  Electrons of dark current (DC), generated in high-gradient superconducting RF
cavities (SRF) due to field emission, can be accelerated up to very high
energies-19 GeV in the case of the International Linear Collider (ILC) main
linac-before they are removed by focusing and steering magnets. Electromagnetic
and hadron showers generated by such electrons can represent a significant
radiation threat to the linac equipment and personnel. In our study, an
operational scenario is analysed which is believed can be considered as the
worst case scenario for the main linac regarding the DC contribution to the
radiation environment in the main linac tunnel. A detailed modeling is
performed for the DC electrons which are emitted from the surface of the SRF
cavities and can be repeatedly accelerated in the high-gradient fields in many
SRF cavities. Results of MARS15 Monte Carlo calculations, performed for the
current main linac tunnel design, reveal that the prompt dose design level of
25 {\mu}Sv/hr in the service tunnel can be provided by a 2.3-m thick concrete
wall between the main and service tunnels.
