Main Steam Safety Valve

Pressurizer Safety Valve Parameters Main Steam Safety Valve Parameters... 

Typical Pressurizer Level Setpoint Program Typical Temperature Control Program Typical Pressurizer Level Error Program Pressure Control Program Primary Safety Valve Main Steam Safety Valve... 

The main steam safety valves are direct acting, spring loaded, carbon steel valves. The valves are mounted on each of the main steam lines upstream of the steam line isolation valves, and outside containment. A schematic drawing of the main steam safety valves is given in Figure 5.4.13-2. The valve parameters are given in Table 5.4.13-2. For a description of overpressure protection equipment and components for the main steam system refer to Section 10.3.2. 

The main steam safety valves will be designed to withstand the following transients without failure or malfunction. 

The main steam safety valves are designed to operate in the environmental conditions outlined below. 

Material specifications for the main steam safety valves are given in Table 5.4.13-2. 

The location of the pressurizer safety valves is shown in Figure 5.1.2-3, They discharge to the In-Containment Refueling Water Storage Tank (IRWST). Figure 10.1-2 shows the location of the main steam safety valves. 

See Table 3.9-13 for loading combinations for piping upstream of the main steam safety valves. 

Implemented System Pressure and Inventory Pressurizer provides principal control method. The Liquid Relief Valves provide Control System overpressure protection together with reactor power stepback In accidents with LOCA crash cooling by opening the Main Steam Safety Valves depressurizes the secondaiy circuit, which cools and depressurizes the primary circuit and helps ECC injection for small breaks... 

ECCS is accompanied by crash cool down of the steam generators, involving blowing off steam to atmosphere through the main steam safety valve. This ensures that the HTS pressure stays below ECCS injection pressure, especially for small LOCA, and also for large LOCA in the long term. 

Generic Safety Issue 126, Reliability of PWR main steam safety valves (NUREG-0933)... 

The failure rate of non-tin doped mercury wetted relays was found increasing significantly. An event happened at Embalse NPP, consisting on the opening of the main steam safety valves due to dormant failures of these relays. Besides, these relays are included on the safety systems of the plant. The corrective actions were to replace such relays by tin-doped relays, which prevents the jumping. Other issues related to l C reliability components are evaluated and considered in the PSAs. 

The decay heat and residual heat could be cooled for about 30 minutes through the natural circulation of primary coolant in the primary system, and through the operation of turbine operation auxiliary water supply pump and the main steam safety valve. Necessary power for the safety protection systems and the turbine-driven auxiliary feedwater systems is supplied from highly reliable batteries to secure the safety of reactor even during the total loss of power. 

The turbine bypass pipe work and valves provide the capability to dissipate heat directly to the condenser during plant start-up, and during the cool down of the reactor coolant system up to the point where the normal residual heat removal system can be placed in service (Section 10.4.4 of Reference 6.1). They also reduce the challenges to the main steam power-operated relief valves, the main steam safety valves, the steam generator level control and the pressuriser safety valves following a reactor trip, rapid load reductions during normal operation and turbine trips without a reactor trip. 

Although the assistant shift supervisor was loudly reading the supplementary actions from the emergency procedure book, the shift supervisor heard the main steam safety valves open again. He knew from experience that something was unusual and instinctively surveyed the control console and panel for a clue. He discovered that both main steam isolation valves (MSIVs) had closed -the first and second of a list of unexpected equipment performances and failures that occurred during the event. 

The secondary-side operator watched the levels in both steam generators boil down he had also heard the main steam safety valves lifting. Without feedwater, he knew that an SFRCS actuation on low steam generator level was imminent. 

Depressurization of the secondary side could result from inadvertent opening of the atmospheric steam discharge valves (ASDVs), the condenser steam discharge valves (CSDVs) or the main steam safety valves (MSSVs) or from failure to unload the turbine after a reactor trip. Pressurization of the secondary side could result from a loss of condenser vacuum. 

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