  The radial thermal expansion {\alpha}r of bundles of single-walled carbon
nanotubes saturated with 3He up to the molar concentration 9.4% has been
investigated in the temperature interval 2.1-9.5 K by high-sensitivity
capacitance dilatometry. In the interval 2.1-7 K a negative {\alpha}r was
observed, with a magnitude which exceeded the largest negative {\alpha}r values
of pure and 4He-saturated nanotubes by three and two orders of magnitude,
respectively. The contributions of the two He isotope impurities to the
negative thermal expansion of the nanotube bundles are most likely connected
with the spatial redistribution of 4He and 3He atoms by tunneling at the
surface and inside nanotube bundles. The isotope effect turned out to be huge,
probably owing to the higher tunneling probability of 3He atoms.
