Two-phase flow models in unbounded two-phase critical flows

Referring to a Loss-of-Coolant Accident situation in LWRs, an analysis of the two-phase region just downstream from the broken pipe, in which a two-phase critical flow takes place, has been performed. A characterization of the flow pattern inside the unbounded two-phase jet is given considering: jet's external shape, obtained by means of photographic pictures; pressure profiles inside the jet, obtained by means of a movable ''Pitot'' gauge; jet phase's distribution information, obtained by means of X-ray pictures. Jet's X-ray pictures show the existence of a central high-density ''core'' gradually evaporating all around, which gives place to a characteristic ''dart flow'' the length of which depends on stagnation thermodynamic conditions. (orig.)