Auxiliary feedwater system

The auxiliary feedwater system is used as a backup to maintain the heat sink capability of the plant in the event that the main feedwater system becomes unavailable. The capability for heat removal of the auxiliary feedwater system may be used to reduce the pressure in the RCS when necessary. 

The auxiliary feedwater system is used to maintain the plant in a hot standby condition for an extended period. It may also have to be used to bring the plant to a cold shutdown. The auxiliary feedwater system should provide 

In indirect cycle reactors (PWRs), if there is no separate emergency feedwater system, the auxiliary feedwater system is used as a safety system (emergency feedwater system) to remove residual heat from the RCS. The transfer of heat to the ultimate heat sink could be effected through the pressure relief devices of the steam generator or through the condenser. 

In BWRs, the auxiliary feedwater system is usually termed the reactor core isolation cooling system. This system is used to maintain the water level in the reactor vessel in the event of a loss of feedwater in hot shutdown conditions (in such an event residual heat is removed from the reactor core by means of the release of steam through safety relief valves to a suppression pool). Another function of this system is to supply the necessary inventory of reactor coolant in the event of a small loss of coolant during normal operation. 

In a BWR, an isolation coohng system for the reactor core should be provided as a standby source of cooling water to provide a capability for feedwater supply whenever the main feedwater system is isolated from the reactor pressure vessel. Abnormal events that could cause such an isolation include the inadvertent isolation of the main steam lines, the loss of condenser vacuum, the failure of a pressure regulator, the loss of feedwater and the loss of off-site power. 

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Table 3.4.4-2 Example Format for an Auxiliary Feedwater System Interaction FMEA ...

Remove core decay heat High pressure injection system Low pressure injection system High pressure recirculation system Core flood tanks Auxiliary feedwater system Power conversion system Remove core decay heat Auxiliary feedwal stem Power conversion m High pressure inj( i system pow. peiuicd relief valves... 

Two valves were closed in the auxiliary feedwater system (a violation of the Tech. Specs, for the operation of the plant),... 

Valve Arrangement of Auxiliary Feedwater System and Main Steam System during Shutdown - the auxiliary feedwater system has six MO Vs in the flow path to the steam generators and the main steam nonreturn valves are normally closed during shutdown. These valves depend on offsite power and are difficult to manually open. 

Example of changes on a system Auxiliary Feedwater System... 

The table 1 below gives an example of changes on the Auxiliary Feedwater System. 

TABLE I EXAMPLE OF CHANGES ON THE AUXILIARY FEEDWATER SYSTEM MAINTENANCE PROGRAMME... 

The active safety systems consist of the safety injection system (SIS), safety depressurization and vent system (SDVS), in-containment refueling water storage system (IRWST), auxiliary feedwater system (AFWS), and containment spray system (CSS). 

Safety injection system Containment spray system Containment hydrogen recombiner system Containment isolation system Auxiliary feedwater system... 

Redundancy and diversity have been considered in the design of the auxiliary feedwater system which includes 2 motor pumps and 2 diesel pumps. After loss of outside power, the feedwater can be provided by the diesel pumps. 

Auxiliary feedwater system A X 2 motor pumps, 2 diesel pumps... 

No specific auxiliary feedwater system is included in the design residual heat can be removed by a number of non safety systems ... 

AUXILIARY FEEDWATER SYSTEM AUTOMATIC INITIATION AND FLOW INDICATION... 

After the Three Mile Island Unit 2 accident the NRC reviewed the auxiliary feedwater system for availability and reliability of components and decay heat removal capability. In particular, the EFW system was scrutinized with regard to the potential for failure under a variety of loss of main feedwater conditions. The safety concern was that a total loss of feedwater, i.e., loss of both main and emergency feedwater, could result in loss of core cooling. The NRC requested operating plants and plants under construction to review both the reliability and the capability of the EFW system to perform its intended safety function i.e., core decay heat removal. The evaluation by the plants was divided into three parts as discussed below. 

After the Three Mile Island Unit 2 accident, the NRC reviewed auxiliary feedwater system designs with respect to timely initiation, as described in 10 CFR 50, Appendix A, (GDC 20), (Reference 3). Upon completion of the review, the NRC determined that new guidance identified in NUREG-0737, (Reference 4) was necessary in order to assure a timely start of the AFW system after a design basis event (e.g., loss of main feedwater). Among this new guidance was automatic system initiation, environmental and seismic equipment qualification, and single failure criterion. 

The acceptance criterion for the resolution of GSI II.E.1.2, are that plants employing auxiliary feedwater systems shall meet the requirements of 10 CFR 50, Appendix A, (GDC 20) by implementing the guidance identified in NUREG-0737 and the design requirements of IEEE Standard 279-1971 (References 3 4, respectively). 

Specifically, the auxiliary feedwater system shall incorporate such design features as automatic system initiation, protection from single failure, and environmental and seismic equipment qualification. 

H2 Procedures used in case of loss of feedwater ARE-ASG (FWFCS (feed water flow control system) -AFWS (Auxiliary feedwater system). Residual heat is extracted using pressurizer relief valves and start up of safety injection and containment spray systems (feed and bleed process). 

The auxiliary feedwater system is connected to emergency power supply in all PWR plants, and is used in upset and emergency conditions when normal makeup system could not be available due to normal power loss. 

Therefore most plants are equipped with a four-train independent emergency feedwater system powered by four dedicated Diesel generators. In addition, these plants dispose of an auxiliary feedwater system (start-up and shut-down system) equipped with two pumps supplied by emergency power supply. As a result there are two diverse, redundant auxiliary/emergency feedwater systems available. 

The operation of auxiliary feedwater system would eliminate thermal shocks to the steam generators by preventing unnecessary startup of the emergency feedwater system. 

At Kozloduy NPP, a project to install 2 additional diesel generators is underway. The additional DGs will back up the makeup system and auxiliary feedwater system. 

The capacity of the emergency power supply (DGs) is high enough to supply also such systems as the high pressure boration system (primaiy circuit makeup) and auxiliary feedwater system (for plant startup and shutdown), although these systems are not classified as safety grade. 

The makeup system of the primary circuit is needed for depressurization and for speeding up the boration of the primary circuit. The activation of the auxiliary feedwater system before the startup of the emergency feedwater system reduces thermal shocks to the steam generators. ... 

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. 

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