and polonium (using tellurium as a polonium simulator) with the purpose to evaluate the radioactivity in the beam tube. All these data, compared with those from literature and the experimental results, give an improvement towards a suitable design of the pumping system for an  DS windowless interface according to technological and nuclear safety constrains.