Patent Number: 054266815
Section: summary

FIELD OF THE INVENTION This invention relates generally to protection systems for shutting down a boiling water reactor (BWR) and maintaining it in a safe condition in the event of a system transient or malfunction that might cause damage to the nuclear fuel core, most likely from overheating. In particular, the invention relates to emergency core-cooling systems (ECCS) for supplying water to the reactor core and containment systems for containing steam and radioactivity escaping from the reactor pressure vessel (RPV) in the event of a loss-of-coolant accident (LOCA) in a BWR. BACKGROUND OF THE INVENTION BWRs have conventionally utilized active safety systems to control and mitigate accident events. Those events varied from small break to design base accidents. Active systems, consisting of both high-pressure and low-pressure pumping equipment, have been the corner-stones of BWR/4 to BWR/6 safety systems product lines. A fully active three-division concept with N+1 capability (i.e., the capability to meet safety requirements despite one disabled division) is incorporated in the Advanced Boiling Water Reactor (ABWR). One alternative to the three active divisions concept is to use four active divisions. The four active divisions concept adds both more ECCS systems and supporting auxiliary systems, which require more maintenance to be performed. This is counter to the objective of reducing maintenance and improving safety. Another alternative to the three active divisions concept is to use passive systems. Totally passive safety systems have been studied for use in BWRs because of their merits in reducing maintenance and surveillance testing of the safety-related equipment, and in eliminating the need for AC power, thereby improving the reliability of BWR operation and safety. Simplified BWRs (SBWRs) have been designed with totally passive safety features that provide more resistance to human error in accident control and mitigation. There are, however, some tradeoffs when employing totally passive safety systems in BWRs. Due to their passive nature, the totally passive system--when designed in accordance with nuclear standards of system separation and diversity--would substantially add to plant size and cost. Therefore passive system applications to BWRs have been limited to small- and mediumsized plants having up to about 1000 MWe output. SUMMARY OF THE INVENTION The present invention is an improved system which combines the advantages of active and passive cooling systems. This invention combines active and passive systems in a single design that meets the safety requirements for BWRs. In addition, the design allows for a safety division out of service for on-line maintenance of the safety equipment during plant operation (N+2 capability). The preferred embodiment of the invention combines three active divisions with a passive fourth division to provide the N+2 capability. In accordance with the invention, an ECCS network is provided which has three active divisions (Divisions I through III) and a passive fourth division (Division IV). The selection of the fourth division to be passive provides diversity in systems and power supply with a resulting increase in plant reliability. The passive system also contains fewer parts so it provides the N+2 capability without a large increase in maintenance. From the licensing standpoint, a design with three active divisions and one passive division contains, at a minimum, the safety elements of the ABWR while introducing the passive features of the SSWR. The active systems include the traditional high-pressure and low-pressure safety systems that derive their power source from either reactor steam or from off-site AC power backed by on-site diesel generator power. The design of the active system divisions is similar to the known ABWR design. The passive division in accordance with the present invention derives its power supply from the plant battery bus. This division (as described herein) incorporates the following passive equipment: a gravity-driven cooling system (GDCS) for both short- and long-term reactor inventory supply; a primary containment cooling system (PCCS); a reactor heat removal condenser (RHR-CND) as a backup for the active reactor heat removal heat exchanger (RHR-HX) coupled to the suppression pool (SP), and the release valves (RLVs) connecting the RHR-CND to the RPV. Both the PCCS and RHR-CND are located in the condenser pool above the drywell. Multiple benefits accrue from combining the active and passive safety system divisions in a single design. First, the resulting system combines the individual merits of the active and passive systems. Second, the passive systems carry the load in the event that active systems are rendered inoperative and also share the load under plant degraded conditions. Therefore the passive systems serve the function of backing up the active systems. Third, the resulting design is feasible because it is based on the experience gained in the ABWR and SBWR designs. Lastly, the resulting reliability of the active/passive design concept is higher than the purely active concept of the ABWR. A BWR design combining the active safety systems of the ABWR with the passive safety features of the SBWR offers a new approach in combining the merits of both designs to provide diversity in core cooling, inventory makeup, depressurization and ultimate heat sink. In addition, the combined active/passive safety system of the invention provides increased plant reliability because of the diversity offered by the passive fourth division. The passive fourth division of the invention in turn incorporates a new system, the RHR-CND, which is used in conjunction with RLVs to provide backup depressurization of the RPV and backup heat removal and inventory control for events such as station blackout and reactor isolation. This is a medium-pressure system which is unique in operation compared to past isolation condenser (high pressure) and passive containment cooling (low pressure) systems. To improve plant availability and utilization of manpower, on-line maintenance is considered desirable for the ECCS network. By making the fourth division passive, on-line maintenance can be achieved without increasing the number of diesel generators, service systems and component cooling water systems.