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Development of wall thinning screening system and its application to a commercial nuclear power plant
Nuclear Engineering and Design 265 (2013) 591–598
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Development of wall thinning screening system and its application to a commercial nuclear power plant Kyung Ha Ryu a , Il Soon Hwang b , Ji Hyun Kim c,∗ a b c
Korea Institute of Machinery and Materials, Daejeon, Republic of Korea Seoul National University, Seoul, Republic of Korea Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea
h i g h l i g h t s • • • •
Wall loss screening system (WalSS) has been developed based on ES-DCPD method. Screening criteria was established based on the thinning of the actual shape that occur in the power plant. With the criteria, the WalSS gives priority of the need for inspection. This technique was successfully applied to commercial nuclear power plant.
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Article history: Received 26 February 2013 Received in revised form 9 September 2013 Accepted 16 September 2013
a b s t r a c t A new non-destructive evaluation (NDE) method has been developed for metal pipes for the detection wall thinning. The method has been showed to be suitable for applications to electric power generation plants where flow accelerated corrosion (FAC) of carbon steel piping is a significant cause of increased maintenance and plant personnel casualty. The wall thinning screening system (WalSS) was developed in two major phases. In the first phase, the equipotential switching direct current potential drop (ESDCPD) method was developed for piping wall (Ryu et al., 2008a, 2010). In the second phase, in this paper, a quantitative detection criteria was developed. The relative ES-DCPD change of 3.8% has been defined as the screening criteria for wall thinning schematization. This criteria means that the component with measured ES-DCPD change greater than 3.8% is called for a more comprehensive examination. In the criteria development, all variables were taken into consideration based on commercial plant piping inspection data such as initial thickness distributions, wall thinning shape and nominal thickness. The developed WalSS based on ES-DCPD was applied to a moisture separator reheater (MSR) drain line of a commercial nuclear power plant (NPP) during a scheduled overhaul. The measured ES-DCPD change was 2.16%, which is lower than the ES-DCPD criteria, identifying the pipe having adequate wall thickness. This is confirmed by site thickness inspection using ultrasonic technique (UT). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Flow-accelerated corrosion (FAC) is a corrosion mechanism in which a normally protective oxide layer on a metal surface dissolves in flowing water. The underlying metal corrodes to re-create the oxide, and thus the metal loss continues. It often affects carbon steel piping carrying pure, deoxygenated water or wet steam. The total length of piping is about 100 km in a typical 1000 MWe pressurized water reactor (PWR) (Choi and Choi, 2004). Except for primary system piping is normally made of stainless steels, most of the secondary nuclear piping in PWR is made of carbon steel. The operating experience shows that the carbon steel piping is very
∗ Corresponding author. Tel.: +82 52 217 2913; fax: +82 52 217 3009. E-mail address: [email protected] (J.H. Kim). 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2013.09.021
susceptible to FAC during nuclear power plant operation (USNRC, 1980a,b, 1989). A total of 1003 piping failure cases has been caused by FAC out of a total of 4064 piping failure cases reported in the United States from 1961 to 1997 (EPRI, 1998). In general, piping failure due to FAC increases with the age of NPPs. So piping failure due to FAC has been regarded as an important safety concern for the extended operations of existing nuclear power plants. With many forms of degradation, there are non-destructive evaluation (NDE) methods such as an UT. The examination process for UT methods includes removal of insulation, the layout of an inspection grid, acquisition of thickness measurement, and input of the data into evaluation programs for predicting repair, maintenance and corrective action. Therefore UT inspection takes significant out of time. Furthermore UT inspection is highly unreliable for small pipes (KHNP, 2008). Hence, by limited overhaul, the numbers of inspected components would be limited.
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Fig. 1. Thinned pipe management procedure.
The difficulty to manage the FAC phenomenon is that the wall thinning progress at the inner wall. A special NDE technique is positively required; it is impossible to tell with the naked eye. Most reported piping ruptures have something in common that have been occurred at locations where never inspected. 2. Objectives of WalSS development The nuclear piping system susceptible to FAC has been extensive on quantity due to aging of NPPs. Within a given downtimes for refueling, the number of components that can be inspected is limited. In addition small pipes are excluded from UT inspection due to technical limitations (KHNP, 2008). Hence many pipes can be left un-inspected, the risk of rupture or accident increase. To prevent these rupture or accident and to reduce unnecessary inspections, in this paper, the piping WalSS has been developed to possess high speed and high reliability. 3. Equipotential switching direct current potential drop (ES-DCPD) method for monitoring piping wall thinning The direct current potential drop (DCPD) method is a traditional method to inspect material properties especially online cracks length monitoring (ASTM, 2003). This method makes electric current flow in the test specimen, measures electric potential drop induced by material degradations. To apply this method to inspection of piping wall thinning, ES-DCPD method was developed (Ryu et al., 2007, 2008a, 2010). The piping system in NPP’s secondary side is connected each other even through grounds and anchors, which makes piping system as electrical closed loop. It is possible to leak current to outside of target monitoring section. Leaked current can cause interferences with NPP’s sensors, electrical risks for workers, as well as errors of DCPD signal. ES-DCPD method uses low voltage interlock, less than 1 V, which guarantees safety of workers as described (Ryu et al., 2008b). To prevent external current leakage, equipotential method was developed by installing two independent current sources with circuits, and by synchronizing its current switching. By maintaining both ends as equipotential, it was possible to achieve almost zero current flows outside of the target monitoring range as demonstrated in earlier study (Ryu et al., 2007, 2008a). This method was verified in dry test loop (DRYTEL) and heavy eutectic liquid metal loop for investigation of operability and safety of PEACER (HELIOS) (Jeoung, 2006). The ES-DCPD method measures three DCPD simultaneously; the target DCPD induced by degradation of target section, the leaked DCPD induced by leakage current, and the DCPD of reference
specimen for normalization of temperature effect. Measuring the leaked DCPD, because the leaked current is almost zero, the measured potential is almost zero. To measure and control leaked DCPD, precise potential measurement is needed. To get higher signal to noise ratio, current switching technique was adopted. By switching current, thermoelectric noise is eliminated and thus standard deviations of signal decreased more than quadruple in the traditional DCPD method (Ryu et al., 2008b). 4. Development of wall thinning screening system (WalSS) The developed ES-DCPD measures the resistance changes induced by wall thinning with measuring potential drop between two points. It is not trivial to determine the wall thinning degree by two-point. ES-DCPD indicates the average wall thinning that can be used to prioritize for more accurate but time consuming inspection technique such as an UT. This is the concept of wall thinning screening by ES-DCPD. In this paper, an wall thinning screening criteria is shown. The material for creating the criteria is thickness measurement results of commercial NPPs. With the prioritized results, UT inspection would be performed. It becomes possible to relatively accurately depicts the shape of the thinning. Knowing the shape of the thinning, life management of the pipe is possible with ES-DCPD between two points. The criteria at this time is a wall thinning maintenance criteria. The wall thinning maintenance criteria should be set conservatively to account for measurement error or the like. 4.1. Thickness criteria of pipe system Fig. 1 shows the thinned pipe management procedure which is suggested by NSAC-202L-R2 (EPRI, 1999). This procedure contains meaningful thickness criteria: 0.875 nominal thickness, critical thickness, and cut-off thickness. In the ASME code case N597 (ASME, 2003), 0.875 nominal thickness (0.875tnom ) is defined as acceptable thickness and 0.3tnom or 0.2tnom is defined as cut-off thickness. If the minimum measured thickness (tmin ) is larger than the 0.875tnom , then the component is allowable. If the tmin is smaller than the 0.3tnom or 0.2tnom , then the component should be replaced. ASME code case N-597 defines cut-off thickness as 0.3tnom for class 1 pipe and 0.2tnom for class 2 pipe. The management program of Korea NPP is adopting 0.3tnom in all cases. Hence, in this paper, cutoff thickness is used only 0.3tnom . NPP piping systems were designed by using ASME pressure vessel code section III (ASME, 1998). A piping component must be designed to safely withstand the internal pressure at which it will operate. The stress resulting from the internal pressure is known
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Fig. 2. Initial thickness distribution of elbow of commercial NPPs.
as the hoop stress. In many piping system, the hoop stress governs the required thickness. Eq. (1) typical of the relationship used in piping code to calculate the critical wall thickness (tcrit ) needed to withstand the hoop stress in straight pipe. The tcrit value of this equation is equally applicable to elbow pipe (EPRI, 1999). tcrit =
PDo 2(S + Py)
(1)
where P is internal pressure, Do is outside diameter, S is allowable stress at designed temperature. The value of allowable stress by the material is shown in ASME section II (ASME, 1998). And y is Di /(Di + Do ) and Di is inside diameter. Thus, if the tmin is under the tcrit , then the component should be repaired unless tmin is larger than 0.875tnom . If tcrit were lower than 0.3tnom , 0.3tnom should become the thickness criteria as a substitute of tcrit . 4.2. Initial thickness profile of NPP secondary side elbow It is obvious that ES-DCPD varies according to the shape of the wall thinning. Moreover, ES-DCPD depends largely not only the shape of the wall thinning but also the initial thickness distribution of the elbow. For this reason, it is shown in Fig. 2 that initial thickness distribution of the elbow section of a commercial of NPPs. The data utilized in the Fig. 2 of the initial thickness distribution is the pre-service inspection (PSI) result of the 98 elbows of NPP A and 5 elbows of NPP B. For NPP A, PSI data of the there are many relatively because it is a new power plant. In the case of NPP B, because it is old plant, material is relatively small by PSI data base that has been measured at the replacement of the elbow. It is confirmed that the initial thickness distribution has the form of three major: the intrados of the elbow is thicker than the extrados named as type A in Fig. 2, the extrados is thicker than the intrados named as type B in Fig. 2, both of intrados and extrados is thicker than the other portions named as type C in Fig. 2.
In the production process of the elbow (bending), the thickness of the intrados of the elbow increases generally and the thickness of extrados decreases. In order to offset these effects, generally, thickness of extrados portion is made thicker than the thickness of the intrados portion in the straight pipe to bending. Therefore, as shown in Fig. 2, the initial thickness distribution of the three appear is presented. 65% of the 103 elbow are type A, 12.6% are type B, and 22.3% are C type. Most elbows are confirmed that they have been made thicker than the nominal thickness. In thicker portion, the thickness is up to 162% of nominal thickness. The thickness of the thin portion is −4.83% minimum value of the nominal thickness.
4.3. Wall thinning screening criteria Using the PSI data of the initial thickness and the ISI data of thickness after some operation, the presentation of thinning shape is possible. Some examples are presented in Fig. 3. The components that were observed noticeable thickness change by ISI compared to the PSI are summarized in Table 1. Noticeable thickness changes are shown in 17 elbows of NPP A and 5 elbows of NPP B. For NPP B, all five elbows that have PSI data show noticeable thickness change. Since PSI data was measured in replaced five elbows that located in interval thinning is active, it seems that thinning was terrible after replacement. With initial thickness distributions and its wall thinning shapes, ES-DCPD changes were analyzed when the wall thinning progresses to the thickness criteria, 0.875tnom , tcrit , and tcutoff by finite element analysis (FEA). The values of thickness requirement are also summarized in Table 1. The analyzed ES-DCPD changes of 21 elbows are summarized in Fig. 4. The information of analyzed elbows is the elbows described in Table 1. In the cases of analyzed ES-DCPD results of tcrit , and tcutoff , the initial thickness distribution does not have a significant impact on ES-DCPD changes. On the other hands, the initial thickness
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Table 1 Information of elbows that analysis has been performed. Plant
A A A A A A A A A A A A A A A A A B B B B B a b c d
Fluid
Single Single Single Single Two Two Two Two Two Two Single Single Single Single Single Single Single Two Two Two Two Two
Diameter
16 16 16 12.75 24 24 4.5 4.5 4.5 4.5 10.75 16 6.625 6.625 6.625 6.625 6.625 10.75 10.75 10.75 10.75 10.75
tnom a
0.500 0.500 0.500 0.688 0.500 0.500 0.337 0.337 0.377 0.377 0.594 0.375 0.280 0.280 0.562 0.562 0.562 0.365 0.365 0.365 0.365 0.365
Initial thickness type
C C C A B B A C C A A B A A A A A A B A A A
Initial thicknessb
Thickness requirement
tmax a
tmin a
0.875tnom a , c
tcrit a , d
tcutoff a , c
0.586 0.602 0.619 0.759 0.814 0.802 0.396 0.420 0.416 0.422 0.789 0.422 0.345 0.331 0.715 0.697 0.696 0.430 0.416 0.431 0.458 0.445
0.512 0.505 0.483 0.687 0.586 0.529 0.338 0.351 0.367 0.369 0.616 0.359 0.283 0.301 0.552 0.528 0.551 0.349 0.329 0.342 0.346 0.353
0.438 0.438 0.438 0.602 0.438 0.438 0.295 0.295 0.330 0.330 0.520 0.328 0.245 0.245 0.492 0.492 0.492 0.319 0.319 0.319 0.319 0.319
0.3408 0.3408 0.3408 0.5260 0.0639 0.0639 0.0120 0.0075 0.0075 0.0075 0.4335 0.1718 0.0110 0.0110 0.2682 0.2682 0.2682 0.0720 0.0720 0.0720 0.0720 0.0720
0.150 0.150 0.150 0.206 0.150 0.150 0.101 0.101 0.113 0.113 0.178 0.113 0.084 0.084 0.169 0.169 0.169 0.110 0.110 0.110 0.110 0.110
The unit that is used in this table is in inches. The initial thickness values are measured values in the field of NPPs with PSI. The value of 0.875tnom and the value of tcutoff is a value that is 0.875 times the nominal thickness and 0.3 times, respectively. The value of tcrit is a value calculated by Eq. (1) based on the temperature and pressure the pipe is used.
distribution seems to be a dominant factor on ES-DCPD changes in 0.875tnom cases as shown in Fig. 4. The dominant factor to thickness requirement of the ES-DCPD may be considered as follows; (1) for 0.875tnom , initial thickness profile, (2) for tcrit , operating pressure as shown in Fig. 4, and (3) for 0.875tnom , tcrit , and tcutoff , thinning shape. In Fig. 5, the ES-DCPD changes are shown with initial thickness type. In type B elbows, the ES-DCPD change seems very large. For type A elbows, it can be confirmed that the change of the DCPD is very small. To eliminate these variations induced by initial thickness distribution, to generalize the ES-DCPD change to the 0.875tnom , the worst initial thickness distribution was assumed. As shown in Fig. 2, most elbow pipe is thicker than nominal thickness in aspect of average thickness. Because, in general, thinner initial thickness lowers the ES-DCPD change to the given thinning, assumption of uniform initial thickness with nominal thickness can be worst case. In Fig. 5, the ES-DCPD changes with this worst initial thickness case are also plotted. These ES-DCPD changes reflect only the effect of the wall thinning shape with conservative small value. The minimum ES-DCPD change was 3.8%. In this paper, this minimum value, 3.8% ES-DCPD, is defined as wall thinning screening criteria. Based on this worst initial thickness assumption, the suggested ES-DCPD criteria can be applied to various components in all initial thickness profile conservatively. Thus the suggested ESDCPD criteria can get more reliability in spite of a little analyzed sample. To examine the worst assumption, lots of analyses were performed in various initial thickness distributions. Because the ES-DCPD change of type A elbow is small as shown in Fig. 5, the initial thickness profile used in this analysis is type A elbow in Fig. 2. The analyzed results are shown in the Fig. 6. The ES-DCPD change shows some variance by initial thickness profile. The minimum ES-DCPD change was 4.63%, which is larger than the worst assumption case, 3.8%. This guarantees that the calculated ES-DCPD change with worst assumption can be said as minimum ES-DCPD change at the given wall thinning shape regardless of the initial thickness profile.
4.4. Wall thinning screening system (WalSS) If the measured ES-DCPD change is lower than the wall thinning screening criteria, 3.8%, the minimum thickness of the component is larger than 0.875tnom . Thus, the component can be categorized as safe; other NDE such as UT can be exempted. If the measured ESDCPD change is larger than 3.8%, additional UT inspection should be required. In the Fig. 7, an example of WalSS is presented for the use of wall thinning screening criteria. If the measured ES-DCPD change is under 3.8%, then there is no need of further actions. When the measured ES-DCPD change becomes 3.8%, UT inspection should be performed. In this case, measured thickness by UT becomes 7.7% thinning. Once UT inspection is performed, wall thinning maintenance criteria can be derived. In the given wall thinning shape, the ES-DCPD change at critical thickness would be 41.0%. To determine the ES-DCPD maintenance criteria, conservative linear fit is recommended because it should be considered that UT inspection error as described (KHNP, 2008). As shown in Fig. 7, ES-DCPD maintenance criteria can be presented with linear fit, 21.4%. With this maintenance criteria, ES-DCPD change fewer than 21.4% can indicate that the component is safe to its lifetime. 5. WalSS application to a commercial nuclear power plant The developed WalSS with ES-DCPD was applied to a domestic commercial NPP secondary side piping during the overhaul. 5.1. Applied component: MSR drain line Moisture separator and reheater (MSR) line have been reported that it is much susceptible to FAC when using ammonia as pH controller. The applied component is 10inches standard elbow, which outside diameter is 273.05 mm and nominal thickness is 9.27 mm. It is ASTM A106B carbon steel standard elbow. Operating pressure is 13.82 atm, operating temperature is 196 ◦ C, and inner fluid is two-phase flow. The tcrit is 1.83 mm, which is lower than the cutoff
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Fig. 4. ES-DCPD changes for each thickness criteria.
Fig. 5. ES-DCPD changes for 0.875tnom .
Fig. 3. Illustrations of initial thickness distribution and its wall thinning shape.
thickness (0.3tnom ), 2.78 mm. So, its lifetime is determined by tcutoff . The allowable thickness (0.875tnom ) is 8.11 mm. After 16 years operation, the component was replaced by severe wall thinning. At this replace time, the baseline thickness distribution was measured. The WalSS was applied during the overhaul after 4.5 years from the replacement time. In this overhaul, UT inspection was also performed to the component. Fig. 8 shows the elbow. The ES-DCPD probe attachment can be shown in Fig. 8(b). In this figure, UT grid point is also shown. To inspect the component, UT should inspect lots of point that makes inspection slow. In this component, 288 UT inspections should be performed. On the other hand, ES-DCPD measures the wall thinning with wide range at a time. It covers all UT grid points containing
Fig. 6. Examination of worst initial thickness assumption.
elbow, inlet pipe (0.5 diameter length) and outlet pipe (2 diameter length). In order to attach the ES-DCPD probes, it is not required to remove whole insulation except a small portion for attaching the probe. Because it is not necessary to further remove the insulation after attaching the probe once, it is possible to significantly reduce the time taken to inspect. Since the excellent in high-temperature
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Fig. 7. Methodology of setting the second criteria.
Fig. 8. ES-DCPD applied elbow. (a) With insulation and (b) without insulation, ES-DCPD probe attachment and grid point for UT inspection can be shown.
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597
Fig. 9. Measured V1 /V2 raw data.
applicability and the signal is not influenced by the temperature as demonstrated (Ryu et al., 2010), online monitoring is possible. Even if there is no online monitoring, inspection during operation without removal of the insulating material is possible, thus applicability in the field is excellent. 5.2. Wall thinning screening result In this application time, the measured V1 /V2 was 4.773. V1 means measured potential at the target elbow and V2 means measured potential at reference region. To get the ES-DCPD, V/V0 , the initial potential value is needed. Actually, this is one of the main limitations of ES-DCPD method. Unless it has been applied since initial time to non-degraded component, the measured V1 /V2 indicates hardly wall thinning. In this measurement time, the initial V0,1 /V0,2 was calculated by finite element analysis (FEA) using the baseline thickness data, where V0,1 means measured potential at the target elbow at initial time, V0,2 means measured potential at reference region at initial time. Based on the value of baseline thickness distribution, the elbow was modeled and the potential value was measured by applying a current, 10 A, to the shape. The measured ES-DCPD change was 2.16%, which is lower than the ES-DCPD criteria. Thus the component can be screened as safe. Fig. 9 shows the measured raw data of potential value. In the plant environment, some environmental noise which is not detected in the laboratory condition (Ryu et al., 2008b) was detected. The 40 signals of 400 total signals showed trend off, as shown as peak in Fig. 9. To remove this environmental noise, statistical data reduction method is used. This method is based on the assumption that measured data sets follows normal distribution. This assumption can be validated with Fig. 10. When the data sets follow the normal distribution, reliability of each data can be indicated. With 99.74% reliability, ±3, 51 data reductions can be performed. To use this method, lots of data sets are required and ES-DCPD measures enough number of signals in short time. The measured ES-DCPD change was 2.16%, which is lower than the wall thinning screening criteria. Thus the component can be screened as safe. The measured minimum thickness (tmin ) by UT was 1.033tnom . There are considerable margin to 0.875tnom . The lifetime of the component is determined by cutoff thickness. The ES-DCPD change can be analyzed as 52.65% at tcutoff by FEA analysis. If conservative linear fit is used, the wall thinning maintenance criteria becomes 20.24%. These results and procedure are illustrated in Fig. 11.
Fig. 10. Histogram of measured potential drop signal.
Fig. 11. Measured ES-DCPD and UT, ES-DCPD second criteria.
6. Summary and future work A quantitative screening criteria was developed for its use for selecting thinned pipes that will be recommended for detailed UT
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inspection. In this step, the thickness database of commercial NPP was analyzed. Because ES-DCPD changes are affected not only by wall thinning profile but also by initial thickness profile, the initial thickness distributions of elbows were estimated using NPP pre-service inspection (PSI) database and the wall thinning shapes is analyzed by thickness changes reported in in-service inspection (ISI) database. The relative ES-DCPD change of 3.8% has been defined as the wall thinning screening, in association with ASME Boiler and Pressure Vessel Code Section XI. The uniform distribution of nominal thickness is assumed as a lower bound of initial thickness. Based on this worst initial thickness assumption, the suggested ES-DCPD criteria can be applied to pipes that have various initial thickness profile. In addition, the suggested screening criteria possesses an sufficient reliability in spite of relatively small number of analyzed sample. The developed WalSS based on ES-DCPD was applied to a commercial NPP at MSR drain line. Because there was no measured initial potential value available, the initial potential value was calculated by FEA using the baseline thickness data. The measured ES-DCPD change was 2.16%, which is lower than the ES-DCPD criteria, judged as acceptable. This is confirmed by separate thickness inspection using UT on site. The developed WalSS with ES-DCPD also has limitations, as follows: First, it is hard to indicate the defect location within a measurement area. WalSS using ES-DCPD has been developed to screen wall thinning caused by FAC. In this case, this limitation does not matter because the location and profile of wall thinning can be within predictions (for example elbow, orifice, valve, etc.). While ES-DCPD can be applied in experience areas of NPP, an extension of application to the various piping system and degradation mechanism such as buried pipe or sea water pipe force same limitation. Second, ES-DCPD needs the initial potential value because ESDCPD measures potential drop that is related to thickness changes from initial condition. If there is no initial measured potential value, the initial value should be calculated using NDE result at initial time or design value. In this process, inaccuracy may be increased. Several additional improvements in the developed wall thinning screening system with ES-DCPD are recommended as future work, as follows: - Additional screening criteria confirmation using more plant data is desirable to assure adequate reliability of the developed screening criteria.
- Additional ES-DCPD database would be required to confirm the reliability. - Application to the piping systems in systems other than nuclear power plants. Acknowledgments This work was financially supported by the R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Ministry of Trade Industry and Energy (MOTIE), and by the Korean Nuclear R&D program organized by the National Research Foundation (NRF) of Korea in support of the Ministry of Science, ICT and Future Planning (MSIP). References ASME, 1998. Boiler and pressure vessel code. Am. Soc. Mech. Eng. ASME, 2003. ASME code case N597, Requirements for analytical evaluation of pipe wall thinning, Section XI Division 1. Am. Soc. Mech. Eng. ASTM, 2003. E1820-01, Standard Test Method for Measurement of Fracture Toughness, Annual Book of ASTM Standards, Document Number E1820-01. Am. Soc. Test. Mater. Choi, S.Y., Choi, Y.H., 2004. Piping failure frequency analysis for the main feedwater system in domestic nuclear power plants. J. Korean Nucl. Soc. 36, 112–120. EPRI, 1998. Nuclear Reactor Piping Failures at US Commercial LWRs: 1961–1997, EPRI Technical Report-110102. Electric Power Res. Inst. EPRI, 1999. Recommendations for an effective flow-accelerated corrosion program, NSAC-202L-R2. Electric Power Res. Inst. Jeoung, S.H., 2006. Development of an Integral Test Loop, HELIOS and Investigation of Natural Circulation Ability for PEACER. Seoul National University (Ph.D. thesis). KHNP, 2008. Optimization of Thinned Pipe Management Program and Application, KHNP-Final Report-A04NT02. Korea Hydro and Nuclear Power Co. Ltd. Ryu, K.H., Lee, N.Y., Hwang, I.S., 2007. A study on the equipotent switching direct current potential drop method for the monitoring of piping thinning. Key Eng. Mater. 345–346 (2), 1331–1334. Ryu, K.H., Hwang, I.S., Oh, Y.J., Lee, N.Y., Kim, J.H., Park, J.H., Sohn, C.H., 2008a. Screening method for piping wall loss by flow accelerated corrosion. Nucl. Eng. Des. 238, 3263–3268. Ryu, K.H., Hwang, I.S., Kim, J.H., 2008b. Verification of the viability of equipotential switching direct current potential drop method for piping wall loss monitoring with signal sensitivity analysis. J. Korean Soc. Nondestruct. Test. 28 (2), 191–198. Ryu, K.H., Lee, T.H., Kim, J.H., Hwang, I.S., Lee, N.Y., Kim, J.H., Park, J.H., Sohn, C.H., 2010. Online monitoring method using equipotential switching direct current potential drop for piping wall loss by flow accelerated corrosion. Nucl. Eng. Des. 240, 468–472. USNRC, 1980a. Pipe Cracking Experience in LWR, NUREG-0679. USNRC, 1980b. Investigation and Evaluation of Cracking Incidents in Piping in PWR, NUREG-0691. USNRC, 1989. Erosion/Corrosion-Induced Pipe Wall Thinning in US NPPs, NUREG1344.
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Development of wall thinning screening system and its application to a commercial nuclear power plant Kyung Ha Ryu a , Il Soon Hwang b , Ji Hyun Kim c,∗ a b c
Korea Institute of Machinery and Materials, Daejeon, Republic of Korea Seoul National University, Seoul, Republic of Korea Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea
h i g h l i g h t s • • • •
Wall loss screening system (WalSS) has been developed based on ES-DCPD method. Screening criteria was established based on the thinning of the actual shape that occur in the power plant. With the criteria, the WalSS gives priority of the need for inspection. This technique was successfully applied to commercial nuclear power plant.
a r t i c l e
i n f o
Article history: Received 26 February 2013 Received in revised form 9 September 2013 Accepted 16 September 2013
a b s t r a c t A new non-destructive evaluation (NDE) method has been developed for metal pipes for the detection wall thinning. The method has been showed to be suitable for applications to electric power generation plants where flow accelerated corrosion (FAC) of carbon steel piping is a significant cause of increased maintenance and plant personnel casualty. The wall thinning screening system (WalSS) was developed in two major phases. In the first phase, the equipotential switching direct current potential drop (ESDCPD) method was developed for piping wall (Ryu et al., 2008a, 2010). In the second phase, in this paper, a quantitative detection criteria was developed. The relative ES-DCPD change of 3.8% has been defined as the screening criteria for wall thinning schematization. This criteria means that the component with measured ES-DCPD change greater than 3.8% is called for a more comprehensive examination. In the criteria development, all variables were taken into consideration based on commercial plant piping inspection data such as initial thickness distributions, wall thinning shape and nominal thickness. The developed WalSS based on ES-DCPD was applied to a moisture separator reheater (MSR) drain line of a commercial nuclear power plant (NPP) during a scheduled overhaul. The measured ES-DCPD change was 2.16%, which is lower than the ES-DCPD criteria, identifying the pipe having adequate wall thickness. This is confirmed by site thickness inspection using ultrasonic technique (UT). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Flow-accelerated corrosion (FAC) is a corrosion mechanism in which a normally protective oxide layer on a metal surface dissolves in flowing water. The underlying metal corrodes to re-create the oxide, and thus the metal loss continues. It often affects carbon steel piping carrying pure, deoxygenated water or wet steam. The total length of piping is about 100 km in a typical 1000 MWe pressurized water reactor (PWR) (Choi and Choi, 2004). Except for primary system piping is normally made of stainless steels, most of the secondary nuclear piping in PWR is made of carbon steel. The operating experience shows that the carbon steel piping is very
∗ Corresponding author. Tel.: +82 52 217 2913; fax: +82 52 217 3009. E-mail address: [email protected] (J.H. Kim). 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2013.09.021
susceptible to FAC during nuclear power plant operation (USNRC, 1980a,b, 1989). A total of 1003 piping failure cases has been caused by FAC out of a total of 4064 piping failure cases reported in the United States from 1961 to 1997 (EPRI, 1998). In general, piping failure due to FAC increases with the age of NPPs. So piping failure due to FAC has been regarded as an important safety concern for the extended operations of existing nuclear power plants. With many forms of degradation, there are non-destructive evaluation (NDE) methods such as an UT. The examination process for UT methods includes removal of insulation, the layout of an inspection grid, acquisition of thickness measurement, and input of the data into evaluation programs for predicting repair, maintenance and corrective action. Therefore UT inspection takes significant out of time. Furthermore UT inspection is highly unreliable for small pipes (KHNP, 2008). Hence, by limited overhaul, the numbers of inspected components would be limited.
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Fig. 1. Thinned pipe management procedure.
The difficulty to manage the FAC phenomenon is that the wall thinning progress at the inner wall. A special NDE technique is positively required; it is impossible to tell with the naked eye. Most reported piping ruptures have something in common that have been occurred at locations where never inspected. 2. Objectives of WalSS development The nuclear piping system susceptible to FAC has been extensive on quantity due to aging of NPPs. Within a given downtimes for refueling, the number of components that can be inspected is limited. In addition small pipes are excluded from UT inspection due to technical limitations (KHNP, 2008). Hence many pipes can be left un-inspected, the risk of rupture or accident increase. To prevent these rupture or accident and to reduce unnecessary inspections, in this paper, the piping WalSS has been developed to possess high speed and high reliability. 3. Equipotential switching direct current potential drop (ES-DCPD) method for monitoring piping wall thinning The direct current potential drop (DCPD) method is a traditional method to inspect material properties especially online cracks length monitoring (ASTM, 2003). This method makes electric current flow in the test specimen, measures electric potential drop induced by material degradations. To apply this method to inspection of piping wall thinning, ES-DCPD method was developed (Ryu et al., 2007, 2008a, 2010). The piping system in NPP’s secondary side is connected each other even through grounds and anchors, which makes piping system as electrical closed loop. It is possible to leak current to outside of target monitoring section. Leaked current can cause interferences with NPP’s sensors, electrical risks for workers, as well as errors of DCPD signal. ES-DCPD method uses low voltage interlock, less than 1 V, which guarantees safety of workers as described (Ryu et al., 2008b). To prevent external current leakage, equipotential method was developed by installing two independent current sources with circuits, and by synchronizing its current switching. By maintaining both ends as equipotential, it was possible to achieve almost zero current flows outside of the target monitoring range as demonstrated in earlier study (Ryu et al., 2007, 2008a). This method was verified in dry test loop (DRYTEL) and heavy eutectic liquid metal loop for investigation of operability and safety of PEACER (HELIOS) (Jeoung, 2006). The ES-DCPD method measures three DCPD simultaneously; the target DCPD induced by degradation of target section, the leaked DCPD induced by leakage current, and the DCPD of reference
specimen for normalization of temperature effect. Measuring the leaked DCPD, because the leaked current is almost zero, the measured potential is almost zero. To measure and control leaked DCPD, precise potential measurement is needed. To get higher signal to noise ratio, current switching technique was adopted. By switching current, thermoelectric noise is eliminated and thus standard deviations of signal decreased more than quadruple in the traditional DCPD method (Ryu et al., 2008b). 4. Development of wall thinning screening system (WalSS) The developed ES-DCPD measures the resistance changes induced by wall thinning with measuring potential drop between two points. It is not trivial to determine the wall thinning degree by two-point. ES-DCPD indicates the average wall thinning that can be used to prioritize for more accurate but time consuming inspection technique such as an UT. This is the concept of wall thinning screening by ES-DCPD. In this paper, an wall thinning screening criteria is shown. The material for creating the criteria is thickness measurement results of commercial NPPs. With the prioritized results, UT inspection would be performed. It becomes possible to relatively accurately depicts the shape of the thinning. Knowing the shape of the thinning, life management of the pipe is possible with ES-DCPD between two points. The criteria at this time is a wall thinning maintenance criteria. The wall thinning maintenance criteria should be set conservatively to account for measurement error or the like. 4.1. Thickness criteria of pipe system Fig. 1 shows the thinned pipe management procedure which is suggested by NSAC-202L-R2 (EPRI, 1999). This procedure contains meaningful thickness criteria: 0.875 nominal thickness, critical thickness, and cut-off thickness. In the ASME code case N597 (ASME, 2003), 0.875 nominal thickness (0.875tnom ) is defined as acceptable thickness and 0.3tnom or 0.2tnom is defined as cut-off thickness. If the minimum measured thickness (tmin ) is larger than the 0.875tnom , then the component is allowable. If the tmin is smaller than the 0.3tnom or 0.2tnom , then the component should be replaced. ASME code case N-597 defines cut-off thickness as 0.3tnom for class 1 pipe and 0.2tnom for class 2 pipe. The management program of Korea NPP is adopting 0.3tnom in all cases. Hence, in this paper, cutoff thickness is used only 0.3tnom . NPP piping systems were designed by using ASME pressure vessel code section III (ASME, 1998). A piping component must be designed to safely withstand the internal pressure at which it will operate. The stress resulting from the internal pressure is known
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Fig. 2. Initial thickness distribution of elbow of commercial NPPs.
as the hoop stress. In many piping system, the hoop stress governs the required thickness. Eq. (1) typical of the relationship used in piping code to calculate the critical wall thickness (tcrit ) needed to withstand the hoop stress in straight pipe. The tcrit value of this equation is equally applicable to elbow pipe (EPRI, 1999). tcrit =
PDo 2(S + Py)
(1)
where P is internal pressure, Do is outside diameter, S is allowable stress at designed temperature. The value of allowable stress by the material is shown in ASME section II (ASME, 1998). And y is Di /(Di + Do ) and Di is inside diameter. Thus, if the tmin is under the tcrit , then the component should be repaired unless tmin is larger than 0.875tnom . If tcrit were lower than 0.3tnom , 0.3tnom should become the thickness criteria as a substitute of tcrit . 4.2. Initial thickness profile of NPP secondary side elbow It is obvious that ES-DCPD varies according to the shape of the wall thinning. Moreover, ES-DCPD depends largely not only the shape of the wall thinning but also the initial thickness distribution of the elbow. For this reason, it is shown in Fig. 2 that initial thickness distribution of the elbow section of a commercial of NPPs. The data utilized in the Fig. 2 of the initial thickness distribution is the pre-service inspection (PSI) result of the 98 elbows of NPP A and 5 elbows of NPP B. For NPP A, PSI data of the there are many relatively because it is a new power plant. In the case of NPP B, because it is old plant, material is relatively small by PSI data base that has been measured at the replacement of the elbow. It is confirmed that the initial thickness distribution has the form of three major: the intrados of the elbow is thicker than the extrados named as type A in Fig. 2, the extrados is thicker than the intrados named as type B in Fig. 2, both of intrados and extrados is thicker than the other portions named as type C in Fig. 2.
In the production process of the elbow (bending), the thickness of the intrados of the elbow increases generally and the thickness of extrados decreases. In order to offset these effects, generally, thickness of extrados portion is made thicker than the thickness of the intrados portion in the straight pipe to bending. Therefore, as shown in Fig. 2, the initial thickness distribution of the three appear is presented. 65% of the 103 elbow are type A, 12.6% are type B, and 22.3% are C type. Most elbows are confirmed that they have been made thicker than the nominal thickness. In thicker portion, the thickness is up to 162% of nominal thickness. The thickness of the thin portion is −4.83% minimum value of the nominal thickness.
4.3. Wall thinning screening criteria Using the PSI data of the initial thickness and the ISI data of thickness after some operation, the presentation of thinning shape is possible. Some examples are presented in Fig. 3. The components that were observed noticeable thickness change by ISI compared to the PSI are summarized in Table 1. Noticeable thickness changes are shown in 17 elbows of NPP A and 5 elbows of NPP B. For NPP B, all five elbows that have PSI data show noticeable thickness change. Since PSI data was measured in replaced five elbows that located in interval thinning is active, it seems that thinning was terrible after replacement. With initial thickness distributions and its wall thinning shapes, ES-DCPD changes were analyzed when the wall thinning progresses to the thickness criteria, 0.875tnom , tcrit , and tcutoff by finite element analysis (FEA). The values of thickness requirement are also summarized in Table 1. The analyzed ES-DCPD changes of 21 elbows are summarized in Fig. 4. The information of analyzed elbows is the elbows described in Table 1. In the cases of analyzed ES-DCPD results of tcrit , and tcutoff , the initial thickness distribution does not have a significant impact on ES-DCPD changes. On the other hands, the initial thickness
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Table 1 Information of elbows that analysis has been performed. Plant
A A A A A A A A A A A A A A A A A B B B B B a b c d
Fluid
Single Single Single Single Two Two Two Two Two Two Single Single Single Single Single Single Single Two Two Two Two Two
Diameter
16 16 16 12.75 24 24 4.5 4.5 4.5 4.5 10.75 16 6.625 6.625 6.625 6.625 6.625 10.75 10.75 10.75 10.75 10.75
tnom a
0.500 0.500 0.500 0.688 0.500 0.500 0.337 0.337 0.377 0.377 0.594 0.375 0.280 0.280 0.562 0.562 0.562 0.365 0.365 0.365 0.365 0.365
Initial thickness type
C C C A B B A C C A A B A A A A A A B A A A
Initial thicknessb
Thickness requirement
tmax a
tmin a
0.875tnom a , c
tcrit a , d
tcutoff a , c
0.586 0.602 0.619 0.759 0.814 0.802 0.396 0.420 0.416 0.422 0.789 0.422 0.345 0.331 0.715 0.697 0.696 0.430 0.416 0.431 0.458 0.445
0.512 0.505 0.483 0.687 0.586 0.529 0.338 0.351 0.367 0.369 0.616 0.359 0.283 0.301 0.552 0.528 0.551 0.349 0.329 0.342 0.346 0.353
0.438 0.438 0.438 0.602 0.438 0.438 0.295 0.295 0.330 0.330 0.520 0.328 0.245 0.245 0.492 0.492 0.492 0.319 0.319 0.319 0.319 0.319
0.3408 0.3408 0.3408 0.5260 0.0639 0.0639 0.0120 0.0075 0.0075 0.0075 0.4335 0.1718 0.0110 0.0110 0.2682 0.2682 0.2682 0.0720 0.0720 0.0720 0.0720 0.0720
0.150 0.150 0.150 0.206 0.150 0.150 0.101 0.101 0.113 0.113 0.178 0.113 0.084 0.084 0.169 0.169 0.169 0.110 0.110 0.110 0.110 0.110
The unit that is used in this table is in inches. The initial thickness values are measured values in the field of NPPs with PSI. The value of 0.875tnom and the value of tcutoff is a value that is 0.875 times the nominal thickness and 0.3 times, respectively. The value of tcrit is a value calculated by Eq. (1) based on the temperature and pressure the pipe is used.
distribution seems to be a dominant factor on ES-DCPD changes in 0.875tnom cases as shown in Fig. 4. The dominant factor to thickness requirement of the ES-DCPD may be considered as follows; (1) for 0.875tnom , initial thickness profile, (2) for tcrit , operating pressure as shown in Fig. 4, and (3) for 0.875tnom , tcrit , and tcutoff , thinning shape. In Fig. 5, the ES-DCPD changes are shown with initial thickness type. In type B elbows, the ES-DCPD change seems very large. For type A elbows, it can be confirmed that the change of the DCPD is very small. To eliminate these variations induced by initial thickness distribution, to generalize the ES-DCPD change to the 0.875tnom , the worst initial thickness distribution was assumed. As shown in Fig. 2, most elbow pipe is thicker than nominal thickness in aspect of average thickness. Because, in general, thinner initial thickness lowers the ES-DCPD change to the given thinning, assumption of uniform initial thickness with nominal thickness can be worst case. In Fig. 5, the ES-DCPD changes with this worst initial thickness case are also plotted. These ES-DCPD changes reflect only the effect of the wall thinning shape with conservative small value. The minimum ES-DCPD change was 3.8%. In this paper, this minimum value, 3.8% ES-DCPD, is defined as wall thinning screening criteria. Based on this worst initial thickness assumption, the suggested ES-DCPD criteria can be applied to various components in all initial thickness profile conservatively. Thus the suggested ESDCPD criteria can get more reliability in spite of a little analyzed sample. To examine the worst assumption, lots of analyses were performed in various initial thickness distributions. Because the ES-DCPD change of type A elbow is small as shown in Fig. 5, the initial thickness profile used in this analysis is type A elbow in Fig. 2. The analyzed results are shown in the Fig. 6. The ES-DCPD change shows some variance by initial thickness profile. The minimum ES-DCPD change was 4.63%, which is larger than the worst assumption case, 3.8%. This guarantees that the calculated ES-DCPD change with worst assumption can be said as minimum ES-DCPD change at the given wall thinning shape regardless of the initial thickness profile.
4.4. Wall thinning screening system (WalSS) If the measured ES-DCPD change is lower than the wall thinning screening criteria, 3.8%, the minimum thickness of the component is larger than 0.875tnom . Thus, the component can be categorized as safe; other NDE such as UT can be exempted. If the measured ESDCPD change is larger than 3.8%, additional UT inspection should be required. In the Fig. 7, an example of WalSS is presented for the use of wall thinning screening criteria. If the measured ES-DCPD change is under 3.8%, then there is no need of further actions. When the measured ES-DCPD change becomes 3.8%, UT inspection should be performed. In this case, measured thickness by UT becomes 7.7% thinning. Once UT inspection is performed, wall thinning maintenance criteria can be derived. In the given wall thinning shape, the ES-DCPD change at critical thickness would be 41.0%. To determine the ES-DCPD maintenance criteria, conservative linear fit is recommended because it should be considered that UT inspection error as described (KHNP, 2008). As shown in Fig. 7, ES-DCPD maintenance criteria can be presented with linear fit, 21.4%. With this maintenance criteria, ES-DCPD change fewer than 21.4% can indicate that the component is safe to its lifetime. 5. WalSS application to a commercial nuclear power plant The developed WalSS with ES-DCPD was applied to a domestic commercial NPP secondary side piping during the overhaul. 5.1. Applied component: MSR drain line Moisture separator and reheater (MSR) line have been reported that it is much susceptible to FAC when using ammonia as pH controller. The applied component is 10inches standard elbow, which outside diameter is 273.05 mm and nominal thickness is 9.27 mm. It is ASTM A106B carbon steel standard elbow. Operating pressure is 13.82 atm, operating temperature is 196 ◦ C, and inner fluid is two-phase flow. The tcrit is 1.83 mm, which is lower than the cutoff
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Fig. 4. ES-DCPD changes for each thickness criteria.
Fig. 5. ES-DCPD changes for 0.875tnom .
Fig. 3. Illustrations of initial thickness distribution and its wall thinning shape.
thickness (0.3tnom ), 2.78 mm. So, its lifetime is determined by tcutoff . The allowable thickness (0.875tnom ) is 8.11 mm. After 16 years operation, the component was replaced by severe wall thinning. At this replace time, the baseline thickness distribution was measured. The WalSS was applied during the overhaul after 4.5 years from the replacement time. In this overhaul, UT inspection was also performed to the component. Fig. 8 shows the elbow. The ES-DCPD probe attachment can be shown in Fig. 8(b). In this figure, UT grid point is also shown. To inspect the component, UT should inspect lots of point that makes inspection slow. In this component, 288 UT inspections should be performed. On the other hand, ES-DCPD measures the wall thinning with wide range at a time. It covers all UT grid points containing
Fig. 6. Examination of worst initial thickness assumption.
elbow, inlet pipe (0.5 diameter length) and outlet pipe (2 diameter length). In order to attach the ES-DCPD probes, it is not required to remove whole insulation except a small portion for attaching the probe. Because it is not necessary to further remove the insulation after attaching the probe once, it is possible to significantly reduce the time taken to inspect. Since the excellent in high-temperature
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Fig. 7. Methodology of setting the second criteria.
Fig. 8. ES-DCPD applied elbow. (a) With insulation and (b) without insulation, ES-DCPD probe attachment and grid point for UT inspection can be shown.
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597
Fig. 9. Measured V1 /V2 raw data.
applicability and the signal is not influenced by the temperature as demonstrated (Ryu et al., 2010), online monitoring is possible. Even if there is no online monitoring, inspection during operation without removal of the insulating material is possible, thus applicability in the field is excellent. 5.2. Wall thinning screening result In this application time, the measured V1 /V2 was 4.773. V1 means measured potential at the target elbow and V2 means measured potential at reference region. To get the ES-DCPD, V/V0 , the initial potential value is needed. Actually, this is one of the main limitations of ES-DCPD method. Unless it has been applied since initial time to non-degraded component, the measured V1 /V2 indicates hardly wall thinning. In this measurement time, the initial V0,1 /V0,2 was calculated by finite element analysis (FEA) using the baseline thickness data, where V0,1 means measured potential at the target elbow at initial time, V0,2 means measured potential at reference region at initial time. Based on the value of baseline thickness distribution, the elbow was modeled and the potential value was measured by applying a current, 10 A, to the shape. The measured ES-DCPD change was 2.16%, which is lower than the ES-DCPD criteria. Thus the component can be screened as safe. Fig. 9 shows the measured raw data of potential value. In the plant environment, some environmental noise which is not detected in the laboratory condition (Ryu et al., 2008b) was detected. The 40 signals of 400 total signals showed trend off, as shown as peak in Fig. 9. To remove this environmental noise, statistical data reduction method is used. This method is based on the assumption that measured data sets follows normal distribution. This assumption can be validated with Fig. 10. When the data sets follow the normal distribution, reliability of each data can be indicated. With 99.74% reliability, ±3, 51 data reductions can be performed. To use this method, lots of data sets are required and ES-DCPD measures enough number of signals in short time. The measured ES-DCPD change was 2.16%, which is lower than the wall thinning screening criteria. Thus the component can be screened as safe. The measured minimum thickness (tmin ) by UT was 1.033tnom . There are considerable margin to 0.875tnom . The lifetime of the component is determined by cutoff thickness. The ES-DCPD change can be analyzed as 52.65% at tcutoff by FEA analysis. If conservative linear fit is used, the wall thinning maintenance criteria becomes 20.24%. These results and procedure are illustrated in Fig. 11.
Fig. 10. Histogram of measured potential drop signal.
Fig. 11. Measured ES-DCPD and UT, ES-DCPD second criteria.
6. Summary and future work A quantitative screening criteria was developed for its use for selecting thinned pipes that will be recommended for detailed UT
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inspection. In this step, the thickness database of commercial NPP was analyzed. Because ES-DCPD changes are affected not only by wall thinning profile but also by initial thickness profile, the initial thickness distributions of elbows were estimated using NPP pre-service inspection (PSI) database and the wall thinning shapes is analyzed by thickness changes reported in in-service inspection (ISI) database. The relative ES-DCPD change of 3.8% has been defined as the wall thinning screening, in association with ASME Boiler and Pressure Vessel Code Section XI. The uniform distribution of nominal thickness is assumed as a lower bound of initial thickness. Based on this worst initial thickness assumption, the suggested ES-DCPD criteria can be applied to pipes that have various initial thickness profile. In addition, the suggested screening criteria possesses an sufficient reliability in spite of relatively small number of analyzed sample. The developed WalSS based on ES-DCPD was applied to a commercial NPP at MSR drain line. Because there was no measured initial potential value available, the initial potential value was calculated by FEA using the baseline thickness data. The measured ES-DCPD change was 2.16%, which is lower than the ES-DCPD criteria, judged as acceptable. This is confirmed by separate thickness inspection using UT on site. The developed WalSS with ES-DCPD also has limitations, as follows: First, it is hard to indicate the defect location within a measurement area. WalSS using ES-DCPD has been developed to screen wall thinning caused by FAC. In this case, this limitation does not matter because the location and profile of wall thinning can be within predictions (for example elbow, orifice, valve, etc.). While ES-DCPD can be applied in experience areas of NPP, an extension of application to the various piping system and degradation mechanism such as buried pipe or sea water pipe force same limitation. Second, ES-DCPD needs the initial potential value because ESDCPD measures potential drop that is related to thickness changes from initial condition. If there is no initial measured potential value, the initial value should be calculated using NDE result at initial time or design value. In this process, inaccuracy may be increased. Several additional improvements in the developed wall thinning screening system with ES-DCPD are recommended as future work, as follows: - Additional screening criteria confirmation using more plant data is desirable to assure adequate reliability of the developed screening criteria.
- Additional ES-DCPD database would be required to confirm the reliability. - Application to the piping systems in systems other than nuclear power plants. Acknowledgments This work was financially supported by the R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Ministry of Trade Industry and Energy (MOTIE), and by the Korean Nuclear R&D program organized by the National Research Foundation (NRF) of Korea in support of the Ministry of Science, ICT and Future Planning (MSIP). References ASME, 1998. Boiler and pressure vessel code. Am. Soc. Mech. Eng. ASME, 2003. ASME code case N597, Requirements for analytical evaluation of pipe wall thinning, Section XI Division 1. Am. Soc. Mech. Eng. ASTM, 2003. E1820-01, Standard Test Method for Measurement of Fracture Toughness, Annual Book of ASTM Standards, Document Number E1820-01. Am. Soc. Test. Mater. Choi, S.Y., Choi, Y.H., 2004. Piping failure frequency analysis for the main feedwater system in domestic nuclear power plants. J. Korean Nucl. Soc. 36, 112–120. EPRI, 1998. Nuclear Reactor Piping Failures at US Commercial LWRs: 1961–1997, EPRI Technical Report-110102. Electric Power Res. Inst. EPRI, 1999. Recommendations for an effective flow-accelerated corrosion program, NSAC-202L-R2. Electric Power Res. Inst. Jeoung, S.H., 2006. Development of an Integral Test Loop, HELIOS and Investigation of Natural Circulation Ability for PEACER. Seoul National University (Ph.D. thesis). KHNP, 2008. Optimization of Thinned Pipe Management Program and Application, KHNP-Final Report-A04NT02. Korea Hydro and Nuclear Power Co. Ltd. Ryu, K.H., Lee, N.Y., Hwang, I.S., 2007. A study on the equipotent switching direct current potential drop method for the monitoring of piping thinning. Key Eng. Mater. 345–346 (2), 1331–1334. Ryu, K.H., Hwang, I.S., Oh, Y.J., Lee, N.Y., Kim, J.H., Park, J.H., Sohn, C.H., 2008a. Screening method for piping wall loss by flow accelerated corrosion. Nucl. Eng. Des. 238, 3263–3268. Ryu, K.H., Hwang, I.S., Kim, J.H., 2008b. Verification of the viability of equipotential switching direct current potential drop method for piping wall loss monitoring with signal sensitivity analysis. J. Korean Soc. Nondestruct. Test. 28 (2), 191–198. Ryu, K.H., Lee, T.H., Kim, J.H., Hwang, I.S., Lee, N.Y., Kim, J.H., Park, J.H., Sohn, C.H., 2010. Online monitoring method using equipotential switching direct current potential drop for piping wall loss by flow accelerated corrosion. Nucl. Eng. Des. 240, 468–472. USNRC, 1980a. Pipe Cracking Experience in LWR, NUREG-0679. USNRC, 1980b. Investigation and Evaluation of Cracking Incidents in Piping in PWR, NUREG-0691. USNRC, 1989. Erosion/Corrosion-Induced Pipe Wall Thinning in US NPPs, NUREG1344.