  We develop a model in the framework of nuclear fragmentation at thermodynamic
equilibrium which can be mapped onto an Ising model with constant
magnetization. We work out the thermodynamic properties of the model as well as
the properties of the fragment size distributions. We show that two types of
phase transitions can be found for high density systems. They merge into a
unique transition at low density. An analysis of the critical exponents which
characterize observables for different densities in the thermodynamic limit
shows that these transitions look like continuous second order transitions.
