The ROSA-V Large Scale Test Facility (LSTF) is the largest facility
in the world to simulate a PWR accident (Fig. 6-1). More than 130 experiments
have been carried out using the LSTF and these have confirmed that Japanese
nuclear power plants have adequate safety margins. Real plant events,
such as the Mihama Unit-2 steam generator tube rupture (SGTR) incident,
were reproduced using the LSTF. The results contributed to the evaluation
of the incident by the Nuclear Safety Commission. Recently, experiments
on accident management measures for the prevention of severe core damage
and for the confirmation of the advanced passive safety PWR (AP600)
have been conducted.
AP600 is the advanced 600 MW PWR designed by the Westinghouse Electric
Corporation and features passive safety systems for mitigating accidents.
The passive safety systems rely either on gravity-driven flow or on
pressure-driven injection to ensure effective operation. The AP600 passive
safety systems are shown in Fig. 6-2. Because only the valves are active
components, the safety systems are not affected by power failure and
there is less possibility of malfunction. However, the performance of
the passive safety systems may possibly be affected adversely by interactions
among various reactor components, because these systems rely on relatively
weak driving forces. The experiments were designed to address such possibilities.
Figure 6-3 shows the pressure at primary coolant loop (primary pressure)
during an experiment which simulated a 1-inch, small break, loss-of-coolant
accident (LOCA) in the AP600. Soon after the break, when the pressurizer
liquid level decreased, the core makeup tanks (CMTs) injected cold water
and the passive residual heat removal (PRHR) systems actuated automatically.
Once the automatic depressurization system (ADS) valves opened, the
primary pressure decreased to near atmospheric pressure and the huge
amount of water in the incontainment refueling water storage tank (IRWST)
flowed into the reactor pressure vessel by gravity. Twenty-four experiments
have been conducted simulating LOCAs and other abnormal events including
the inadvertent opening of ADS valves and a SGTR. In these experiments,
the passive systems maintained adequate cooling of the core. |
