REACTOR CORE SUBSYSTEM

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 Reactor System, and particularly the Reactor Core Subsystem, design has evolved to the current level of design detail, as described below, through a series of design choices which individually, and in combination, emphasize passive or inherent safety, and which assure that both user and top-level regulatory requirements will be met by the design. 

The Reactor Core Subsystem (RCSS) consists of fuel elements, hexagonal graphite reflector elements, plenum elements, startup sources, and reactivity control material, all located inside a reactor pressure vessel. The RCSS, together with graphite components of the Reactor Internals Subsystem, constitutes a graphite assembly which is supported on a graphite support structure and restrained by a core lateral restraint structure. (See Figures 4.1-1 and 4.1-2). 

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

All instrumentation and control required for the operation of the Reactor Core Subsystem is provided by the Neutron Control Subsystem (NCSS) and discussed in Section 4.3.4.4. 

Interface requirements imposed on other systems by the Reactor Core 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. 

Reactor Core Subsystem - Consists of the reactor core, cladding and internal structural supports. 

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 Reactor Internals Subsystem (RISS) consists of the core lateral restraint (CLR), permanent side reflector (PSR), graphite core support structure (GCSS), metallic core support structure (MCSS), upper plenum thermal protection structure (UPTPS), and the hot duct. Figure 4.4-1 illustrates the location of the components of the RISS within the Reactor System. 

The principal function of the Reactor Internals Subsystem is to provide support and lateral restraint for the reactor core. Other important functions are to channel the primary coolant flow to the core, to control the amount of core coolant bypass flow, and to mix the core exit coolant flow. The reactor internals also augment shielding of the reactor vessel from core radiation. 

Metallic structures of the Reactor Internals Subsystem consist of the core lateral restraint, metallic core support structure, upper plenxim thermal protection structure and hot duct assembly. Materials specifications for these structures are listed in Table 4.4-1. 

The typical ECCS for BWRs is also composed of three subsystems with equipment and flow path redundancy. The first subsystem is a high-pressure spray system that sprays water on the reactor core. For small pipe breaks, the high-pressure spray can maintain the water level in the reactor vessel so that the core remains covered. For larger breaks, this system cools the core by spraying water on the fuel rods. The second subsystem is a low-pressure spray system that delivers a large-volume water spray to the top of the core. The third subsystem is a low-pressure core injection system that provides a large flow of water to the reactor vessel to reflood and cover the core. 

As a direct cycle system, a BWR relies on the main feedwater subsystem to supply cooling water to the reactor, and the main steam subsystem to remove heat from the reactor core during normal operation, In the event of an accident or a serious natural disaster, both of these subsystems may be unavailable. To cope with these accident and disaster scenarios, GE and other BWR designers developed alternate heat removal systems (US Nuclear Regulatory Commission, n.d.b). A major goal of a nuclear reactor s emergency procedures is to keep the fuel elements cool, despite their continued production of decay heat energy,... 

Emergency core cooling system consists of two trains. Each train meets the single failure principle. System includes high and low pressure sub-system. High-pressxue subsystem includes passive (hydro-accumulators) and active pumps and water storage tanks) features for water injection in reactor. Low-pressure sub-system ensures returning a condense accumulated in containment, into the reactor by recirculation pumps. 

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. 

The outer control rod drives (CRDs) shall be designed to permit the Safety Protection Subsystem to interrupt the power supply to the drives when reactor trip levels are reached, causing the control rods to drop by gravity into the core. The inner control rod drives shall operate in a similar manner but are tripped from the Investment Protection Subsystem. This trip command shall override all other commands. The reserve shutdown material shall be stored in hoppers above the core and released to fall into the core upon receipt of a signal from the Safety Protection Subsystem. 

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 capacities of the emergency core cooling systems suffice to provide water under all postulated pipe break conditions. This statement is also valid assuming that only two of the four redundant subsystems are operable. The postulated loss-of-coolant conditions include a hypothetical 80 cm leak at the bottom of the reactor vessel In this context, it can be noted that the capacity of the low pressure coolant injection punq)S has been reduced for BWR 90, following comprehensive core cooling analyses. As a secondary effect, it has been possible to simplify the auxiliary power supply systems. 

The Automatic Depressurization Subsystem (ADS) consists of the eight SRVs and six depressurization valves (DPVs) and their associated instrumentation and controls. The ADS quickly depressurizes the RPV in sufficient time for the Gravity-Driven Cooling System (GDCS) injecting flow to replenish core coolant to maintain core temperature below design limits in the event of a LOCA. It also maintains the reactor depressurized for continued operation of GDCS after an accident without need for power. 

The second subsystem includes three motor-driven pumps of 10 m /h capacity and of 3.0 MPa pressure head. Its water inventory is 50 m. Each system has two channels. A special system for recirculation of water from the condensate collectors back into the reactor is provided using glandless motor pumps. A special structure excludes loss of coolant and ensures long-term passive heat removal from the core and reactor vessel during beyond design base accidents. 

The plant model includes eight different safety systems that are mostly four-redundant. The safety systems are divided into two separate subsystems Reactor Protection System (RPS) and Diverse Protection System (DPS), which are implemented on different automation hardware. The RPS safety systems are automatic depressurisation system (ADS), component cooling water system (CCW), emergency core cooling system (ECC), service water system (SWS) and residual heat removal system (RHR). The DPS safety systems are emergency feed water system (EFW), and main feed water system (MFW). In addition, the AC power system belongs to both RPS and DPS. The model describes the operation logic of the safety systems, the hardware equipment used to implement each system, and the associated failure modes for each piece of equipment. 

The core make-up tanks subsystem is a passive, subsystem that injects borated makeup water into the reactor coolant system. The core makeup tanks are coimected to the reactor coolant system through a discharge injection line and an inlet pressure balance line connected to a cold leg. 

Reactor shutdown is assured by two independent and diverse basic shutdown systems which guarantee very high reliability so as to relegate shutdown failure into the domain of residual risk. In the domain of residual risk, however, the "Third Shutdown Level" becomes effective. It consists of a bundle of additional engineered safety features which are incorporated in the design as a result of extensive risk-minimization studies. The system consists of active and passive subsystems and is supported by beneficial natural core behaviour. For example, the following features of the absorber rod actuators are part of the "Third Shutdown Level" ... 

The I C safety systems include those systems that provide the protection functions. These functions are typically provided by a system known as the reactor protection system, or by the I C subsystems of special safety systems, such as reactor shutdown systems, the emergency core cooling system and containment isolation systems. I C safety systems may also fulfil post-accident monitoring functions and support functions (for example, essential data communication systems for the protection systems or the special safety systems). 

This is the preferred low power mode because it reduces core and plant temperatures. Turbomachinery stresses, and achieves a significant reduction in reactor power. The actual low power setpoint had not been determined and may be limited by material temperature limitations in certain plant locations, turbomachinery performance considerations, and/or PCAD subsystem design considerations. 

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