The study of NC is of primary interest in the nuclear technology. In the case of PWR, primary circuit layout is designed to optimize the NC performance. Following accidents originated by recirculation pumps trip or even small break Loss of Coolant Accidents, NC may constitute the main mechanism to transfer energy from the core to the steam generators, therefore keeping the NPP in a safe condition. In addition, the ‘quasi-steady’ thermalhydraulic configuration of the reactor loops makes easier the assessment of system code capabilities used for simulating the evolution of generic transient. Owing to the above reasons, all Integral Test Facilities (ITF) built so far that simulate PWR have been used to characterize the NC. A wide experimental database has been gathered and is available.

In the present context, only data measured in ITF designed following the criteria ‘time­preserving’, ‘power-to-volume-scaling’ and ‘elevati on-scaling-factor-equal-to-unity’ are taken into account. The main features of these facilities are summarized in Table I. Additional details can be found in Refs. [8] and [9]. The actual Kv, last row of Table I, gives the ratio between the volume of the reference reactor for the largest ITF (i. e. Lstf) and the volume of the considered facility. An idea of the relative dimensions of the loops and, therefore, of the electrical core power can be derived. NC experiments have been performed in all these facilities with core power close to the decay power value typical of NPP, as documented in Ref. [1].

The NC experiments of interest, around ten, have been conducted with primary loop in single­phase and two-phase conditions:

— at constant pressure of the primary system, close to the saturation pressure of the hot leg in nominal conditions;

— with core power in the range 1%-5% of nominal value as already mentioned;

— with the SGs at nominal conditions of level and pressure;

— having available feedwater flowrate and temperature suitable for removing core power;

— by stepwise draining of primary coolant achieving a quasi steady-state at the end of each draining step.

The NC flow patterns or regimes a) to d) defined in Section 1. have been identified. Based on the results of computer codes calculations and of experiments performed in the PWR simulators (see Table I), the NC regimes are characterized in Fig. 1, taken from Ref. [9]. The mass flowrate at core inlet is given as a function of the primary system mass inventory. Other than the flow regimes, the transition zones and the occurrence of dryout situation at mass inventory roughly below the 40% of the nominal value, can be noted. The dryout occurs owing to the sharp decrease in the heat transfer coefficient in the core when void fraction and mass velocities reach a lower boundary. The wideness of the transition zones comes from uncertainties of the database, generally originated by lack of quality assurance, as well as from differences in some boundary and initial conditions. More details related to each flow regime are given below.

TABLE I. RELEVANT HARDWARE CHARACTERISTICS OF THE PWR SIMULATORS CONSIDERED FOR NATURAL CIRCULATION