Seven commercial NPP systems and three ITF, not used for setting up the database presented in Fig. 2, are considered for demonstrating the use of the NCFM, Ref. [3]. Main characteristics of the NPP and of the ITF can be drawn from Tables 2 and 3, respectively.

TABLE II. RELEVANT CHARACTERISTICS OF NPP CONSIDERED FOR THE APPLICATION OF THE NCFM

TABLE III. RELEVANT CHARACTERISTICS OF ITF CONSIDERED FOR THE APPLICATION OF THE NCFM

Reactors 1 to 4 (Table II) have been built and are in operation. Reactors 5 to 7 are in a more or less advanced design stage. The geometric layout of primary systems for reactors 1, 2, 5, 6 and 7 is similar to the geometric layout of ITF originating the database for the NCFM. However, differences are present in the relative elevations between core and SG. Reactors 3 and 4 are equipped with OTSG and HTSG, respectively. So the geometric layout of the primary system is different from the geometric layout of ITF originating the database for NCFM.

Pactel and RD14M (Table III) are experimental simulators of WWER-440 and CANDU NPP, respectively. Their geometric layout is different from those of a PWR. In the case of WWER — 440, six loops equipped with HTSG are connected to the vessel, though only three are simulated in Pactel. Horizontal core configuration characterizes the CANDU design, that otherwise is equipped with UTSG.

A comparison has been made between measured (case of ITF) and calculated (case of NPP) system behaviours during NC and the data that characterize the NCFM. To this aim, code calculations assuming stepwise draining of primary system fluid mass inventory have been performed (case of NPP) and relevant NC experimental data are utilized (case of ITF).

In the case of NPP, the qualification level of the adopted code and nodalisation affects the calculated NCP. Furthermore, in the case of new generation ‘passive’ safety reactors 6 and 7, the emergency loops connected with the primary system are assumed to come into operation once the coolant draining process is initiated.

Calculated or measured transient scenarios reflect the NC flow regimes identified in Fig. 1, Ref. [3]. The RCNC regime is not achieved in the systems equipped with HTSG and OTSG and is not evident from the RD14M database. The SCNC regime is also not evident in all the calculations or available experimental databases. Mostly SPNC is calculated in the NPP 6 and 7.

Significant results are shown in Figs 4 to 6. The following observations apply:

• NCP of UTSG equipped PWR and of WWER-1000 is qualitatively similar (Fig. 4). Therefore the last generation of HTSG WWER shows a reasonable NCP. The good performance of the PWR-2 can be noted (low power NPP equipped with four UTSG).

• The OTSG equipped PWR show an ‘early’ NC flowrate decrease and an early stop of NC due to void formation in the ‘candy-cane’ and the rising part of the hot leg (Fig. 4).

• NC flowrate in AP-600, as expected, is not affected by draining because of liquid mass supplied by the ‘passive’ emergency cooling loops (Fig. 5). This behaviour does not show up in the case of EP-1000 presumably due to lack of qualification of the adopted code model.

• The WWER-440 simulator (Pactel) exhibits a decrease of the NC flowrate at relatively high mass inventories of the primary loop. The presence of the hot leg loop seal is at the origin of a partial flow stagnation (Fig. 6). Removal of the hot leg loop seal is effective in improving NCP as shown by the calculated WWER-1000 transient. The consideration of a passive system also improves the NCP of the Pactel.

• The CANDU simulator exhibits two different behaviours depending upon the considered experiment (Fig. 6). This shows the need of a deeper investigation before drawing conclusions. It may be noted that larger driving forces characterize CANDU systems for NC compared with PWR systems, owing to the larger distance of heat source and sink. However, larger pressure drops are also expected owing to the longer core and to the presence of small equivalent diameter pipes at core inlet and outlet.

Any attempt to judge the results, i. e. the NCP of involved NPP and ITF, should consider the quality of the used databases (DB). The result of a two step evaluation can be found in Table IV. The second column considers the demonstration of the quality of the starting DB. If ‘N’ appears in the second column, the evaluation in the third column is not meaningful. The third column considers the quality demonstration of the used DB, e. g. the nodalisation in the cases of code use and, finally, the NCP judgement (more details are provided in Ref. [3]).

The ‘NS’ mark for PWR-3 is due to the poor NCP also at high values of the RM inventory caused by the long vertical hot leg. Quality of data is assumed to be suitable. The ‘N’ in the case of EP-1000 (third column) is related to the available nodalisation. The dash (-) in the case of RD14M comes from the contradictory DB available at the moment.

TABLE IV. SUMMARY OF NCP EVALUATION FOR THE CONSIDERED NPP AND ITF