An HPI/makeup nozzle safe end from a B&W plant type was selected as one of the base case systems.

F.3.4.1 Dimensions and Welds — This type of component was considered in Reference F. 5, which identifies the component as 2 Уг inch schedule 160 pipe fabricated from Type 304 austenitic stainless steel. The location considered in Reference F.5 is in the safe end at the nozzle, which has a thickness of 11.1 mm (0.4375 inches) and a mean radius of 32.5 mm (1.28 inches) at the location of high stresses.

F.3.4.2 Stresses and Cycles -_As shown in Reference F.5, the cyclic stress history is dominated by two types of transients, with the amplitudes and frequencies shown in Table F.14.

Table F.14 Stress History for HPI/Make Up Nozzle
from NUREG/CR-6674 [F.5]

Name	Stress Amplitude ksi	Number in 40 years
HPI actuation A/B	221.24	33
Test Null	169.31	7

The deadweight and restraint of thermal expansion stresses for this location under normal operation that were used in Reference F.5 are

Odw-0

Cte= 63.1 MPa (9.16 ksi)

As discussed above, these stresses were composed of 310 MPa (45 ksi) uniform and the remainder the generic gradient of Equation F.6. These stresses are believed to be very conservative and are for the thermal sleeve being intact.

F.3.4.3 Results — The version of PRAISE that considers fatigue crack initiation was run for the HPI/make up nozzle. The stresses of Table F.14 were taken to be axisymmetric. Due to the small line size, only 4 initiation sites around the circumference were considered. Table F.15 summarizes the results.

Table F.15 Cumulative Probability PRAISE Results for HPI/Make Up Nozzle

(Intact Thermal Sleeve)

Condition	From Reference F.5	Here
О A	25		1.004×10-5
40	0.00210	6.08×10-4
60	0.0309	1.04×10-2
		Inel4a2
О О л	25	—	4.5×10-8
40	—	4.9×10-7
60	—	1.79×10-5
		Inel4a1
>1500	25	—	2.0×10-8
40	—	2.10×10-7
60	—	4.56×10-6
		Inel4a2

Table F.15 shows a cumulative leak probability of 10-5 in 25 years, which is quite low. However, leaks in this component have been observed in service, in which case the thermal sleeve in the component was failed. The results of Table F.15 use the stresses for an intact sleeve, and the stresses will be altered if the sleeve fails. A failed thermal sleeve is now considered.

F.3.4.4 Failed Thermal Sleeve — There is a thermal sleeve at the HPI nozzle, and the results in Table F.15 are for the case of the thermal sleeve not failing. The thermal sleeve has been observed to fail in service, which changes the stresses in the component.

In order to model the failure of the thermal sleeve, the following steps were taken:

1. Once the thermal sleeve fails, assume that a crack of the “initiation size” immediately appears. This size is a depth of 3.0 mm (0.12 inches). The WinPRAISE default distribution of the aspect ratio is used, as in other components.

2. A WinPRAISE run with this initial crack is performed, with the stresses that were present before the crack initiated (Table F.14), plus a uniform cyclic stress cycling each hour of sufficient amplitude to result in a high leak probability at not long times. This defines the uniform stress.

3. Use WinPRAISE to compute the leak frequencies for larger leak rates.

This procedure provides the results shown in Table F.16.

Table F.16 Cumulative PRAISE Results for HPI/Make Up Nozzle with
Failed Thermal Sleeve and Additional Uniform Cyclic Stress, ou

		Intact Sleeve	With Initial Crack and Original Stresses, Ou=0	With Initial Crack and ou = 8 ksi	With Initial Crack and ou = 12 ksi	With Initial Crack and ou = 25 ksi
О A	5	—	5.67×10-5	<10-2	0.047	0.18
25	1.004×10-5	3.69×10-3	0.032	0.14	0.727
40	6.08×10-4	1.26×10-2	0.129	0.33	0.909
60	1.04×10-2	2.98×10-2	0.161	0.47	0.909
>100	25	4.5×10-8	6.49×10-4			<10-5
40	4.9×10-7	2.68×10-3
60	1.79×10-5	5.31×10-3
>1500	25	2.0×10-8	—
40	2.10×10-7	—
60	4.56×10-6	—
break	25	—	6.49×10-4
40	—	2.68×10-3
60	—	5.31×10-3

Table F.16 shows that a uniform stress of some 170 MPa (25 ksi) is needed to result in an appreciable leak probability within 25 years. However, the frequency of larger leak rates is actually reduced by imposing the uniform stress that is necessary to produce the high leak probabilities seen in service. This uniform stress grows cracks to leaks, so that the larger leak rate frequencies are reduced. The least favorable condition for larger leaks is a failed thermal sleeve with the original stresses (cu = 0).

F. 3.5 Recirculation Line — 12 inch

The recirculation line is one of the base case systems for a BWR. This system has developed leaks in the past due to intergranular stress corrosion cracking (IGSCC). The 12 inch line has some of the highest stresses, so is considered here. The recirculation system also has 28 inch lines, which can contribute to larger flow rate failures than possible from a 12 inch line. Hence, the 28 inch line is also considered in subsequent sections.

F.3.5.1 Dimensions and Welds — The layout of the recirculation system is given in isometrics made available to panel members. There are two recirculation loops, which are very similar to one another. There are 121 welds in this system, including field welds, shop welds and safe ends. The piping is fabricated from A-358 Class 1 Type 304, and the piping is of diameters 12, 22 and 28 inches — all schedule 80.

F.3.5.2 Stresses and Cycles — IGSCC will be the dominant degradation mechanism. Hence, time at stress is of major concern, and the number of stress cycles is of secondary importance. Estimated stresses at the highest stressed locations for the two pipe sizes of interest are given in Table F.17.

Table F.17 Stress Information for Two Recirculation Joints OD, inch Thickness, inch Ono, ksi Seismic o, ksi 12.75 0.687 20.41 20.41 28 1.201 9.48 10.60

The normal operating stress (cNO) is the sum of the pressure stress, deadweight stress and restraint of thermal expansion stress. A value of 14 MPa (2 ksi) for the deadweight stress is assumed. The seismic stress is the normal operating stress plus the seismic-induced stress. Note that the seismic stresses are small in this case. The magnitude of the seismic event is unknown.

The time at stress is important for this case, with the cycling frequency being of less importance. Consistent with what is used for the PWR, the cycling is considered to be composed of heat up and cool down at 3 per year. The parameters related to stress corrosion cracking are summarized in Table F.18.

Table F.18 Stress Corrosion Cracking Parameters

Oxygen at startup (PPM) = 8.0 Oxygen at steady state (PPM) = 0.20 Heat up (100-550F) time (hrs) = 5.00 Coolant conductivity (ps/cm) = 0.20 Degree of sensitization (C/cm2) = 7.04

Residual stresses will be important, and the default residual stress distributions in pcPRAISE, which are documented in Reference F.2, are used when no remedial treatments are performed. In order to include remedial treatments that have been performed in service, a weld overlay at 20 years will be considered. This alters the thickness, crack growth kinetics (post-treatment analyses use Type 316NG crack growth defaults in PRAISE) and residual stresses. The axisymmetric through-wall residual stress distribution of Figure F.4 is employed. This figure is from Reference F.15. PRAISE can not treat the actual gradient, so the linear approximation in this figure is used. The linear gradient employed underestimates the beneficial effect of the weld overlay.

Figure F.4 Through-Wall Residual Axial Stress Distribution from Weld Overlay [F.15]

F.3.5.3 Results — Table F.19 summarizes the results obtained for the 12 inch weld in the recirculation system.

Table F.19 Cumulative Probability PRAISE Results for the 12 inch Recirculation Line Weld, with
and without Weld Overlay at 20 Years (ano = 141 MPa [20.41 ksi])

OD=12.75 inches, h=0.687 inches, wrought austenitic,
stress corrosion crack initiation and growth

		Base	Overlay at 20 years	Overlay & aDL @ 39 years
О A	25	0.3674	0.2967	0.2968
40	0.5986	0.3803	0.3872
60	0.7435	0.4241	0.4253
>100	25	0.1682	0.1427	0.1429
40	0.2452	0.1622	0.1632
60	0.2872	0.1693	0.1708
>1500	25	0.1529	0.1066	0.1078
40	0.2193	0.1250	0.1276
60	0.2534	0.1312	0.1343
break	25	0.1529	0.0490	0.0502
40	0.2193	0.0674	0.0700
60	0.2535	0.0736	0.0767

5000 trials 304 full residual stress
cdw=2.0 ksi cte=13.32 ksi aDL=11.67ksi 3 HU-CD/yr p=1125 psi

The beneficial effect of the weld overlay at 20 years is not readily apparent from the results in Table F.19; such benefits are shown more clearly in Figure F.5, which provides a plot of the cumulative probability of a leak exceeding 380 lpm (100 gpm) as a function of time.

12 in recirc, overlay at 20 years

Figure F.5 Cumulative Probability of a Leak Exceeding 100 gpm as Functions of Time for the 12
inch Recirculation Line Weld with and without Weld Overlay at 20 Years

The slopes of the lines in Figure F.5 are the leak frequencies, and the slope at 40 years with no overlay is about 7 times that with overlay.

F.3.5.4 Summary of Observations from Service — Leak frequencies due to IGSCC in recirculation lines were estimated from service experience and reported in Reference F.16. Figure F.6 is Figure F.12 from that reference. With some exceptions, the results in Figure F.6 are between 10-4 and 10-3 per weld — year. The results are for times up to 15 years and do not include remedial actions. No strong dependencies on time or line size are apparent, but the smaller diameter lines appear to have a somewhat higher failure frequency.

Table F.20, which is from Charts 2 and 3 of Reference F.17, summarizes the depth distribution of observed cracks per weld-year for various pipe sizes in recirculation lines in BWRs. The remedial action of Reference F.17 is considered to consist of a weld overlay at 20 years. Observed crack sizes without remedial action, as reported in Reference F.16, are shown in Figure F.7.

Figure F.6 Leak Frequencies as a Function of Time and Pipe Size (from Reference F.16)

Table F.20 Observed Crack Depth Frequencies in Various Line Sizes in Recirculation Lines as
Percentages of the Wall Thickness (from Reference F.17)

No Remedial Action

Size	> 10% >	20% >	30% >	40% >	50% >	60% >	70% >	80% >	90%
NPS12	2.06E-03	1.62E-03	7.28E-04	3.64E-04	2.00E-04	1.46E-04	1.09E-04	7.28E-05	3.64E-05
NPS22	1.63E-03	1.11E-03	6.48E-04	3.21E-04	1.90E-04	1.24E-04	9.81E-05	6.54E-05	3.27E-05
NPS28	2.12E-03	1.50E-03	1.04E-03	5.99E-04	2.57E-04	1.84E-04	6.12E-05	3.67E-05	1.22E-05

With Remedial Action Size > 10% > 20% > 30% > 40% > 50% > 60% > 70% > 80% > 90% NPS12 1.95E-04 1.60E-04 1.04E-04 8.31E-05 6.73E-05 4.61E-05 2.78E-05 1.90E-05 1.03E-05 NPS22 3.29E-04 2.74E-04 1.70E-04 1.32E-04 1.01E-04 8.62E-05 4.43E-05 2.95E-05 1.48E-05 NPS28 3.95E-04 2.84E-04 1.77E-04 9.15E-05 6.66E-05 3.82E-05 2.08E-05 1.24E-05 3.95E-06

Figure F.7 Observed Crack Sizes as Reported in Reference F.16

F.3.5.5 Comparisons with PRAISE — The normal operating stress in Table F.17 of 20.41 ksi is for the highest stressed joint in the 12 inch recirculation line, whereas the observations are for all joints, including lower stressed locations. In order to generate PRAISE results that would be more representative of the population, runs were made with various stresses. Table F.21 summarizes the results.

Table F.21 Cumulative PRAISE Results for a 12 inch Recirculation Line Weld for Various Normal Operating Stresses (Remedial Action at 20 Years) Cumulative Mean cno 10 12 15 20 COV 0.0 0.3 0.0 0.0 Mean cte 3.32 5.32 8.32 13.32 о A 25 1.42×10-3 1.54×10-2 9.36×10-2 0.2967 40 1.46×10-3 1.89×10-2 0.1473 0.3803 60 1.46×10-3 2.08×10-2 0.1781 0.4241 >100 25 4.90×10-4 7.59×10-3 3.90×10-2 0.1427 40 4.90×10-4 8.86×10-3 5.35×10-2 0.1622 60 4.90×10-4 9.48×10-3 6.11×10-2 0.1693 >5000 25 3.50×10-4 5.80×10-3 3.19×10-2 0.1066 40 3.50×10-4 7.06×10-3 4.53×10-2 0.1250 60 3.50×10-4 7.684×10-3 5.27×10-2 0.1312 DEPB 25 1.00×10-4 2.70×10-3 2.12×10-2 0.0490 40 1.00×10-4 2.96×10-3 3.46×10-2 0.0674 60 1.00×10-4 4.58×10-3 4.20×10-2 0.0736

The results for 138 MPa (20 ksi) correspond to those in Table F.19. The normal operating stress was taken to be deterministic, except for the case of 83 MPa (12 ksi), in which case the normal operating stress is normally distributed with a mean of 83 MPa (12 ksi) and a standard deviation of 0.3x(36.7+13.8) MPa (0.3x(5.32+2.00) ksi) = 15.2 MPa (2.20 ksi).

The cumulative results from Table F.21 can be compared with the observed frequencies in Table F.20 by converting the cumulative results to a frequency by dividing by the time increment involved. In the current case, the increase in the cumulative following the remedial action is relatively small, as seen from Figure F.5. Hence, the cumulative results at 25 years from Table F.21 should be divided by 20 to provide frequencies for comparison purposes. This provides the results in Table F.22.

Table F.22 Estimated Leak Frequencies Prior to Remedial Action, from Table F.21

Mean cno	10	12	15	20
COV Cno	0.0	0.3	0.0	0.0
Mean cte	3.32	5.32	8.32	13.32
Frequency	7.11×10-5	7.69×10-4	4.68×10-3	1.48×10-2

This table shows that the mean normal operating stress of 83 MPa (12 ksi) with some variance provides the best agreement with the results of Figure F.6. This is the case that will be used for benchmarking against observed cracks.

The following steps were followed in order to provide PRAISE results for comparison with observations of part-through cracks:

1. The WinPRAISE software was modified to print out the sizes of cracks present at each time step in the analysis. The depth and length of the deepest crack and the longest crack at that time step are printed into a file, along with the number of cracks present at that time. This file contains at most a number of lines equal to the number of Monte Carlo trials times the number of time steps (which can be a lot of lines).

2. The WinPRAISE file from step 1 is then processed to provide another file that includes only the sizes of part-through cracks present at the time of interest (25 years in this case). (Cracks of zero depth, leaks and other times are eliminated.)

3. The crack size file from step 2 is then loaded into a histogram, which provides the number of cracks present at 25 years that fall within a certain depth range.

4. Since Reference F.17 reports detected cracks, the detection probability (Equation F.1) must be accounted for. This is accomplished by multiplying the number of cracks in each bin by the detection probability for a crack of depth equal to the midpoint of the bin. This provides the number of detected cracks in this bin. The contents of each bin are then divided by the number of trials times the time (25 years) to provide the crack sizes per weld year.

5. The histogram is then converted to a complementary cumulative form, which is then directly comparable to results from Reference F.17.

Figure F.8 presents the crack size results for the benchmark case. Once again, not many deep cracks are observed. A pattern is observed in Figure F.8 which shows a preponderance of cracks below about 2.5 mm (0.1 inches). This pattern is due to cracks growing to a depth of 2.5 mm (0.1 inch) and then slowing

down or arresting, which is most likely due to the transitioning from growth of “initiating cracks” to “fracture mechanics cracks” that occurs in the PRAISE modeling of initiation and growth. The transitioning criteria are discussed on page 42 of Reference F.2, and one of the criteria is “If the depth of the crack is greater than 2.5 mm (0.1 inch), its growth will always be by fracture mechanics velocity”.

Figure F.9 is a plot of the predicted complementary cumulative number of observed cracks for the benchmark case, along with a comparison with reported observations. The outstanding inspection parameters of Table F.1 were employed. In this figure, Reference F.17 results from Chart 1 (prior) and Chart 2 (posterior) are both shown, since the analysis mixed with and without remedial action (weld overlay at 20 years). The analysis results fall midway between the two results, except for shallow cracks.

о

&

nj

13

M

Figure F.8 Crack Sizes After 25 Years for the Benchmark Case (Mean oNo = 12 ksi) with Weld

Overlay at 20 Years

a/h

Figure F.9 Comparison of Results for the Benchmark Case Predicted for Outstanding Inspection
Quality with Reported Prior and Post Observations [F.17]

The agreement shown in Figure F.9 is felt to be quite good, and indicates that the PRAISE model best fits the observed crack depths when the mean stress of 83 MPa (12 ksi) is used. Figure F.10 shows that the stress has an important effect, because the agreement is not so good when a stress of 103 MPa (15 ksi) is employed.

12 in, med stress random

Figure F.10 Comparison of Results for the High-Stress Case Predicted for Two Inspection
Qualities with Reported Observations (the Square Data Points are the Observations [F.17], no

Weld Overlay)

The results of Figure F.7 are for observed (detected) cracks, whereas Figure F.8 has not had the nondetection probability applied. A direct comparison is therefore not possible, and the two figures are plotted on much different scales. However, the PRAISE predictions contain a much greater proportion of short and shallow cracks than the observations. This could be somewhat affected by the nondetection probabilities, but the differences would not be removed by applying the nondetection probabilities to the cracks predicted by PRAISE.

The question immediately arises regarding the results to be used in the estimation of the recirculation system reliability; the results of Table F.20 for the highly stressed joint or the results of Table F.21 for the benchmarked stress of 83 MPa (12 ksi). Interestingly, the system and average weld leak frequencies are nearly the same whether the 83 MPa (12 ksi) or 140 MPa (20 ksi) weld is used, because the number of joints involved also depends on the stress. Table F.23 summarizes this comparison. This table is in terms of the leak frequency per year, which is obtained from the cumulative results given above by use of Equation F.2.

The system frequency and average frequency per weld are nearly the same for both cases.

Table F.23 System and Average Weld Leak Frequencies for Two Cases
of the 12 inch Recirculation Line

Mean cNO, ksi	12	20
Mean Cte, ksi	5.32	13.32
COV	0.3	0
Std Dev of ate, ksi	1.6	0
	per we	d joint
0 — 25 years	6.15×10-4	1.19×10-2
25 — 40 years	2.36×10-4	5.57×10-3
40 — 60 years	9.25×10-4	2.19×10-3
Number dominant. joints	49	2
Number in system	49	49
	System (times number dom. joints)
0-25	3.01×10-2	2.38×10-2
25-40	1.16×10-2	1.11×10-2
40-60	4.53×10-3	4.38×10-3
	Average per joint (F 49)
0-25	6.15×10-4	4.86×10-4
25-40	2.36×10-4	2.24×10-4
40-60	9.25×10-4	8.94×10-5

Of the two cases in Table F.23, the case of a mean stress of 83 MPa (12 ksi) and coefficient of variation of 0.3 (on cte + Cdw) is more representative of the population of joints as a whole, so is preferred for comparisons with observations of part-through cracks.