Emergency feedwater

K. Parsaye and K. Y. Lin, "An Expert System Stmcture for Automatic Fault Tree Generation for Emergency Feedwater Systems for Nuclear Power Plants," in Proceedings of the Second IEEE Westex Conferences Anaheim, Calif., June, 1987. 

The safety improvements use redundancy and diversity to prevent and mitigate lents. The safety injection system (SIS) and emergency feedwater system (EFWS) are dedicated four train systems. Containment spray and safety injection pumps take water from the in-containment water storage tank (IRWST), thus, eliminating the need to switch from an external source and provide a. semi-closed system with continuous recirculation. Emergency core coolant flows direc nto the... 

The component hardware data were well-based because they are derived from 22 years of records. It is also useful because of the structure and depth of the presentation for example, the Inclusion of data on pumps in different systems (emergency feedwater, condensate, service water). 

In the previous stage of the reconstruction programme for Units 1 to 4, several technical measures to improve nuclear safety during low power and shutdown conditions were implemented for example, installation of a cold overpressurization device in all units, development of a feed and bleed procedure, and installation of complementary emergency feedwater system located outside the turbine hall were completed. 

F. Seven cycles of adding 40 F feedwater at 800 gpm to the steam generator through the downcomer feedwater nozzles during a steam line break. This provides for one steam line break incident with the emergency feedwater cycled a maximum of seven times (faulted condition) ... 

The Shutdown Cooling System (SCS) is used in conjunction with the Main Steam and Main or Emergency Feedwater Systems (see Sections... 

Additionally, the SCS is used in addition to the SG atmospheric steam release capability and the Emergency Feedwater System to cooldown the RCS following a small break LOCA (see Section 6.3). The SCS is also used subsequent to steam and feedwater line breaks, steam generator tube ruptures, and is used during plant startup prior to RCP restart to maintain flow through the core. 

Examples of plant parameters monitored that are needed to identify a Loss-of-Feedwater event are steam generator pressure and level (wide range) main and emergency feedwater flow and reactor coolant pressure, temperature and degree of subcooling. 

The System 80+ Standard Design also incorporates a dedicated safety-related Emergency Feedwater System, as described in CESSAR-DC, Section 10.4.9. This system is not required for normal operation but significantly reduces the probability of a Loss-of-Feedwater event occurring. 

Generic Safety Issue (GSI) 124 in NUREG-0933 (Reference 1), addresses Emergency Feedwater System reliability and availability and its impact on mitigating core-melt frequency. 

The acceptance criterion for the resolution of GSI 124, is that the Emergency Feedwater System shall be designed so that its unavailability is no more than 1 x E-4 per demand after accounting for EFW support systems, common cause failures, and operational errors. Furthermore, this reliability goal should be demonstrated by PRA calculations consistent with the guis ance provided in SRP 10.4.9, Rev. 2. 

Because of the improved reliability of the Emergency Feedwater System design, the unavailability for the system was estimated from PRA studies to be in the range of 1 x E-4 to 1 x E-5 per demand as described in CESSAR-DC, Section 10.4.9.1.2. Analysis identified in CESSAR-DC, Appendix lOA, which was developed using generic data assesses the systems ability to function on demand and demonstrates its compliance with the above unavailability range. Therefore, the EFW system meets the recommended unavailability goal of 1 x E-4 per demand identified in SRP Section 10.4.9, Rev. 2. 

Since the Emergency Feedwater System meets the recommended unavailability goal specified in SRP Section 10.4.9, Rev.2, Subsection II, paragraph 5c, this issue is resolved for the System 80+ Standard Design. 

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. 

The plant design incorporates an Emergency Feedwater System which provides an independent safety-related means of supplying quality feedwater to the steam generator(s) for removal of heat and prevention of reactor core uncovery during emergency phases of plant operation. The EFW System is a dedicated safety-related system which has no functions for normal plant operation (See CESSAR-DC, Section 10.4.9). 

Thus, sufficient emergency feedwater can be provided at the required temperature and pressure even if a steam line or feed line pipe break is the initiating event, if any one EFW pump subtrain fails to deliver flow (the EFW train for each steam generator has two full capacity pump subtrains), and if no... 

Also, an adequate emergency feedwater supply is available to allow the plant to remain at hot standby for 8 hours followed by an orderly cooldown to the primary system pressure and temperature at which the Shutdown Cooling System (SCS) can be initiated to continue cooldown to cold shutdown conditions. Level instrumentation and a low level alarm are provided on each EFWST to help the operator align the EFWST from the other train to preclude the tank from being emptied before the changeover to SCS cooling can be effected. 

Specifically, the Feed and Condensate, Emergency Feedwater, Main Steam, Safety Injection, Containment Spray, Shutdown Cooling,... 

Main feedwater flow during plant startup is only delivered to the economizer inlet feedwater nozzles of the steam generators at temperatures at or above 200°F, which minimizes the probability of condensation-induced water hammer in the economizer sections of the generators. Below a predetermined power level, main feedwater is delivered to the downcomer feedwater inlet nozzles. Changeover to the economizer nozzles at this power level is effected by the Main Feedwater Control System. Emergency feedwater is always delivered to the downcomer inlet nozzles. (See CESSAR-DC, Sections 10.4.7.2.ID and 7.7.1.1.4.)... 

To meet section (c)(1) ( the mitigation requirement ) of 10 CFR 50.62, plant equipment must automatically initiate emergency feedwater and turbine trip under conditions indicative of an ATWS. This equipment must function reliably and must be diverse and independent from the RTS. 

In summary, the System 80+ Standard Design includes a control grade APS that supplements the RPS and provides a diverse and independent means of reactor trip. Also, the APS supplies a control grade AFAS which maintains a diverse and independent method of automatically initiating emergency feedwater. Since the APS is designed to meet 10 CFR 50.62 as identified in CESSAR-DC Section 7.7.1.1.11, this issue is resolved for the System 80+ Standard Design. 

New design features (e.g., relocating emergency feedwater tanks to protected areas, increasing the monitoring, separation and independence of plant protection systems, providing additional back-up sources of power) which provide countermeasures to sabotage must be consistent with plant safety requirements. 

Redesign of the emergency feedwater system (EFW) to function in the event of a loss of offsite and onsite power. This design accommodates coincident failure of a single active mechanical or electrical component or the effects of a high or moderate-energy pipe rupture. 

Locating the emergency feedwater storage tanks within the auxiliary building where access is restricted. 

Also, there is a requirement that the emergency feedwater system shall have sufficient inventory to permit operation at hot shutdown for at least 4 hours, followed by a cooldown to the conditions permitting operation of the RHR system. The inventory needed for cooldown, shall be based on the longest cooldown required with either onsite or offsite power available with an assumed single failure. 

The System 80+ Standard Design utilizes the Shutdown Cooling System (SCS), the Reactor Coolant Gas Vent System (RCGV), the Safety Depressurization System (SDS), the Atmospheric Dump Valves (ADV), and the Emergency Feedwater Systems (EFW) as the preferred means to bring the reactor plant from hot standby to a cold shutdown condition within a reasonable period of time. These safety-related systems are normally operated from the control room and are described in CESSAR-DC, Sections 5.4.7, 10.1, and... 

One turbine-driven emergency feedwater pump is included for each steam generator. (These are in addition to the two motor-driven emergency feedwater pumps.) In previous designs one turbine-driven pump was shared by both steam generators. 

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