Separate effects tests were carried out for the AP600 design to demonstrate the feasibility of using a passive core cooling system to mitigate all design basis accidents. There were also confirmatory tests to verify the performance of the various system components. These included: passive residual heat exchanger, automatic depressurisation, passive core cooling system check valve and core make-up tank tests.

In addition to separate effects tests, there were also passive core cooling system tests to demonstrate the overall system performance for both pressurised and de-pressurised conditions. The test facility for this programme was the Oregon State University APEX facility, and the programme was carried out within a Westinghouse/USDOE collaboration.

There were a number of thermal-hydraulic facilities commissioned and operated during the 1980s and 90s in support of the needs of currently operating plant. Many of these facilities have been dismantled but others remain either in standby or in operation to service the needs of evolutionary water reactors. Facilities include PKL, SPES for PWR, PIPER-ONE for BWR, PACTEL and PMK for VVER and PANDA for BWR (Addabbo et al., 2001).

The SPES facility (Bacchiani et al., 1994) at the SIET facilities in Piacenza, Italy was modified to include a passive core cooling system and used for high-pressure system loop thermal -hydraulic tests in support of AP600. All the safety systems were simulated and a series of tests addressed LOCA, steam generator tube rupture (SGTR) and SLB thermal — hydraulic issues. PKL is currently in use to simulate boron mixing effects, in connection with a present day reactor transient issue involving boron dilution during reflux condensation in a LOCA.

Although configured for VVER geometry, PACTEL tests (Kervinen et al., 1990), have been carried out to simulate passive injection during a LOCA, which is of relevance to the AP600 safety system function.

15.10.1 Containment Tests

Many of the confirmatory tests for AP600 were in justifying the passive containment cooling system. Separate-effects tests to characterise the decay heat removal character­istics of the containment design were carried out. These tests included the investigation of heat removal from wetted steel plates simulating the containment surface. Also containment external cooling air flow path pressure drop tests were carried out to characterise the frictional losses. Steam condensation tests on surfaces at different angles were performed to simulate condensation inside the containment in the presence on non­condensable gases.

Composite containments, including a steel inner liner and an outer concrete shell, have been considered to meet potential European requirements for licensing. The outer concrete shell provides greater strength to mitigate the consequences of some severe accidents. Experiments to establish passive containment cooling for such containments were carried out in the PASCO facility at FZK, Germany (Erbacher et al., 1995).

Passive systems are a feature of a number of advanced evolutionary LWRs, both for primary coolant system heat removal and for containment cooling. Tests are in progress in the PANDA facility in Switzerland in the EC TEMPEST programme (Wichers et al., to be published), to resolve outstanding issues of the effects of light gases for confirming the long-term LOCA response of the passive containment cooling systems for SWR100 and ESBWR.