AN RCS LEAK RATE CALCULATION PROGRAM

Transcription

1 AN RCS LEAK RATE CALCULATION PROGRAM Baik S.J., Choi N.I., Yune S.J., Suh S.K., Cho J.H., Seo J.T. Korea Power Engineering Co. Ltd., Daejeon, Korea sjbaik@kopec.co.kr 1. Introduction Leak rate from the reactor coolant system (RCS) is routinely calculated to ensure that the leakage remains within the allowed limit which is specified in technical specifications (TS). If the leakage exceeds the limit, the plant shutdown is required for repairs. TS requires the periodic surveillance of the identified and unidentified RCS leakage typically by the RCS inventory balance method. [1,2] At Korean Standard Nuclear Power Plant, TS limits are 10 gpm for identified RCS leakage and 1 gpm for unidentified RCS leakage. [3] Since the accuracy of leak rate calculation is dependent of the plant operating condition, the change in the RCS temperature, inventory, and the transient operating condition should be avoided during the measurement period. NUREG-1107 recommends four (4) hour period for the 0.2 gpm accuracy with stable plant operating conditions. [4] US NRC s recent review on the reactor pressure vessel head degradation of the Davis-Besse plant points out that the existing monitoring program used by the utilities may need to be enhanced to ensure the early detection of leakage and reactor coolant pressure boundary integrity. [5] Also NUREG/CR-6582 indicates that the conventional inventory balance methods that are used for measuring RCS leakage are not sufficiently sensitive to detect small leakage. [6] The Action plan for addressing Davis-Besse lessons learned task force recommendations regarding assessment of barrier integrity requirements includes the recommendations for inspection guidance pertaining to unidentified RCS leakage that include action levels to trigger NRC interaction with licensees in response to the increasing levels of unidentified RCS leakage. [7] The on-line computer program calculating the RCS leak rate based on the NUREG-1107 has been used for YGN 3&4 and UCN 3&4 since 1994 and 1997, respectively. However, this program using a conventional inventory balance method with a snapshot approach with one (1) hour test period instead of the recommended four (4) hour period can calculate the RCS leak rate only at the very stable condition (i.e., normal power operation within 0.2 % of power variation) with a large uncertainty, thus its fluctuating calculation results sometimes can not be relied on. The operation of the makeup of the borated water into the RCS and the diversion of the inventory to the outside of the RCS boundary makes it difficult to maintain the plant stable over an hour. Due to the large fluctuation of the calculated leak rate, it is sometimes hard to know the trend of the leakage as well as the instantaneous leak rate. An advanced on-line computer program for the RCS leak rate calculation is introduced herein. The new leak rate calculation program, C-LEAK, uses an improved calculation algorithm that enhances the accuracy of the leak rate calculation not only for the steady state operations but also for the major transients. C-LEAK is capable of accounting for RCS makeup and/or letdown operations and normal power maneuvering. It also has a useful function of estimating leak rate during a steam generator tube rupture (SGTR) event and a small break loss-of-coolant event with the safety injection pumps in operation. C-LEAK, which is designed to enhance the plant safety, is applicable to all pressurized water reactors. 2. Improvement of program C-LEAK adopts a new calculation method and various monitoring functions for the improvement of the RCS leak rate calculation program. To get an accurate calculation result the sensitivity study of the measured parameters, the calculation time interval & the data acquisition time interval, and the comparison of the snapshot method, moving average & linear regression have been performed.

2 2.1 Development of calculation algorithm To evaluate the effect of the uncertainties of the key input parameters on RCS leak rate, sensitivity analysis for each input signal has been carried out. Table 1 shows the results of these analyses which indicate that the prime contributor to leak rate inaccuracies is the fluctuation of RCS average temperature, and the next ones in order are pressurizer level, pressurizer pressure, volume control tank level and so on. Table 1. Effect of key input parameter values on RCS Since the leak rate uncertainty calculated ( is the number of input data used to by the previous program using snapshot obtain the averaged value.) method has been considerably large, the method to reduce it is required. To determine which method would be the most accurate, several methods including Moving Average (MA) and Linear Regression (LR) have been reviewed. [8] After running some sample cases by these methods, it has been found that the LR method is the most accurate in calculating the leak rate. MA is an average of data for a certain time period, and it "moves" as time goes by. LR is a widely used statistical method of developing a straight line equation for correlated data points. In this study, LR has been applied to all process instrumentation signals which are important to the leak rate calculation. Since this LR algorithm needs a lot of computer memory, proper number of data, scanning time, calculation interval, etc. should be determined in consideration of computer system capability. The effect of the various calculation time intervals such as 10 minutes, 30 minutes, one hour and two hour has been investigated for operator s convenience. For example, calculation time interval of 10 minutes could be used for monitoring the sudden change of leak rate due to the outbreak of the leakage, but its calculation uncertainties were comparatively large. On the other hand, two hour calculation time interval could be used for measuring and reporting the accurate leak rate during normal power operation since its calculation uncertainty was as small as ±0.05 gpm. 2.2 System boundary and operating condition Input Parameters Max - Min Value (Converted Volume Change, gallon) =1 =10 =20 PZR Level(%) (13.367) (1.497) (0.668) VCT Level(%) (0.549) (0.101) (0.077) RDT Level(%) (0.763) (0.073) (0.039) EDT Level(%) (1.376) (0.137) (0.068) PZR Pressure (kg/cm 2 ) (3.713) (0.303) (0.152) RCS Temp.( ) 0.090(22.645) (7.798) 0.013(3.525) During the normal power operation, reactor coolant is contained in the RCS and chemical and volume control system (CVCS). The RCS consists of reactor vessel, pressurizer, reactor coolant pumps, steam generators, and connecting piping. The CVCS consists of the VCT, letdown heat exchangers and connecting piping. Reactor drain tank (RDT), equipment drain tank (EDT) and safety injection tanks (SITs) are also included within the system boundary for calculation. When the operation of the makeup of the borated water into the RCS and/or the diversion of the inventory to the outside of the RCS boundary takes place during the calculation interval, it is not easy to calculate the leak rate correctly. Special algorithm has been developed to make it possible to calculate the leak rate during the transients such as makeup operation or diversion. The status of the valves related to makeup or diversion has been incorporated in the program. C-LEAK is capable of accounting for RCS makeup and/or letdown operations and normal power maneuvering. Also it can provide useful estimates of leak rate during a SGTR event and a small break loss-of-coolant event with the safety injection pump in operation. To include safety injection operation, the status of the high pressure safety injection (HPSI) pump(s) operation and HPSI header isolation valve position are monitored and reflected in the C-LEAK program. The leak rate calculation with C-LEAK can be performed continuously over the plant operation MODES 1, 2, and 3. RCS leakage was measured using a mass balance of the RCS and CVCS with the correction of density differences in the two systems. All mass within the calculation boundary should be converted to volume at a standard temperature and pressure condition.

3 2.3 C-LEAK program features The improved RCS leak rate calculation program has the following features: a. Continuous on-line calculation of the RCS leak rate b. Improved reliability of the calculated result c. Trending of an RCS leak rate change d. Applicability to non-steady state operational mode such as: makeup, diversion, power maneuvering, heatup/cooldown and a hot-standby condition e. Improved performance during transient conditions f. Monitoring of volume change in key control volumes g. Monitoring of charging and letdown flow rate h. Off-line manual calculation of the leak rate i. Operator-friendly-interface j. Print-out of the calculated results for a TS surveillance compliance k. Capability of running on multiple platforms, either PC-based or plant-computer. The compact graphical display of C-LEAK provides the operator with trending functions for all key parameters, mass changes in various portions of the RCS control volume and the calculated leak rate over the user selected several time interval (Figures 1 and 2). On-line program annunciates alarm when the leak rate exceeds the alarm set-point. The information about the leakage and its trend is sufficient for the routine TS surveillance, and this program can print out the results for the history logging (Figure 3). RCS LEAK RATE SURVEILLANCE (ON- LINE, 2 HOURS) DATE : 01/17/2006 Figure 1. The RCS leakage monitoring program 1. RCS LEAK RATE UNIDENTIFIED RCS LEAK RATE [GPM] 0.01 IDENTIFIED RCS LEAK RATE [GPM] 0.02 TOTAL RCS LEAK RATE [GPM] VOLUME CHANGE VCT VOLUME CHANGE [GAL] RDT VOLUME CHANGE [GAL] 3.15 PZR VOLUME CHANGE [GAL] EDT VOLUME CHANGE [GAL] 0.64 RCS VOLUME CHANGE [GAL] SIT VOLUME CHANGE [GAL] TOTAL LEAKAGE [GAL] 3.59 IDENTIFIED LEAKAGE [GAL] MEASURED DATA PARAMETER START END PARAMETER START END TIME [Hour, Min., Sec.] 08:58:02 10:58:02 TIME [Hour, Min., Sec.] 08:58:02 10:58:02 REACTOR POWER [%] RCS AVG TEMP [ ºF] VCT LEVEL [ %] EDT LEVEL [%] VCT TEMP [ºF] EDT TEMP [ºF] PZR LEVEL [%] SIT1 LEVEL [ %] PZR TEMP [ºF] SIT1 PRESS [PSIG] PZR PRESS [PSIA] SIT2 LEVEL [%] RCS HOT LEG [ºF] SIT2 PRESS [PSIG] RCS COLD LEG [ºF] SIT3 LEVEL [ %] RDT LEVEL [ %] SIT3 PRESS [PSIG] RDT TEMP [ºF] SIT4 LEVEL [ %] CNMT TEMP [ºF] SIT4 PRESS [PSIG] Figure 2. Trend of the RCS temperature, Pressurizer level and VCT level Figure 3. SUPERVISOR : / / NAME SIGNATURE DATE Print-out of the RCS leak rate calculation

4 3. Results and discussion At first the computer simulation with various scanning time periods was carried out to obtain a reasonable one for leak rate calculation, and the scanning time of ten (10) second period was chosen in consideration of plant computer system performance of UCN 3&4. Table 2 shows the fluctuation band of the leak rate calculation by linear regression and snapshot methods during normal power operation with different calculation time intervals. The fluctuating band by linear regression method ( 0.05 gpm with 120 minutes of calculation time interval) was reduced to about 1/7 of the band by snapshot calculation ( 0.36 gpm with 120 minutes of calculation time interval). The leak rates during the transient like power decrease operation also can be calculated by using the improved program with an acceptable accuracy. Calculation Time Interval (Min.) Table 2. Fluctuation Band by Linear Regression (gpm) Fluctuation Band by Snapshot (gpm) Fluctuation band with various time intervals Figure 4 shows the calculated leak rates with snapshot, moving average and linear regression methods during normal power operation. As discussed in Section 2.1, the leak rate calculated by linear regression method is most reliable. TOTAL LEAK RATE (gpm) Figure 4. Comparison of Snapshot, moving average and linear regression method (60 minute interval) LT010 LT030 LT060 LT120 The leak rate comparison with different time intervals during normal power operation is given in Figure 5. This figure shows that the longer calculation time interval of 60 minutes or 120 minutes gives more reliable calculation results than others. It is noted that the leak rate calculation with 10 minute interval is suitable for finding the sudden leakage quickly, and the leak rate calculation with 60 minute or 120 minute interval is good for TS surveillance during normal power operation. 12:10:00 12:24:24 12:38:48 12:53:12 13:07:36 13:22:00 13:36:24 13:50:48 14:05:12 TIME Figure 6 shows the leak rate with -0.5 different calculation time intervals during SGTR event. This indicates -1 also that calculation with 10 or 30 Time minute interval can find quickly a -1.5 large scale leakage during an accident such as SGTR. Figure 5. Linear regression method with various calculation Furthermore, the leak volume as well time intervals of 10, 30, 60 and 120 minutes as the leak rate during SGTR can be estimated by the improved program. The peak flow rate during SGTR can be estimated with 10 minute interval while the longer interval calculations give an averaged flat peak flow rate.

5 SHT010 SHT030 SHT060 SHT120 Total Leak Rate During SGTR (gmp) Even though it has been known that the conventional inventory balance method is not sufficient for the small leakage detection, the integrated leakage monitoring system with improved inventory balance method can provide the operators with the useful operation :30:00 17:58:48 18:27:36 18:56:24 19:25:12 19:54:00-50 Figure 6. Time RCS leak rate during SG tube rupture (10, 30, 60 and 120 minute intervals) The most significant improvement of the C -LEAK is the high accuracy which makes the small leakage of as low as 0.05 gpm detectable. Therefore, the accurate, stable and continuous on-line RCS leak rate monitoring capability with a graphic display of the C-LEAK program is expected to enhance the plant safety by enabling an early detection of leakage and prompt operator action. Also, the calculated result is easily printed out for a TS surveillance compliance. Since C-LEAK program has been successfully demonstrated at the UCN 3&4 and YGN 3&4 in Korea, it will be installed in the other plants in near future. 4. Conclusions On-line computer program calculating the RCS leak rate has been developed. From the results of this study, it is concluded that: a. The accuracy has been improved to 0.05 gpm during normal power operation with a linear regression of the input signals. b. Leak rate can be calculated during makeup and diversion of inventory, power maneuvering, and transients as well as normal plant operation. c. Calculation time periods of 10, 30, 60 and 120 minutes can provide the operators with prompt and accurate leakage information as needed. d. Graphical display of the leakage trend and other parameters can provide the integrated information of the plant operating status regarding the RCS leakage. 5. References [1] US NRC, Reactor Coolant Pressure Boundary Leakage Detection Systems, Reg. Guide 1.45, NRC, [2] EPRI, Advanced Light Water Reactor Utility Requirements Document, Vol. II, Rev.7, [3] KHNP, Technical Specifications for Ulchin Nuclear Power Plant Units 3 and 4, Korea Hydraulic and Nuclear Power Co., [4] US NRC, Reactor Coolant System Leak Rate Determination for PWRs, NUREG-1107, NRC, [5] US NRC, NRC Review of Responses to Bulletin , Reactor Pressure Vessel Head Degradation and Reactor Coolant Pressure Boundary Integrity, NRC Regulatory Issue Summary , NRC, [6] US NRC, Assessment of Pressurized Water Reactor Primary System Leaks, NUREG/CR -6582, NRC, [7] US NRC, Degradation of the Davis-Besse Nuclear Power Plant Station Reactor Pressure Vessel Head Lessons-Learned Report, NRC, [8] Craig T. Olsen and Myra S. MaCarthy, Linear Regression Leak Rate Calculation Helps Avoid Premature Shutdown, Power Engineering, 1994.