TAEA participated to the OECD/NEA International Standard Problem No:42 (ISP-42) which is hosted by the Paul Scherrer Institut (PSI), Switzerland. The ISP-42 test was performed in the PANDA test facility, at the PSI, as a sequence of Phases A through F, representing typical passive safety system operating modes covering certain specific phenomena. The configuration used for ISP-42 was corresponding to the European Simplified Boiling Water Reactor containment and passive decay heat removal system at about 1:40 volumetric and power scale, and full scale for time and thermodynamic state.

1 n

< X ^—4

Mean Deviation^— > abs n 1

250000

225000

200000

cT 175000 £

5 150000 gj 125000

Ї 100000

(6

03

x 75000 50000 25000 0

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

Axial Position (m)

FIG. 4. Heat flux distribution along the condenser tube (Pn=4 bar, Rev=77000-86000 andRev

= 45000for Xi=52 %) [4].

250000

200000

СМ

Е

^ 150000

х

3

100000 га ш X

50000 0

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

Axial Position (m)

Axial Position (m)

FIG. 6. Effect of system pressure (pure steam) [4]. TABLE I. COMPARISON OF CORRELATIONS

PURE STEAM	Mean Deviation**, %
fexp>1-4	fexp< 14
Present Study	5.26	7.18
UCB	10.57
STEAM+NC GAS Based on Xg	Mean Deviation, %
Xg<0.1	Xg>0.1
Present Study	10.23	18.39
UCB	12.41	20.58
STEAM+NC GAS Based on Sh Number	Mean Deviation, %
	Shrr<5	5<Shrr<25	Shrr>25
Present Study	17.22	16.17	9.16

Both the experimental results and prediction of the RELAP5/mod3.2.2 code reveals the fact that the system behaviour during Phase-A is highly affected by the performance of Passive Containment Cooling System (PCCS) heat exchangers. The objective of Phase-A is to investigate the start-up of passive cooling system when steam is injected into a cold vessel (dry-well) filled with air and to observe the resulting gas mixing and associated system behaviour. This simulation demonstrates the importance of both pure steam condensation and steam condensation in the presence of air for natural circulation in the system which in turn governs the realistic system behaviour. The system transient has been developed into two distinct parts: first, system heat-up and pressurization period (~3800 s) due to evaporation in the reactor pressure vessel with constant heat input from the heaters in the core and weak heat removal rate from PCC heat exchangers as the result of high air mass fraction; second, system pressure stabilization period (from 3800 s to the end of analysis) during which PCC heat exchangers become active as the result of venting of air from PCC tubes. The results of RELAP5 predictions for PCC-1 heat exchanger are presented in Figs. 7 and 8 for time 1500 s and 5000 s, respectively. These two distinct times are selected to demonstrate the PCC heat exchanger performance during aforementioned two distinct parts of the transient. In these figures, only the results of 5 tubes (out of 20 tubes) which were lumped to single pipe consisting of 10 control volumes were shown. Two parameters are essential with respect to PCC heat exchanger performance; local heat flux and air mass fraction. As given in Section

4.1, the system pressure is also an important parameter for the rate of condensation. However, the effect of the system pressure is expected to be small in these two figures since the pressure difference is small, i. e. 0.7 bar.

As could be seen in these figures, the local heat flux is affected by the presence of air inside the PCC heat exchanger tubes, as expected. Since the local air mass fraction is about 0.94 (almost pure air) and constant throughout the length of the condenser tubes as predicted at t=1500 s (Fig. 7), the local heat flux values are suppressed to about 0.2 % of the local heat flux values predicted at t=5000 s during which condenser tubes are full of almost pure steam down to 1.3 m (about % of total length). The maximum air mass fraction at the bottom of the condenser tubes is less than 0.3 at 5000 s. It is to be noted that some amount of air is accumulated in bottom part of tubes and lower drum of PCC-1 after 3800 s due to terminated vent flow from PCC lower drum to the wet-well tanks. The accumulation of air at the bottom of tubes shorten the active condensation length to about % of total lenght, as seen in Fig. 8, and this reveals the fact that percent of shortening of active condensation length is also the function of system pressure and differential pressure developed between PCC lower drum and wet-well tanks. In other words, system pressure and differential pressures developed between components in a system operating under natural circulation could highly affect the rate of vent of air from PCC tubes and in turn the effective condensation length.

6. CONCLUSIONS

In this paper, activities of the TAEA concerning condensation in the presence of NC gas were given. In the experimental analysis, it was observed that the overall agreement between the analytical analysis and the experimental data obtained for heat flux or heat transfer coefficients is reasonably good. For example, the heat flux significantly decreases as the inlet air mass fraction increases. Moreover, it could be promulgated that the effect of superheating of inlet stream possesses no considerable effect on the heat flux. Another conclusion emerging from experimental studies is that the local heat flux values for pure steam and mixture runs come closer towards the bottom of the condenser tube.

PCC-1,t= 5000 s
P= 2.8 bars, Re^^l 0,000

The correlations obtained from UCB database show that the mixture side Reynolds number is also a strong parameter affecting heat transfer coefficient. However, it should be noted that the given correlations stay behind the real phenomenon occurring inside the tube. Because, neither interfacial waviness nor the suction effect is taken into account. At the same time, these correlations depend on the flow regimes of either phase. If turbulent — turbulent flow regime is in question, these correlations fail. Therefore, the studies have been concentrated on an analytical solution in which a film-wise condensation of a down-flow steam/NC gas mixture in a vertical tube is considered. In this analytical model, the mixture side is treated as turbulent flow. The effect of Prandtl number, interfacial shear stress, interfacial waviness, entrainment and deposition and especially the suction have been covered in our model. The two-fluid formulation constitutes the main routine. The interfacial temperature is estimated using the stagnant film theory. Moreover, for the mixture side, the turbulence model is developed in order to elaborate suction effect, which is one of the primary reasons of the enhancement of the mixture side heat transfer coefficient. Finally, it should be stated that the diffusion layer theory is superimposed into the model to define the closure relations.

The condensation is an important heat transfer mode for natural circulation in innovative systems nuclear reactor systems like the Simplified Boiling Water Reactor design. The realistic prediction of local heat flux in heat exchangers of passive containment cooling system is essential and due to this reason physical models in computer codes for condensation and effect of NC gases on condensation should be assessed. Though very preliminary, ISP-42 study on PANDA reveals us the fact that the realistic prediction of the performance and behaviour of PCC heat exchangers could affect the overall system behaviour and the rate of condensation heat transfer is the function of air mass fraction at the inlet of