Main feedwater line

Generic Safety Issue (GSI) 125.11.07 in NUREG-0933 (Reference 1), addresses the need for owner-operators and plant designers to reassess the benefits of automatically isolating the emergency feedwater (EFW) system after a main steam line or main feedwater line break. 

The System 80+ Standard Design has a Main Feedwater Isolation System (see CESSAR-DC Section 10.4.7.2.2) to protect the SGs from overfill. The system includes redundant remotely operated isolation valves in each main feedwater line to each SG. The valve actuation system (see CESSAR-DC Section 7.3.1.1.10.3) is composed of redundant trains A and B, and each train s instrumentation and controls are physically and electrically separate from and independent of those of the other train. A failure of one train will not impair the action of the other. The main feedwater isolation valves are automatically actuated by a Main Steam Isolation Signal (MSIS) from the Engineered Safety Features Actuation System (ESFAS, see CESSAR-DC Section 7.3.1). High SG water level, in a 2-out-of-4 logic, is one of the initiators for the MSIS. The main feedwater isolation valves can be in-service tested in accordance with ASME Code Section XI, Subsection IWV. A Technical Specification (CESSAR-DC Chapter 16) will establish testing requirements for the valve actuation system. These requirements will also be incorporated into the plant maintenance procedures. 

These design criteria include the capability of the system to withstand the effects of earthquakes and missiles shared systems and components prompt shutdown of the reactor from the control room system decay heat heat removal capacity considering a main feedwater line break redundancy reliability in-service inspection and functional testing. 

Finally, the EFW system is designed to provide decay heat removal capability for 8 hours at hot standby and then support an orderly cooldown to shutdown cooling system entry, even if the initiating event is a main feedwater line break (see CESSAR-DC, Section 10.4.9.3). 

The KSNP has a main feedwater isolation system to protect the SGs from overfill which includes redundant remoteh operated isolation valves in each main feedwater line to each SG. The valve actuation system is composed of redundant trains A and B, and each train s instrumentation and controls are physically and electrically separate from and independent of those of the other train. 

Main feedwater is supplied to each of the two steam generators through its own main feedwater line. Each of the two lines is anchored at the interface between the auxiliary building and the turbine building, and has sufficient flexibility to provide for relative movement of its associated steam generators resulting from thermal expansion. Each main feed line contains a control valve, a non-return and an isolation valve. These valves are installed in the line before it enters the containment. The main feed lines are a closed system inside the reactor containment, and so they do not require any further valves in the line inside the containment. This is because any radioactivity within the contaimnent atmosphere has no way of passing into a closed system. 

Each main feedwater line also includes a check valve. During normal and abnormal conditions, this check valve prevents reverse flow fi om the steam generator should the feedwater pumps be tripped. In addition, the valves stop the uncontrolled blowdown from the steam generators in the event of a feed line break outside the contaimnent, and they stop more than one steam generator from blowing down in the event of feed line break inside the contaimnent. 

There are two start-up feedwater lines, with the same disposition of valves as the two main feedwater lines. Each of these lines supplies its own steam generator through an injection nozzle at the same elevation as the main feedwater nozzle but rotated circumferentially away from the main feedwater nozzle. During start up, feed is supplied through the start-up feed water control valves until the capacity limit of the start-up pumps is qjproached, at which point control is automatically transferred from the start-up feed water control valves to the main feedwater control valves, and the start-up feed isolation valves are then closed. 

The plant control system has been designed in a similar way to that of BWRs [36-39]. It is shown in Fig. 1.14. The plant transient analysis code SPRAT-DOWN was developed and used in the design work. The node-junction model, shown in Fig. 1.15, contains the RPV, the control rods (CRs), the main feedwater pumps, the turbine control valves, the main feedwater lines, and the main steam lines. The characteristics of the turbine control valves and the changes of the feedwater flow rate according to the core pressure are given in the calculation. 

The basic SPRAT was modified to deal with water rods cooled by downward flow [6]. It is called SPRAT-DOWN [7-9], The coolant flow scheme is shown in Fig. 4.3. The fuel channel and the water rod chaimel are modeled as single channels with 20 meshes. At normal operation, 30% of the feedwater is led to the water rod channel through the upper dome and the control rod guide tube. The lower plenum, including the downcomer, is divided into 20 meshes. The upper plenum, including the main steam line, is also divided into 20 nodes. The main feedwater line and the... 

Inner diameter of main feedwater lines/main steam line (m) 0.27/0.46... 

Seismic design basis for the main coolant loop piping and pumps, and for typical category I piping, e.g., the auxiliary feedwater line. 

Core Make-up Tanks (2) 19. Main Control Room 29. Feedwater Line... 

F-3 a Feedwater line rupture between steam drum and inlet isolation valve F-3b Feedwater line rupture in main incoming header... 

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. 

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 turbine house is located opposite the auxiliary building and connected to the SGBs by feedwater and main steam lines. Tunnels are used for cables and pipes to connect the different buildings. 

The main steam lines (MSLs) are designed to direct steam from the RPV to the TMSS the feedwater lines (FWLs) to direct feedwater from the C FS to the RPV the RPV instrumentation to monitor the conditions within the RPV over the full range of reactor power operation. 

The main steam and feedwater lines is equipped with three system-fluid-operated isolation valves, one of every three being of a different design from that of the other two valves. Closure of the main steam valves to isolate the containment is based on the fail-safe principle employing solenoid pilot valves or through passive actuation. 

The plant designer shall therefore define the mass and energy input to containment to include flow of emergency feedwater to the affected steam generator following a main steam line break. It should be assumed that the operators will not take action to terminate the flow of emergency feedwater to the affected steam generator within 30 minutes of the break, based upon current industry recommendations. 

I) Concrete containment 2) Containment steel shell 3) Polar crane 4) Reactor pressure vessel S) Control rod drive mechanism 6) Spent fuel pool 7) Refuelling machine 8) Steam generator 9) Pressurizer 10) Pressurizer relief tank 11) Main coolant pump 12) Main steam line 13) Feedwater line 14) Concrete shield IS) Accumulator 16) Personnel lock 17) Mate rials lock 18) Lifting gantry 19) Fresh fuel assembly storage 20) Borated water storage tank 21) Residual heat cooler 22) Component cooler 23) Safety injection pump (By courtesy of Siemens/KWU)... 

Reactor core 2) Upper cover plate 3) Water-steam lines 4) Water-steam separator 5) Main coolant pumps 6) Feedwater lines 7) Refuelling machine 8) Condensation chambers... 

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