  Variable Conductance Heat Pipes (VCHP) are mainly employed to cool down
electronic systems in spacecraft applications, as they can handle high
temperature fluctuations in their cold source, preventing thus the systems from
damaging. These fluctuations, as well as ultra-low temperatures, are always
present in outer space, and one of the key steps in a VCHP design is therefore
to make sure that they endure these conditions. However, not much has been
written about their resilience during and after a long exposition to
subfreezing conditions, i.e. temperatures lower than the freezing point of the
working fluid. In this paper we implement and validate a computational routine
based on a modified Flat-Front Approach to predict the VCHP temperature profile
and to determine the location of the gas-vapor front. Then we continuously
expose an ammonia/stainless-steel VCHP to temperatures below the ammonia
freezing point for 211 hours, to later examine the formation and subsequent
dynamics of a thin block of frozen ammonia which is diffused into the inactive
part of the heat pipe condenser. We describe as well how a strong correlation
between the adiabatic section and the reservoir temperatures is maintained (or
broken) upon the occurrence (or absence) of the blockage of frozen ammonia.
