Schematic overview of volatile shielding by magnetic field. Three critical radii are denoted by black lines: the corotation radius R_cor, the magnetosphere radius R_mag, and the ice line radius R_ice. The gradually circularized orbits of tidal fragments of a comet are denoted by gray dashed lines. In the process of the orbital circularization due to the magnetic drag near the pericenter, icy fragments gradually lose their water content by evaporation after crossing the ice line. Small fragments are dried out earlier than large fragments. Provided R_mag > R_cor, the vapor gas between R_cor and R_mag is propelled to infinity by the magnetic field, leading to a truncation of gas near R_mag Thus, the vapor gas is shielded by this propeller effect. If fragments vaporize all of their water content outside the corotation R_cor, then only refractory materials cross the magnetosphere and finally get accreted onto the white dwarf. In this case, volatile accretion does not occur.