Safety-related isolation valves

The hydrogen gas from the chemical reaction increases the pressure within the vessel. When the pressure within the SG reaches a predetermined higher level, the rupture disks burst and the sodium within the SG drains rapidly into a sodium dump tank. For such large sodium/water reaction events, it is also necessary to seal off the reactor/SG system for the purpose of containment isolation. An SG isolation system (SGIS) is provided to terminate a sodium/water reaction (by rapidly removing the water from the SG) and to isolate the water side with safety-related isolation valves which serve a safety-grade barrier for the purpose of containment isolation. 

Safety-related layout of the plant comprises double-walled pipes in product gas carrying ducts and deployment of isolation valves. Released process gases should be routed immediately into the atmosphere to avoid accumulation inside the reactor building. 

It has been required to take into account the feedback of experience related to the mechanical problem of isolated safety injection lines when small leaks through the isolation valves may occur ( Farley-Tihange problem responsible for several small breaks on the safety injection piping). Implementation of on-line temperature and pressure measurements will allow to identify any small leak through the isolation valve to the primary circuit and the dedicated corrective actions can be taken to evacuate the leak to the Chemical and Volume Control System. ... 

The normal decay heat removal function of the steam and feedwater systems is not safety related. The closure of both steam and feedwater lines isolation valves allows a complete separation of the NSSS from the non safety grade BOP. No need for pressure relief is anticipated, since the system is designed for the maximum expected pressure. 

The System 80+ Standard Design is designed to preclude water spray from the fire protection system onto safety-related equipment. The sprinkler systems protecting the safety-related equipment is of the automatic sprinkler type. Actuation of these sprinkler systems requires the opening of the fusible link sprinkler heads and detection by combustible-products and/or heat detectors. In addition, the operator has the capability of isolating flow from the control room by isolating the Sub-sphere Building headers or, locally by manual isolation valves. 

The RCGV function provides a safety-related means of venting remotely from the control room, non-condensible gases from the reactor vessel upper head and the pressurizer steam space during post-accident conditions (see CESSAR-DC, Section 6.7.1.2.1). Positive indication of vent isolation valve position is displayed in the control room (see CESSAR-DC, Section 7.5, Table 7.5-2). 

Over the years, many failures of plant systems and components have been attributed to solenoid-operated valve (SOV) problems. Several events have occurred in which SOV failures affected redundant safety components, multiple trains of safety systems, or multiple safety systems. SOVs are in wide-spread use in each nuclear power facility. They are used in safety-related systems indirectly as pilot operators working with control system fluid (such as pneumatic or hydraulically operated isolation valves) and directly in fluid systems (such as to vent the reactor vessel head or to supply air to the starting system for emergency diesel generators). Many SOVs are also used in non-safety-related systems that can significantly affect safety systems (such as plant instrument air drier systems). 

Operation of the main control room habitability system is automatically initiated on high-2 particulate or iodine radioactivity set point, or low pressuriser pressure, a safety related signal is generated to isolate the main control room from the nuclear island non-radioactive ventilation system and to initiate air flow from the main control room habitability system storage tanks. Isolation of the nuclear island non-radioactive ventilation system consists of closing valves in the supply and exhaust ducts that penetrate the main control room pressure boundary. Main control room habitability system airflow is initiated by a signal which opens the isolation valves in the main control room habitability system supply lines (see Section 6.4 of Reference 6.1). 

Particular attention was paid to fire safety for the primary and secondary sodium equipment and pipelines. Technical measures are provided to limit the scope of sodium leaks and fires. Thus all reactor-related auxiliary sodium system pipes leaving the reactor vessel are jacketed up to a second isolation valve (including the valve casings) and are located in partially leak-tight rooms. 

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