After the GDCS injection period is terminated, the core starts to boil again due to the long term decay heat. The long term DW pressure increase is mitigated through PCCS condensers. The PCCS removes the core decay heat energy, released to the containment from the RPV, to outside of the containment. Steam condensate from the PCCS is returned to the RPV via GDCS tanks and noncondensable gases are vented to the SP.

The water inventory of the GDCS pools is slowly replenished with condensate draining from the PCCS. For all design basis events, the closed loop of PCCS condensation and GDCS drainage to the RPV results in long term coverage of the core. Beyond design basis events, the GDCS equalization flow may be necessary where multiple failures are assumed. When the vessel water level reaches 1 m above TAF and at least 30 minutes has passed since the Level 1 signal is confirmed, the GDCS equalization line will open to inject water from the SP to the RPV.

VI-4. Conclusion

The ESBWR program is based on the earlier SBWR program, which was sponsored by the US Department of Energy (DOE). The ESBWR program was started in 1993 to improve the economics of the SBWR. There are some significant differences between the SBWR and ESBWR designs. The major differences are: 1) increased core thermal power, 2) much higher core power density, 3) considerably reduced DW and WW volumes relative to the reactor power, 4) increased GDCS tank volume, and 5) increased component numbers of the PCCS and ICS in the ESBWR compared with those in the SBWR. In addition to these, the number of the main steam lines was increased to four in the ESBWR. A multi-year, four-phase program was defined to complete the technology, develop a detailed design, and secure certification with regulatory bodies. Evaluation of the overall design showed that the plant was considerably simplified and that the overall material quantities were significantly lower than those for the SBWR design and other GE designs.

The ESBWR is equipped with a passive safety system that is basically similar to that of the SBWR. This reactor design features the simplification of the coolant circulation system and implementation of passive safety system. There are several engineered safety systems and safety-grade system in the ESBWR which are directly related to the relevant issues and objectives of the present program. The performance of these safety systems under a LOCA and other important transients is a major concern. Since the ECCS is driven by the gravitational head, interactions between the ADS, GDCS, PCCS and other auxiliary systems are important. The safety systems and various natural circulation phenomena encountered after initial blowdown in the ESBWR are somewhat different from the system and phenomena studied by the nuclear community in the existing commercial nuclear reactors.

Comprehensive integral system and separate effects testing have been conducted to verify the functionality of passive safety system [1]. GE performed tests to assess the GDCS performance in a low pressure full-height GIST facility. Results of this study demonstrated the feasibility of the GDCS concepts. GE also performed tests to assess the PCCS performance in a low pressure, full-height Toshiba GIRAFFE facility in Japan. A PANDA facility in Switzerland, with a low pressure and full — height, was built for testing the PCCS performance and containment phenomena in the SBWR. Later, PANDA was modified to partially simulate the ESBWR configuration.

Purdue University designed and constructed an integral test facility, called PUMA (Purdue University Multi-dimensional integral test Assembly), sponsored by the U. S. NRC. Originally, the PUMA facility was designed to address the functionality of SBWR safety system in 1994. The facility was modified to simulate the safety system in the ESBWR in 2006. The facility contains all of the important safety systems of the ESBWR that are pertinent to the postulated LOCA transient.

The design and technology program for the ESBWR involves several utilities, design organizations and research groups. In mid 2002, the technology base of the ESBWR was submitted to the U. S. NRC for review with the objective of obtaining closure of all technology issues. This was a first and necessary step toward obtaining NRC design certification.

REFERENCES TO ANNEX VI

[1] ISHII, M., et al., Second Scaling and Scientific Design Study for GE ESBWR Relative to PUMA Facility with Volume Ratio of 1/475, Purdue University Report PU-NE-04-04 (2004).

[2] ISHII, M., et al., Scientific Design of Purdue University Multi-Dimensional Integral Test Assembly (PUMA) for GE SBWR,” Purdue University Report PU-NE-94-01, U. S. Nuclear Regulatory Commission Report NUREG/CR-6309 (1996).

[3] GAMBLE, R., ESBWR Technology Program: Test Program, NRC-GE Meeting, Rockville, Maryland, USA (2002).