  This work constitutes one part of an investigation of the low-temperature
changes of the properties of the eta' meson. In turn these properties are
strongly tied to the U(1)_A anomaly of Quantum Chromodynamics. The final aim is
to explore the interplay of the chiral anomaly and in-medium effects. We
determine the lifetime of an eta' meson being at rest in a strongly interacting
medium as a function of the temperature. To have a formally well-defined
low-energy limit we use in a first step Chiral Perturbation Theory for a large
number of colors. We determine the pertinent scattering amplitudes in leading
and next-to-leading order. In a second step we include resonances that appear
in the same mass range as the eta' meson. The resonances are introduced such
that the low-energy limit remains unchanged and that they saturate the
corresponding low-energy constants. This requirement fixes all coupling
constants. We find that the width of the eta' meson is significantly increased
from about 200 keV in vacuum to about 10 MeV at a temperature of 120 MeV.
