DUAL PURPOSE SYSTEMS IN PROBABILISTIC RISK ASSESSMENTS ARKANSAS NUCLEAR ONE - UNIT 2

摘要

This paper describes the results of a performance monitoring system to quantitatively measure the effectiveness of the Chemical and Volume Control System (CVCS) at Arkansas Nuclear One (ANO) - Unit 2 and provide input into the Unit 2 Probabilistic Risk Assessment. The CVCS has a dual purpose of 1.) maintaining Reactor Coolant System chemical and volume parameters within acceptable conditions during normal power operation and 2.) providing backup reactivity control following plant transients when the normal reactivity control does not function. An Availability Model was constructed to monitor CVCS performance during normal power operation. The overall system availability has been 100% over the time period covered by this paper, January 1,1989 to September 30,1994. This excellent record has been achieved only because there are three CVCS charging pumps in the system with only one pump being necessary during normal operation. The availability of the individual pumps ranged from 40% to 100% over this time period as measured by a 12 month sliding value (see Figure 1). A Conditional Probability Model was constructed to monitor the ability of the CVCS to respond to transients when the normal reactivity control equipment does not function. The conditional probabilities would be used in the Probabilistic Risk Assessment of the Unit. The conditional probabilities of the three CVCS charging pumps varied from 70% to 97% when measured by sliding values (see Figure 3). Only one of the three pumps is necessary for backup reactivity control. The CVCS was not called upon to provide backup reactivity control during the time period covered by this paper. During the beginning of the time period covered by this paper, there were a number of occasions when Unit 2 entered into a Limiting Condition of Operation (LCO) due to the unavailability of two of the CVCS charging pumps at the same time.