Abstract: The Westinghouse AP1000 nuclear power plant in the United States adopts a digital instrumentation and control system, implemented using Common Q+Ovation. This integrated instrumentation and control system design reduces the number of interfaces and software platforms, providing an optimized structure and performance. The Common Q platform is Class 1E, and the reactor protection and safety monitoring system of the AP1000 nuclear power plant is implemented on this platform. This paper focuses on introducing the structure of the Common Q platform. Keywords: AP1000 nuclear power plant; Common Q; digital instrumentation and control 1 Background In the competition with EPR, Westinghouse's AP1000 nuclear power technology successfully won the bid, becoming the third-generation nuclear power technology introduced to China. The AP1000 has distinctive design features. Based on mature pressurized water reactor technology, it adopts a "passive" safety system, which has brought about innovative changes in the design of nuclear power plant safety systems. In the instrumentation and control system, an important component of the nuclear power plant, a digital control and protection system is adopted, integrating various process systems of the power plant and providing a unified interface for the control, protection, and operation of the power plant. The AP1000's digital instrumentation and control system is implemented through Westinghouse's Common Q + Ovation platform. The Ovation platform is non-Class 1E, used for normal operation instrumentation and control functions. It is a commercially available, mature product primarily used in power generation and industrial control. Many conventional power plants in China have adopted the Ovation platform. The Common Q platform is Class 1E, used for safety instrumentation and control functions. The equipment has seismic resistance requirements, has undergone nuclear power application environment suitability testing, and has been evaluated through approved verification and validation (v&v) procedures and quality assurance procedures. The AP1000 nuclear power plant's reactor protection and safety monitoring system (PMS) utilizes the Common Q platform . 2. Common Q System Hardware Structure Since Common Q is a Class 1E platform, all components of the platform are Class 1E. Its main components include: AC160 controller (with PM646 processing module), input/output modules, human-machine interface, interface and test processor, component interface modules, input/output terminal units, integrated communication processor, power supply modules, etc. The AC160 controller is a modular controller with multi-channel processing capabilities, used to execute protection algorithms for safety-related systems and supporting hot-swapping. The processor is programmed using the ABB programming language (AMPL), providing programming support for the AF100 network, global memory, input/output, and HSL interfaces, in addition to the logic structure programming. The processor has a built-in, independent watchdog timer. If a protection function fails due to a processor malfunction, the watchdog can issue an alarm and provide channel tripping functionality. The processor module used in the AC160 controller is the PM646, developed from the 32-bit Motorola MC68360 processor. The PM646 module consists of two 32-bit microprocessor boards: a processor section and a communication section. Its processor section contains application code, enabling self-diagnostic functions, and storing and retrieving data from the dual-port memory. Its communication section handles two HSL communication ports and an RS422 interface. The AC160 controller uses the S600 series I/O modules. The S600 series I/O modules include all traditional cards, such as analog inputs (including differential inputs, thermocouples, and RTDs), analog outputs, digital inputs and outputs, speed sensor inputs, and pulse counters. During startup and normal operation, the system software automatically monitors the status of the I/O modules, while more detailed diagnostic information can be queried through the MTP (Service and Test Panel). S600 I/O modules are hot-swappable. However, for the PMS system, channels and sequences affected by I/O module replacement should be bypassed before maintenance begins. The Common Q HMI is a flat-panel display system, including a touchscreen display and a PC node box. The PC node box acts as the host of the industrial PC, and its QNX operating system provides a graphical interface for online and testing purposes. The CI527 communication interface module is used to access the AF100 network. The Ethernet interface provides an interface to external non-secure systems. It also includes an optical drive, keyboard, mouse, etc., with functions equivalent to an industrial PC, which will not be described further here. When installed in a control cabinet, the flat panel display system is called the Maintenance and Test Panel (MTP); when installed in the main control room as part of the PMS, it is called the Operator's Module (OM). Through the MTP, technicians can perform tasks such as monitoring, modification, configuration, and testing. The MTP has graphical and trend display capabilities. The OM performs the same functions as the MTP, but with certain limitations to prevent unintentional modifications by the operator. There is no direct electrical connection between the OM and the safety system. The Interface and Test Processor (ITP) is used to test and diagnose system operation. The ITP receives data from the AFIO0 bus and transmits it via unidirectional HSL to the ITPs of other PMS channels for continuous online cross-comparison to detect faults in field sensors, loop power supplies, input signal acquisition, and D/A conversion. During normal operation, it monitors fault and diagnostic information from other channels and issues alarms in its own channel, thereby improving the maintainability of the protection system. The ITP transmits test and maintenance information to the operator interface via the MTP. The Component Interface Module (CIM) receives component control signals from safety systems and various non-safety-related systems, and votes on signals according to user-selected priorities (e.g., safety system priorities or component status priorities) before sending them to the power plant equipment. Additionally, the CIM can receive remote manual component control signals and treat them as the highest priority signal. The CIM can also continuously test safety system commands from the controller to the CIM module. The CIM can accept up to eight feedback input signals for providing component interlocking (e.g., torque switching control of electric valves) or for displaying equipment status limit indicators. The CIM provides a separate connection that can be used to send component status information to the power plant computer. The Input/Output Terminal Unit (I/O) provides an interface between the S600 I/O module and field circuits. Different terminal units correspond to different I/O modules. Each type of terminal unit provides termination points for different field wiring, including their respective cable shielding. Each type of terminal unit also provides signal breakpoints and test points for system testing and maintenance. The Integrated Communications Processor (ICP) system receives data from the AF100 fieldbus and converts it into analog or digital signals. These signals are then sent to the PLS via independent hardwiring. The PLS receives data from the AF100 fieldbus and transmits it to other ICP channels via HSL communication, while simultaneously receiving data from the other three ICP channels. The software uses the Common Q platform and the AMPL Control Configuration (ACC) software configuration tool, programmed on a PC with an Intel processor. AMPL operates based on predefined function blocks called Process Control (PC) elements and Database (DB) elements. The PC and DB elements work together to configure complete control functions. The AC160 software includes a real-time operating system, task management program, diagnostic program, communication interface program, and user application program. All these programs are stored in the PROM of the PM646 processor. The application development and modification of the system are completed by the ACC configuration tool. From the initial configuration to internal testing, to on-site debugging and maintenance of the working system, all are completed by this tool. The ACC software includes an application generator and a function diagram generator. The application generator is used for project management and software integration, and the function diagram generator is used for program development. 4 Communication The Common Q platform mainly uses three communication methods: AFIO0 (Advant Fieldbus 100) for communication within the channel; High Speed ​​Link Communication (HSL) for data transmission between channels and data transmission between different controllers in the same channel; Ethernet for data communication with extended systems, such as power plant computer systems. The Zhejiang Sanmen Nuclear Power Plant and Shandong Haiyang Nuclear Power Plant in China are the independent support projects for AP1000. At present, the construction of various projects is proceeding according to plan. It is believed that with the continuous deepening of the project, the Common Q safety-level platform will receive more attention. With the efforts of all parties, we will fully digest and absorb this technology and finally achieve localization. References [1] Westinghouse AP1000 Design Control Document Rev. 16Chapter2． [2]Westinghouse AP1000 Design Control DocumentRev. 16 Chapter 7． [3]ABB·CE, CENPD-396-P, Rev. 1. Com on qualified platform．