reactor pressure decreases, until finally only one low pressure condensate pump is left in service.

After flashing and warming the residual heat removal system at a reactor pressure of 0.93 MPa (gage) or less, reactor cooling is started at 0.75 MPa (gage) in the shutdown cooling mode (from the reactor to the residual heat removal system heat exchanger and back to the reactor). As long as the reactor water temperature is 80° C or more, the reactor water is deaerated and the condenser vacuum is maintained. Alien, the turbine system is separated from the reactor system, the noncondensable gas in the main condenser is exhausted through the off-gas system, the main condenser vacuum breaker is opened, and last, the main condenser is taken out of service.

3. Chemistry control during plant shutdown

For chemistry control during plant shutdown, control of corrosion product generation should be especially taken into consideration. If large amounts of corrosion products are produced in the feedwater and condensate systems during plant shutdown, they could become a source of impurities to be introduced into the reactor after plant startup and they could pose problems to the system cleanup operation before plant startup.

Therefore, it is important to control corrosion of piping and components of the feedwater and condensate systems by conducting the following layup control.

1. Formation and retention of a protective film by continuous water flow

2. Wet layup using deaerated water or dry layup to prevent rust development when continuous water flow is impractical

2. Plant Maintenance

Maintenance of BWR NPPs in Japan has a history of nearly 40 years beginning with the commissioning of Tsuruga Unit 1 of the Japan Atomic Power Company Co., Ltd. in 1970. During that period, stress corrosion cracking of the reactor feedwater sparger and the reactor primary system, etc. were experienced, and studies on plant maintenance optimization have been conducted together with the efforts to improve component materials and designs.

As a result, the number of plant shutdowns due to failures during plant operation is small these days, and the continuous operation length also has become longer.

Additionally, introduction of the devices to facilitate periodic inspections has also shortened periodic inspection duration, and the equivalent doses to workers have also been reduced.

1. Concept of Maintenance Management (1) Classification of maintenance

Maintenance is carried out to keep structures, systems and components in operable condition and includes preventive maintenance and corrective maintenance as shown in Figure 4.2.1.

[Source] Japanese Standards Association

(JIS 28115-2000)

Figure 4.2.1 Administrative classification of maintenance

In order to stably operate NPPs, it is natural to pay careful attention to operation, but it is also necessary to pay careful attention to maintenance in order to avoid a component failure during operation. For this reason, the mainstream in maintenance management is preventive maintenance which means maintenance done to avoid a component failure.

Preventive maintenance includes predictive maintenance and planned maintenance. The former monitors conditions of components in service, finds indications of performance degradation and/or failures, and performs maintenance in advance, and the latter performs maintenance on a periodic basis at established intervals. This planned maintenance is applied to many facilities and components during periodic inspection.

NSRA, Japan

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