The similitude of the geometry and of the operational parameters of the PWR simulators (or Integral Test Facilities, ITF) allowed a direct comparison between results of NC experiments. The database that was gathered from ten experiments performed in the six ITF listed in Table I has been used to establish a Natural Circulation Flow Map, Refs. [2] and [3]. In all the considered ITF, the NC regimes discussed in the previous chapter are experienced. The linear power of fuel rod simulators, the fraction of nominal core power and the primary system pressure mainly differentiate the considered experiments. In relation to pressure, PKL experiments have been performed at a pressure value roughly one half the value adopted in the other facilities. The identified differences and the ranges of design and of other operational parameters (e. g. pipe diameter, system volume, number of steam generators, heat losses to the environment, not explicitly discussed here), produce an envelope for any expected NC situation in a typical PWR. The envelope for NC system behaviour is assumed applicable when decay heat removal conditions are established.

Measured values of core inlet flowrate (G, Kg/s), core power (P, MW), primary system fluid mass inventory (RM, Kg) and net volume of the primary system (V = const., m3), have been used for setting up the NCFM. The diagram G/P versus RM/V has been preferred for the NCFM over other possible choices including non-dimensional quantities. The experimental database from ITF (six ITF, ten experiments) and the envelope of curves are given in Figs 2 and 3, respectively. The envelope in Fig. 3 is assumed to constitute the NCFM of PWR at decay core power.