Mass balance issue during phase change (water -> steam)

Dear users,

In TOUGH/ TOUGHREACT when we consider water-steam flow in a pipe (EOS1) and assume at given conditions for pressure and temperature the water changes its phase to vapour.

If I have a control volume of 1 litre and this cell is completely saturated with water (1 litre of water), usually when the phase transition happens, I form more than 1 litre of steam (lets say 1000 litres of steam for example). That is, 999 litres of gas phase more than what the cell can handle. How to deal with such mass balance issues? I am looking at reactive transport in such cases where phase transition occurs.

Thanks,

Sai