2. Turbine system

This system consists of a steam turbine, its associated equipment, a condenser, a circulating water system and a condensate feed water system. The ABWR turbine system is designed to achieve a larger capacity and higher plant efficiency based on experiences with the conventional BWR. This improves the overall plant thermal efficiency by about 1% as compared with the conventional 1,100 MWe-class BWR as a result of adopting 52-inch last stage turbine blades, moisture separator reheaters and higher inlet pressures.

The steam generated in the reactor flows into the main steam header via four main steam pipes, and enters the high pressure turbine via four main steam pipes, a main steam stop valve and a turbine steam control valve. The high pressure turbine exhaust is fed into the low pressure turbine via the cross around piping, a moisture separator reheater, an intermediate stop valve, and an intercept valve, before being led to the condenser. Butterfly valves are adopted as the intercept valve and the intermediate stop valve at the low pressure turbine inlet This reduces the pressure loss to improve the thermal efficiency as compared with conventional combination intercept angle valves.

The condensate water is boosted by a low pressure condensate pump, and processed in the condensation purification system via the steam jet air ejectors and a gland steam condenser. Then, the purified condensate is boosted by a high pressure condensate pump, heated by low pressure feed water heaters, pumped by a feed water pump, heated again by high pressure feed water heaters, and returned to the RPV.

The low pressure feed water heater system has three lines, each with four stage heaters, whereas the high pressure feed water heater system has two lines, each with heaters in two stages. The condensate feed water is heated in the feed water heater system by the steam extracted from the high pressure turbine. The plant thermal efficiency is also increased by the “heater drain pump-up method,” which leads the drain condensed by the feed water heater systems to the drain tank and returns it to the condensate feed water system.

The feed water flow is controlled by the three element control system, using the reactor water

level, the main steam flow rate and the feed water flow rate as the input

A turbine bypass system is installed to directly release the steam from the reactor to the condenser, in a turbine hip, bypassing the turbine system.

Figure 2.12.7 illustrates the turbine system configuration.

3. Enhanced Safety

The ABWR has been demonstrated to have safety, reliability and other excellent characteristics, as shown in Figure 2.12.8, by reflecting its advantageous features: unique technologies to the ABWR such as the RIPs, or the improved CRD (FMCRD); and those of the conventional BWR

1. Improved reactor shutdown capabilities

The ABWR is able to mitigate the influence of abnormal power excursions, as in the conventional BWR by means of its inherent safety features of self-controllability such as the Doppler effect, or void effect. Further in an ABWR, the FMCRD immediately shuts down the reactor, in an abnormal rapid power increase or heat removal capability drop, by rapidly inserting the control rods into it, as soon as high neutron fluxes, low reactor water level, high drywell pressure and other appropriate signals are detected. The FMCRD has two diversified power sources for control rod insertion: the electric drive function and the hydraulic one (the conventional BWR has only the hydraulic power source). With the adoption of step motors, its fine motion control (about 18mm/step) is made possible, which increases fuel reliabilities during normal operations. When the fuel power densities change with the control rod withdrawals, fuel is exposed to the stresses corresponding to this change. However, the fine control motions reduce this power change and the stresses in the fuel accordingly. Consequently, the reactor operating performance has been greatly improved together with the fuel performance parameters themselves. The FMCRD is also effective for preventing the occurrences of reactivity initiated events in the reactor and mitigating their consequences. A reactivity initiated event is a hypothetical event, in order to evaluate safety of reactor facilities, which assumes a rod drop (out of the core) when disconnected from the drive

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NSRA, Japan