Simulink, XSteam and numerical/solver problems

Hi!

I am trying to develop a dynamic model for a steam generator in Simulink. In order to do so, I'm adopting a lumped-parameter moving-boundary approach: there are three levels (subcooled water, two-phase and superheated steam) which vary in time, based on temperature and pressure conditions. I implement non-linear differential equations (ODEs) and algrbraic relationships to calculate temperatures and pressure. The water side is coupled with energy balance equations for the wall, which is in turn connected to energy balance equations for the primary coolant side. The coupling is therefore axial and radial. Since temperature and pressures are necessary to calculate water properties (I use XSteam), I encur in numerical problems and the simulation aborts when the system is coupled. There is extreme sensitivity to the selected solver (only ODE15 appears to work) and to the simulation parameters (e.g., not able to reduce the relative tollerance below 1e-4). I've tried to assign IC to almost all variables in the system and it appears to work, until I try to connect the steam generator block to the rest of the power system: it is sufficient that I put both the steam generator block and any other (much simpler) block on the same Simulink file, without even connecting them, that the simulation aborts immediately.

How should I proceed in order to try to solve these issues? Can anybody help)

Thank you very much in advance!! Sara