NC experiments have been conducted in all ITF for characterizing the loop features and for constituting databases suitable for code assessment. NC experiments have been conducted in the Lobi, Pkl, Bethsy Pmk and Pactel facilities, the first three simulators of PWR and the last two simulators of WWER-440. In relation to all the ITF, NC experiments with decreasing mass inventories of the primary coolant system have been carried out. The electrical power supplied to the core simulator corresponded to the decay power and ranged between 1.5% and 5%, roughly of the nominal core power. In these situations the NC scenario can be characterized by a diagram showing the core power as a function of the primary system mass inventory.

The applications of system codes have been of help for interpreting the experimental scenarios, for optimizing the features of the nodalizations and for characterizing the code capabilities. A list of significant achievements is given below together with references where details of the analyses can be found.

a) The codes have been used to distinguish five main NC flow patterns depending upon the value of the mass inventory of the primary loop (see also Ref. [7]):

— Single phase NC with no void in the primary system excluding the pressurizer and the upper head;

— Stable co-current two-phase NC with mass flow rate increasing when decreasing primary system fluid inventory;

— Unstable two-phase NC and occurrence of siphon condensation;

— Stable reflux condensation with liquid flowing counter-current to steam in the hot legs: flow-rate is sufficient to remove core power till loop mass inventory achieves values as low as 30-40% of the nominal values;

— Natural circulation with part of core rods in dry-out condition not favorable from the current technological and safety point of view.

b) The code has been used for characterizing the oscillations (third dashed item in the list above), showing the different role of the counter-current flow limitation (CCFL) at the entrance of the U-tubes, the siphon effect and the steam condensation both in the rising part of the U-tubes. The flow reversal and the different behaviour of parallel groups of U- tubes could also be observed by the help of the code, Ref. [8].

c) Different codes used by different European organizations have been applied to the comparative analysis of the A2-77 experiment carried out in Lobi facility. Capabilities of the codes were characterized as well as the influence upon the results that should be expected owing to the code-user effect, Ref. [9]. Deficiencies were observed in the predictions of pressure drops at low value of the Reynolds number.

d) The application to the study of NC in WWER-440 showed the code capability in predicting the flow stagnation and the consequent rise in system pressure caused by the loop sealing present in the hot leg of this reactor type, Ref. [10]. Clearing of loop seal occurs before dangerous situation for the system are reached and is also predicted by the code.

e) The code has been used to determine the maximum core power at which the PWR systems or PWR simulators can be operated in NC keeping sub-cooled the core. The limit was found close to 10% of the core nominal power, Ref. [11]. Higher limits were found fixing different thresholds for the system operation.

f) The code has been successfully used in predicting NC phenomena measured in systems that are relevant to the AP-600, including NC across the PRHR (pressurized residual heat removal), and the CMT (core make-up tank) systems, Refs [12] and [13].