The results presented in the following chapters were gained within the project "European BWR-R&D Cluster for Innovative Passive Safety Systems (INNO-IPSS)" supported by the EU within the 4th framework programme.

2.1. The Emergency condenser

The design of the Emergency Condenser, as already used in the Gundremmingen Unit A Power Plant, is shown in Fig. 2. The tubes are about 10 m long. The tube inner diameter is

38.7 mm and the wall thickness 2.9 mm. Because the wall thickness resulted in a thermal conductivity of about two thirds of the total thermal conductivity a second bundle was designed with an inner diameter of 44.3 mm and the minimum acceptable wall thickness of 2 mm. Tests with the second bundle were performed with the bundle oriented in a vertical position, under an angle of 40.9° and in a horizontal position. The orientation under an angel of 40.9° should decrease the height between a flooded bundle (with zero energy transfer) and an empty bundle (with maximum energy transfer to the pool); in addition the mixing within the pool should be enhanced.

The tests have been performed under the same thermal conditions as in a real BWR; the pressures chosen were 7, 5, 3 and 1 MPa. The tests were performed in the NOKO facility, see Appendix A. The test procedure started with a flooded bundle, then the water level in the pressure vessel was decreased stepwise until the bundle was empty.

In Fig. 3 and 4 the experimental results are given. It should be added that the accuracy of the results ranged between less than 10 per cent for high powers and several ten per cent for power levels below 1 MW; the conditions for a flooded bundle are — on the other hand — relatively accurate.

In principle, the dependence of the transferred power on the water level in the pressure vessel is similar for both bundles. The influence of the orientation of the bundle is only minor, see Fig. 4.

Post-test calculations with ATHLET are shown in Fig. 3 for the first bundle and in Fig. 5 for the second bundle; the agreement is good. The distortion at about 5 m is due to the vertical part of the bundle.

It was shown that despite the good agreement for the (integral) transferred power the local deviations in the heat transfer during condensation as calculated by the ATHLET and CATHARE, see Fig. 6, are large.

FIG. 5. NOKO-2 power levels from tests and calculations for the bundle in vertical position

—I——— 1——— 1——- 1——— 1——— 1——— 1——— 1——— 1

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Tube length [m]

FIG. 6. Comparison between ATHLET and CATHARE calculation residts.

Therefore, for orientation tests were performed with a single tube (dimensions of the first bundle) instrumented with needle probes for the identification of the film thickness. To better study the film thickness a test tube with a tube of the second bundle and movable film probes are underway.

It was a result of the single tube tests that non-condensables were accumulated in front of the condensed water rather than been distributed along the tube.