  The KArlsruhe TRItium Neutrino experiment (KATRIN) aims to measure the
effective electron anti-neutrino mass with an unprecedented sensitivity of 0.2
eV/c$^2$, using beta-electrons from tritium decay. Super-conducting magnets
will guide the electrons through a vacuum beam-line from the windowless gaseous
tritium source through differential and cryogenic pumping sections to a high
resolution spectrometer. At the same time tritium gas has to be prevented from
entering the spectrometer. Therefore, the pumping sections have to reduce the
tritium flow by at least 14 orders of magnitude. This paper describes various
simulation methods in the molecular flow regime used to determine the expected
gas flow reduction in the pumping sections for deuterium (commissioning runs)
and for radioactive tritium. Simulations with MolFlow+ and with an analytical
model are compared with each other, and with the stringent requirements of the
KATRIN experiment.
