REACTOR INTERNALS 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 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. 

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 only instrumentation required for the Reactor Internals Subsystem is a pressure transducer(s) to monitor the pressure in the space between the double-walled bellows of the hot duct. The transducer indicates a leak in the bellows by indicating a pressure increase or decrease. 

Interfacing requirements imposed on other systems by the Reactor Internals Subsystem are identified in Table 4.1-2, which also includes a description of the interface and a quantitative expression for the interface. 

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

The ex-vessel neutron detectors are involved in AOOs. The ex-vessel detectors are unaffected by the transient imposed on the vessel internals by the rod withdrawal event (AOO-3) because they are located outside the vessel. Their function is carried out when they detect a high power level and send the signal to the Safety Protection Subsystem which combines it with a flow signal resulting in a reactor trip command. 

The primary function of the IVMS is to provide data from which changes in the motion of the reactor vessel internals can be detected. The LPMS is designed to detect the presence of a loose part within the reactor coolant system pressure boundary and is designed to meet the guidance of Regulatory Guide 1.133, Rev. 1. Further details regarding these subsystems of NIMS may be found in CESSAR-DC, Section 7.7.1.6 and in the resolution of GSI B-60. The ALMS is used to detect leaks from the RCS at specific locations and therefore does not apply to this issue but is addressed in GSI I.D.5 (3). 

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