Auxiliary Feedwater Reliability

NUREG-0800, Standard Review Plan, Section 10.4.9, Rev. 2, "Auxiliary Feedwater Reliability", U.S. Nuclear Regulatory Commission. 

Turbine-driven auxiliary feedwater pump reliability-from design and maintenance is... 

Improved feedwater reliability via two independent sources of high pressure auxiliary ieedwater, one seismically qualified, the other uses an auxiliary diesel-driven pump to cope with LOOP... 

The AFWS is a 2 division and 4 train system. The AFWS is designed to supply feedwater to the SGs for RCS heat removal in case of loss of main/startup feedwater systems. The reliability of the AFWS has been increased by use of two 100% motor-driven pumps, two 100% turbine-driven pumps and two independent safety-related auxiliary feedwater storage tanks as a water source instead of condensate storage tank. 

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. 

In performance of the analyses it is also necessary to take into account the measures proposed by the given programme such as installation of common unit diesel generator station providing power supply to such important systems as makeup blowdown of the primary circuit and auxiliary feedwater. Connection of the primary circuit makeup blowdown system to reliable power supply does not increase the risk related to possible dilution of the primary coolant with pure condensate because ... 

USNRC Information Notice 88-09, Reduced reliability of steam driven auxiliary feedwater pumps caused by instability of Woodward PG-PL Type Governors, March 18, 1988. 

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 next three subsections describe the role that quantitative risk estimates played in addressing and resolving three important regulatory issues Anticipated Transients Without Scram, Auxiliary Feedwater System Reliability, and Station Blackout. Following these discussions, current policies and practices of the NRC regarding the use of quantitative risk estimates are discussed in subsections addressing the Safety Goal Policy, the Backfit Rule, and Individual Plant Examinations. 

The Reactor Safety Study found the AFWS to be important in preventing certain core damage scenarios, and, the loss of auxiliary feedwater at TMI-2 reinforced concerns regarding the reliability of the AFWS. Prior to the accident at TMI-2 there was wide variance in design philosophy for auxiliary feedwater systems. In particular the degree of diversity and redundancy varied widely. Some multiplant sites had only one auxiliary feedwater pump per plant with interconnections between units. Other plants had two motor driven and one turbine-driven pump. 

The USNRC staff determined that all PWRs in the US should meet the reliability criterion (lO to 10 5 unavailability/demand) for the auxiliary (emergency) feedwater system. A reliability analysis and/or any necessary system modifications and procedural or maintenance changes should be reviewed by USNRC staff. 

The major auxiliary systems of SMART consist of a component cooling system (CCS), purification system and make-up system. The function of the CCS is to remove heat generated in the main coolant pumps (MCPs), control element drive mechanisms (CEDMs), pressurizer (PZR), and the internal shielding tank. Feedwater supplied from the condensate pump of the turbo-generator is used as the coolant to remove heat. The purification system purifies the primary coolant and controls water chemistry to provide reliable and safe operation of the reactor core and all equipment in any mode of operation. The make-up system fills and makes-up the primary coolant in case of a primary system leak and supplies water to the compensating tanks for the PRHRS it consists of two independent trains, each with one positive displacement makeup pump, a makeup tank, and piping and valves. 

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