NEUTRON CONTROL SUBSYSTEM

The seismic qualification of the Neutron Control Subsystem includes the control rod assemblies and the reserve shutdown control equipment (RSCE) assemblies and their associated electrical controls. 

The electrical panels associated with the safe shutdown function of the Neutron Control Subsystem control equipment will be seismically qualified by test. (Ref. 9)... 

Safety Classification for Neutron Control Subsystem Equipment... 

The Reactor System (RS) is identical for each of the four modules of the 4 X 350 MWt Standard MHTGR plant. It consists of three subsystems, i.e., Reactor Core Subsystem (RCSS), Neutron Control Subsystem (NCSS), and Reactor Internals Subsystem (RISS). These subsystems are described in detail in Sections 4.2, 4.3, and 4.4, respectively. 

The instrumentation and control required for operation of the RS is provided by the Neutron Control Subsystem (NCSS) and Is discussed in Section 4.3.4.4. 

Interface requirements imposed on other systems or subsystems within other systems by the Neutron Control Subsystem are identified in Table 4.1-2, which also includes a description of the interface and a quantitative expression for the interface. 

The Neutron Control Subsystem (NCSS) consists of the drive mechanisms for positioning the control rods, the rod controls, the reserve shutdown control equipment (RSCE) with its controls, and the instruments for measuring neutron flux levels within the reactor vessel (i.e., in-vessel flux mapping units and startup detectors) and around the perimeter of the reactor outside the vessel (i.e., ex-vessel flux detectors). The control rods and the reserve shutdown material are part of the Reactor Core Subsystem (Section 4.2). Most of this equipment is configured into assemblies which are normally installed in penetrations in the top or bottom of the reactor vessel. These assemblies are periodically removed either to provide access to the core for refueling or for maintenance of the equipment. 

The radionuclide control function performed by the Neutron Control Subsystem is to control heat generation assuring that control with movable poisons is accomplished to shut down the reactor. These functions are accomplished by ... 

The Neutron Control Subsystem also has the functions of controlling direct exposure to operating personnel and of controlling transport of radionuclides during handling operations. 

The Neutron Control Subsystem is safety related. The classification of specific components is given in Table 4.3-1. 

The Neutron Control Subsystem uses five types of assemblies to monitor neutron flux, and to move control rods and insert reserve shutdown material in response to signals generated by the NSSS Control Subsystem (NCS) and the PPIS. 

The mechanical arrangement of the Neutron Control Subsystem is illustrated in Figure 4.3-3, 4.3-4, 4.3-5, and 4.3-6. Figure 4.3-3 shows typical ONCA and INCA equipment installed in their respective penetrations in the top head of the reactor vessel. The neutron control assemblies are supported on ledges in their respective penetrations while the lower portions of the neutron control assemblies extend down into the control channels of the core sector below the penetration. 

The operating modes of the Neutron Control Subsystem, in conjunction with the Reactor Core and Reactor Internals Subsystems, are discussed in Section 4.1.4.3. 

The failure modes and effects discussion of the Neutron Control Subsystem is divided into separate discussions of the control rod drives, reserve shutdovm control equipment, IFMUs, SDAs, and ex vessel detectors. 

SAFETY CLASSIFICATION FOR THE NEUTRON CONTROL SUBSYSTEM EQUIPMENT... 

Signals to the Plant Protection and Instrumentation System (PPIS) and the NSSS Control Subsystem (NCS) are supplied by neutron detectors. During power operation, the neutron flux levels are monitored by detectors located in wells between the reactor vessel and the concrete cavity wall. These detectors are distributed symmetrically around the reactor vessel at about the core midplane. During low power operation, starting up, shutting down, and while shut down, the neutron flux levels are monitored by source-range detectors, located in selected side reflector elements near the bottom of the active core. 

Quantity The NSSS Control Subsystem shall provide setpoint signals to the neutron flux controller for use in automatically controlling the rods. 

The ex-vessel neutron detectors provide signals to the Safety Protection Subsystem, the NSSS Control Subsystem, and the rod drive control equipment from the startup range to as high as 200 percent power. Two detectors in each of six wells feed the Safety Protection Subsystem and one from each well feeds the NSSS Control Subsystem and rod drive control equipment. The NCS and rod drive control equipment use the signals to control reactor power through the flux controllers while the Safety Protection Subsystem signals are used to provide protection for abnormal plant conditions. 

The ex-vessel neutron detection equipment consists of fission chamber neutron detectors mounted in six equally spaced vertical wells located just outside the reactor vessel as illustrated in Figure 4.3-4. The signals from these detectors are supplied to the nuclear instrumentation cabinet and Safety Protection Subsystem equipment located primarily in the reactor building. These data are used by the automatic control systems to operate the control rod drives or the reserve shutdown equipment, thereby changing the neutron flux levels within the reactor core. 

The remaining 12 ex-vessel neutron detectors (top and bottom) provide linear power signals to reactor trip portions of the Safety Protection Subsystem. (The signals are combined into four groups of three for use in the two-out-of-four trip system.) The detectors and circuitry used for protection are separated from the detectors and circuitry utilized for control. 

The nuclear instrumentation must be operable prior to reactor startup. The automatic rod control during startup will not operate if more than one of the three ex-vessel wide-range channels is out of service. The Safety Protection Subsystem requires at least three of its four nuclear input channels operating. The power range neutron flux control will not operate automatically with more than two of the six input channels out of service. 

The protection system was designed to avoid any unsafe condition. It was subdivided into two subsystems, the nuclear detection subsystem and the interlock subsystem. The nuclear detection subsystem is used to monitor neutron flux level and period. It is composed of 8 nuclear channels to monitor the neutron flux from start up to 100% of full power (100 watts), including comparators and isolation ampliBers. Three channels are used in the start-up region, and the others in the intermediate and power regions. In each region we have three measurements of the neutron flux (power) and three measurements of the period. The nuclear channels are complemented by two linear channels, used (alternatively) to control the reactor in automatic mode. Figure 5 shows the relative location of the detectors, and the operational interval of them. 

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