A molecular transport model for the Europa UV Spectrograph instrument has been developed to predict optical throughput over the course of the mission lifetime. At beginning of life, internal surfaces will be covered with a thin layer of non-volatile residue (NVR); after launch, contaminants from the electronics components will diffuse out of their parent materials, adding to the overall contaminant environment. The transport of these contaminants, and accumulation onto optical elements, is dependent on geometry, temperature, transport kinetics, and contamination process control during instrument build. Quantitative thermogravimetric analysis (QTGA) was used to estimate the distribution of activation energies for desorption of contaminant from electronics, and that of NVR. An assessment of the effectiveness of different lengths and frequencies of decontamination cycles was performed, and a 12-hour decontamination sequence was effective at removing accumulated contaminant. We found that the combined effects of temperature and view factors resulted in the curious result that whether the telescope aperture door was open or closed had insignificant effect on optics cleanliness. Allowing the door to be closed through much of mission life, in turn, protects the spectrograph from being contaminated by thruster plumes or contaminant from the spacecraft environment. Finally, a parametric analysis of the effect of activation energy distribution was performed. If a lower energy distribution, characteristic of electronics outgassing was used for NVR transport, the throughput margin would be reduced significantly, but the coverage would still be below that at beginning of life.