Main steam line

The safety/relief valves are dual-function valves discharging directly to the pressure suppression pool. The safety function provides protection against overpressure of the reactor primary system. The relief function provides power-actuated valve opening to depressurize the reactor primary system. The valves are sized to accommodate the most severe of the following two pressurization transient cases determined by analysis  

For the safety function, valves open at spring set point pressure and close when inlet pressure falls to 96% of spring set point pressure. 

Selected safety/relief valves are associated with the automatic depressurization of the primary system under assumed LOCA conditions. These valves have two independent logic channels powered from different power sources, either of which can initiate depressurization. Valves open automatically and remain open imtil the pressiue falls to a preset closure pressure. These valves open automatically upon signals of high drywell pressure and low reactor water level and confirmation of one LPCI function of the RHR system or LPCS system running. Initiation signals need not be simultaneous. The valves remain open until the primary system pressure is reduced to a point where the LPCI function of the RHR system and/or the LPCS system can adequately cool the core. The initiation of automatic depressurization is delayed from 90 to 120 s to allow the operator to terminate the initiation should the HPCS system initiation and acceptable reactor vessel level have been confirmed. 

The valves used for automatic depressurization can be manually power actuated to open at any pressure. The signal for manual power actuation is from redundant control room switches from different power sources. 

In the unlikely event that the RHR shutdown suction line is unavailable during reactor shutdown to cool reactor water and during the period when the LPCI function of the RHR system and/or the LPCS system pumps are injecting water into the reactor vessel, safety/relief valves used for automatic depressurization can be used to pass water from the reactor vessel to the suppression pool via valve discharge lines. For this to occur, the reactor vessel floods to a level above the vessel main steam line nozzles, selected safety/relief valves are opened from the control room to pass reactor water to the suppression pool. 

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A steam heating line used to warm up the main steam line and equalize the pressure around the main steam stop valve. 

A drain line used to remove condensate from the main steam line. Freeboard ... 

Nuclear Boiler Assembly. This assembly consists of the equipment and instrumentation necessary to produce, contain, and control the steam required by the turbine-generator. The principal components of the nuclear boiler are (1) reactor vessel and internals—reactor pressure vessel, jet pumps for reactor water circulation, steam separators and dryers, and core support structure (2) reactor water recirculation system—pumps, valves, and piping used in providing and controlling core flow (3) main steam lines—main steam safety and relief valves, piping, and pipe supports from reactor pressure vessel up to and including the isolation valves outside of the primary containment barrier (4) control rod drive system—control rods, control rod drive mechanisms and hydraulic system for insertion and withdrawal of the control rods and (5) nuclear fuel and in-core instrumentation,... 

Personnel Hatches (2) 18. Technical Support Center 28. Main Steam Line... 

F-4a Saturated steam line rupture F-4b Main steam line rupture... 

F-4c Rupture between superheater module outlet and superheater outlet isolation valve F-4d Rupture between superheater outlet isolation valve and main steam line... 

EDRS Eme t ency Decay Heal Removal System CWCS Containment Water Cooling System RHRS Residual Heat Removal System MSL Main Steam Line FVn.. Feed Water line... 

Main steam line break accident Power range neutron flux high 13.8 2.31... 

Venting area currently included in the design of the Reactor Building is that for the blowdown from a double-ended guillotine break of a main steam line. 

Main steam line rupture inside containment at zero power ... 

Other components, such as main steam lines or feedwater lines, whose dis-lodgement or failure might put in jeopardy the systems mentioned in items (1) and (2) above. 

PRV—pressure regulating valve Steam pressure regulation Overheating of evaporator Provide safety valve on main steam line also... 

Example of a category 4 accident main steam line break... 

Figure 4-9. Main steam line break at full power with external electric power supply available primary pressure as a function of time.

A Radiological Consequences of Main Steam Line Failures Outside Containment of a PWR... 

B. 1 - Installation of Fast Closing Main Steam Line Valves ... 

The existing analysis for the uncontrolled depressurisation of all steam generators is based on a scenario where neider main steam header separation nor steam generator isolation occurs, enabling all steam generators to blow down through the ruptured main steam line (i.e. the isolation valve on the ruptured line fails to close), however the faulted SG is automatically isolated on feed side by the logic. For this scenario, the analysis was finalised to include safety injection termination criterion for boron concentration. 

For RBMKs of the first generation, guillotine breaks of the main feedwater pipe and of the main steam line should be treated as BDBAs [6]. Intensive in-service inspection and other feasible actions are undertaken to prevent breaks in these pipes (with the break probability reduced to below 10 per reactor-year). 

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 Line Isolation System (see Section 10.3.2 for more details) is composed of portions of the Main Steam System and the Engineered Safety Features Actuation System. Discussed here are those portions of these systems that respond to a Main Steam Isolation Signal, as defined in Section 7.3. A discussion of radiological considerations is provided in Section 12.3. 

A. The Main Steam Line Isolation Valves are designed to isolate the steam generators and the main steam lines in the event of a main steam line rupture. 

B. The Main Steam Line Isolation Valves are designed to perform containment isolation functions for the main steam lines in the event of a design basis accident, as discussed in Section 6.2.4. In the event of a steam line break outside the containment, the isolation function serves to reduce the potential leakage of radioactivity to the environment. 

Each of the four main steam lines is provided with a power-actuated Main Steam Isolation Valve designed to stop flow from either direction when it is tripped closed. Each valve is located outside containment and is provided with means of actuation from the Engineered Safety Features Actuation System, meeting the requirements of IEEE Standard 279. 

The logic circuitry required to isolate the main steam lines is discussed in Section 7.3. The main steam system valves and... 

The main steam line isolation system components are qualified to serve in the environment specified in Section 3.11. 

Overpressure protection for the shell side of the steam generators and the main steam line up to the inlet of the turbine stop valve is provided by the secondary safety valves. 

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