R/B:reaclor building T/B:turbine building

C/B:control building S/B;$eivice building RW/B radwaslc treatment building

Figure 2.12.1 ABWR plot plan (a twin-unit plant)

a twin ABWR.

At the center of the reactor building stands the RCCV accommodating the RPV and other structures. The cylindrical RCCV is mounted on the reinforced concrete base-mat and integrated with the reactor building. The RIP use eliminates the need for recirculation piping. This facilitates lowering the positioning of the RPV in the reactor building and the center of gravity of the entire reactor building.

Consequently, the building height is about 10m lower than the conventional building for a 1,100 MWe class BWR, enhancing the earthquake resistance.

Figure 2.12.2 compares cross-sectional views of a conventional 1,100 MWe BWR (an improved MARK- II containment vessel) and the ABWR reactor building.

Dimensions of the turbine building are also

Figure 2.12.2 Reactor buildings (a 1,100 mWe bwr and an abwr)

reduced by cutting the main steam piping space by the side entry arrangement, and positioning the main steam piping and the high pressure turbine side by side.

The turbine building is partitioned into the turbine area and the sea water heat exchanger area. The turbine area accommodates the turbine generator, the condenser, the feed water heaters, the reactor feed water pumps and other units, and is equipped with an overhead crane above the operation floor. The sea water heat exchanger area accommodates the heat exchangers of the RCW system, the sea water pumps, the circulating water pumps and other units. Those facilities shared by two units, such as the main control room, the radioactive waste treatment facilities, the service facilities and other common facilities, are centralized between the two units to reduce the lengths of connection piping and cables.

Figure 2.12.3 shows cross-sectional elevation and horizontal views of an ABWR reactor building and turbine building.

2. System design

In an ABWR, the conventional recirculation pipe break is excluded from the design basis event, taking advantage of the RIP use. This simplifies the ECCS for optimization.

Figure 2.12.3 Cross-sectional and horizontal views of ABWR reactor and turbine buildings

NSRA, Japan

2-120

The ECCS design adopted the probability safety analysis (PSA) approach from the design stage*⁸.

The ECCS is composed of: three independent divisions (two sets of the HPCF system and the RHR system in the LPFL mode, and one set of the RCIC system and the RHR system in the LPFL mode), and the ADS. Hie HPCF system has been improved to increase the resistance against transients and small or medium pipe breaks.

Consequently safety in the ABWR has been enhanced, in combination with the diversified control rod drive mechanisms using the FMCRD.

2. Advanced core

BWR core and fuel have been constantly improved for higher reliability (fuel integrities, fewer fuel failures, etc.) and improved economy (higher burn­up, higher availability, reduced fuel cycle costs, uranium saving technologies, fuel recycling, etc.). Advance fuel design or core concepts can be also used in existing reactors (back fitted).

The most up-to-date core/fuel design is applied to the ABWR and contributes to the improved plant capacity factors, operability, economy and other features.

2. Reactor system

1. Reactor internal pumps (RIPs)

An RIP is a single-stage mixed flow pump driven by a wet motor without shaft seals. Its diffuser, impeller and pump shaft are made of stainless steel, and its motor unit is housed in the motor casing located at the RPV bottom head. The wet motor is cooled down by a dedicated cooling heat exchanger set up for each RIB

The reactor power is controlled using the characteristic that the reactor power change is almost proportional to the reactor core flow change. The reactor core flow is controlled by adjusting the RIP speed. The pump speed is adjusted by changing its drive motor power frequency by means of the solid state, variable frequency power source.

Figure 2.12.4 shows the RIP configuration.

2. Fine motion control rod drive (FMCRD)

The FMCRD is driven by two diversified power sources: electrically in normal operations, and hydraulically in scram conditions. Its key

Figure 2.12.4 Internal pump system configuration

elements are the coupling, ball screws, ball nuts, hollow piston, outer tube, spool piece, motor, etc.

These elements are accommodated in three portions: one is the housing at the lower part of the pressure vessel; the second, the spool piece below it, includes the drive shaft; and the third is the electric drive motor. Bolts fix the FMCRD to the flange at the lower end of the housing. The drive motor is equipped with a control rod position detection device which measures the drive shaft rotation.

A bayonet coupling connects the control rods to the FMCRD. A position detection probe to indicate the control rod position when scrammed is mounted outside the housing. Also, a separation

*⁸⁾ [Translator’s note] The PSA approach in nuclear plant management has been tried in accident management and periodic safety reviews (Basic policies on introducing the “Risk Informed Regulation” approach in future nuclear regulation processes (The Nuclear Safety Commission, 2003)), or preventive maintenance planning. The Nuclear Safety Commission foresees expanded use of risk information in regulation, starting with the operation and maintenance area and seismic aspects of nuclear facilities. The comprehensive use of PSA in regulation, including design and construction, is considered to follow in later days (http://www.nsc.go.jp/NSCenglish/topics/rir.htm).

2- 121

NSRA, Japan