dependent on plant power output capacity. For example, ten SRNM channels and 208 (52 x 4) LPRM channels are installed in a 1,350 MWe ABWR. In addition, the ABWR NMS is a microprocessor­based digital system and employs automatic LPRM gain adjustment which has to be performed periodically. An ABWR plant is capable of operating multiple control rods at a time in the gang mode operation so that the conventional RBM functions are enhanced to form a multi-RBM (MRBM) for neutron flux monitoring in gang mode control rod operation.

2. Main Control Room

The instrumentation and control system of a nuclear power plant is designed to adequately monitor and control the plant during normal operation and in the event of operating condition changes, electric power output changes and other transients or disturbances which are anticipated to occur. Most monitoring and control, including that for reactor and turbine-generator systems, is primarily carried out at the main control room (MCR) in a centralized manner. Figure 2.6.8 shows the basic monitoring and control structure in a nuclear power plant. Control panels in the MCR are comprised of control benchboards and vertical panels including sequence control panels and instrument panels. The control benchboards provide overall plant monitoring and individual device operations such as starting a pump motor. The sequence control panels perform logic controls to operate individual devices such as pumps and valves according to the commands from the control benchboards. The instrument panels process and convey local sensor signals such as flows and pressures to the control benchboards, and provide alarm checks when measured values exceed their limits. Some devices are monitored and controlled not in the MCR but at local panels. Early in the MCR development, the main functions of a process computer system were limited to such functions as data acquisition, alarming and recording of plant operating parameters and reactor core performance calculation for reactor monitoring.

After the TMI accident in 1979, extensive efforts were made to enhance monitoring and control in the MCR through development of better human­

machine interfaces (HMIs) and broader utilization of computers, which thus led to the second generation MCR. Main features of the second generation MCR include substantial use of video display units (i.e. CRTs) for monitoring and the well-designed configuration of one main and two auxiliary benchboards. These features provide simple and secure monitoring over the many complex and diverse plant systems, and reduce operators’ physical work load.

50. Structure and functions of control panels

A nuclear power plant is organized into a variety of systems. Control panels are categorized basically by systems. As summarized below, of particular note are the appropriate redundancy and independency which are incorporated into the design of control panels composing the RPS.

1. Redundancy in the RPS

The RPS is designed to ensure that the required safety and protection functions are operable even in the event of any single failure of components such as detectors and power supplies or any malfunction of any single channel which is one of the circuits to actuate a trip signal according to detector signals.

2. Independency in the RPS

Any one channel including its components is electrically and physically separated from other channels in the same system so as not to hinder the safety and protection functions.

In implementing these design points to the RPS control panels, sufficient redundancy is employed for safety-related components such as detectors and power supplies, and separate control panels are provided where separation is required. Additionally, panels are installed in such a manner that proper distance between separate panels is assured in light of signal cables routing as well. Electrical separation is still considered for devices that are to be separated but mounted unavoidably in the same control panel.

High reliability is required of the control panels particularly for the primary systems whose failure may cause output power decrease or immediate plant shutdown. To this end, i.e. to achieve high plant availability, redundant detectors, control circuits and power supplies are provided in these

2 — 67

NSRA, Japan

1. Second Generation MCR

Figure 2.6.10 shows a second generation MCR, which has advanced points with respect to monitoring and control capability plant operating reliability and plant operation management, compared with a first generation MCR Measures to achieve these advanced points in the second generation MCR are shown in Figure 2.6.11. In addition, the second generation MCR includes functions, which are not provided in the first generation MCR, such as automatic operations in plant startup/shutdown, automatic power adjustment, operation guidance for plant startup or shutdown, and automatic voice announcements in the MCR for major events and parameters concerning plant operation.

While CRTs and computers are utilized in improving the HMI in the second generation MCR, the following design points remain basically the same as those applied to the first generation MCR

Control benchboards

valves

Figure 2.6.8 Basic instrumentation and control structure in a nuclear power plant

NSRA, Japan

2-68

Figure 2.6.9 Integrated digital control system in an ABWR

2-69

NSRA, Japan

Figure 2.6.10 Second generation main control room (MCR)

Objectives

Measures

• Enhancement of plant monitoring and control

• Rearrangement of monitoring and control

• Intensive comprehension of plant situations

• Prevention of erroneous operations

• Prevention of erroneous judgments

• Improvement of plant operating reliability

• Prevention of erroneous operations

• Intensive comprehension of plant situations

• Enhancement of plant operation management

• Operator consoles (with CRTs)

• Data recording

+ Main and auxiliary control benchboards

• Functions allocated based on the analysis in terms of importance, urgency and frequency

Enhancement of CRT displays and computer system

-11 CRTs

• Displays during normal operation

• Displays in an emergency

• Standby system conditions monitor (ECCS, condensate feedwater systems)

• Plant automation and operation guidance

• Surveillance test guidance for ESFs

Easier identification of devices

• Shape, color and layout

Easier identification of alarms

• Layout and color

Improvement of data recording

• Substantial expansion of computer inputs

• High speed printer and CRT hardcopy

Figure 2.6.11 Features of second generation MCR

NSRA, Japan

2-70

are designed from viewpoints of operators or control panel users. The control panels have shapes and dimensions that allow easy operator access. Indicators and recorders are appropriately arranged and mounted on the control panels so that operators can easily and correctly read them, and switches are sorted out by shape and color for distinct identification to avoid erroneous operation.

2. Operation supporting systems

Broader utilization of the process computer system with its additional functions provides significant operation support for better quality HMIs. The major operation supporting functions of the process computer system are described below.

1. Plant operating conditions displays

Arrays of graphical CRT displays are provided to present plant parameters and equipment operating conditions in an intensive manner.

2. Surveillance test guidance

Hie ESFs are periodically tested for readiness (standby conditions), i.e. surveillance tested. On the CRT displays, the process computer system displays surveillance test procedures with associated system conditions, checks test progress in accordance with the procedures, and automatically records test results.

3. Plant operation automation

Automatic operation is done under the control of the process computer system for the standard plant operations during plant startup/shutdown or power level adjustment Within the automated scope, the process computer system gives triggers to start/stop appropriate machines and to open/close target valves, or to change the set points of relevant control systems. Additionally, plant conditions under such automated operations are put on the CRT displays and computer driven voice announcements are made about major operational events. As a sort of safety measure, the automated plant operation process is appropriately divided into several segments by interruption points so that, if completion conditions are not met at one interruption point, then further automated operation is suspended; and in addition, at any time manual operations override automated operations by the process

computer system.

4. Reactor core performance prediction

In addition to the reactor core performance calculation for present power distributions in the reactor core, core performance prediction functions are provided. Core power distributions at some future points are calculated with a BWR simulator in the process computer system, assuming that the reactor is operated in accordance with the preset operation schedule for prediction purposes such as achieving the effective fuel burnup. CRT displays show the predicted results.

5. Rod worth minimizer (RWM)

The RWM prevents control rod operation during plant startup or shutdown from erroneously forming a control rod pattern with high control rod worth. The purpose of the RWM is to minimize the effect of a control rod drop accident in concert with control rod drop velocity limiters. The RWM is basically a kind of interlock function, and it prevents the actual control rod pattern from deviating from the preinstalled control rod patterns that are calculated offline, separately in advance, and confirmed to be appropriate in terms of safety,

3. Third generation MCR

ABWR-type MCR panels (i.e. the third generation MCR panels) are installed in an ABWR plant. While the design of the ABWR-type MCR panels basically follows the design principles for the second generation MCR panels, two design principles are added: to provide easier and more reliable monitoring and control in accident situations; and to enhance the style of plant operation.

Figure 2.6.12 shows the transition of MCR panels design principles. Accordingly, the following design conditions are incorporated in accordance with these principles.

1. To reduce operators’ actions and items to be checked immediately after a reactor scram

2. To share information important to safety with every member of the MCR crew

3. To minimize the directional flow of operators’ actions to the extent that monitoring and control can be covered by one operator

Based on these design conditions, the following

2-71

NSRA, Japan