20.08.2015   Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process

. Data for LOCA Frequency Estimation

Using the information in Section D.4, this section documents the input data to the LOCA frequency model. A Bayesian update is performed to develop posterior weld failure rates. The frequency of leaks exceeding Technical Specification limits are developed through estimates of the conditional probability of a large leak given a small through-wall flaw.

D.5.1 Posterior Weld Failure Rates

The failure rate calculation involves two factors, the number of applicable failures and the exposure data. To account for uncertainty in the exposure data, which could influence the failure rate calculation the following process is used. First, a best estimate update is performed using the appropriate number of failure events and the number of welds of exposure. To account for plant-to-plant variability in the weld exposure term, a second update is performed using the same failure data but an exposure estimate that is 50% higher, and a third update using an exposure estimate that is 50% lower. Each of the three updates is combined in a posterior weighting process using the following weights: 50% for the best estimate, 25% for the high exposure case and 25% for the low exposure case. The result is an uncertainty distribution for each failure rate, which reflects greater uncertainty than the best estimate data would imply alone. The results are given in Tables D.12 and D.13; Attachment B includes the input to the failure rate calculation. Figure D.19 displays posterior IGSCC flaw frequencies. Figures D.20-D.23 compare the prior and posterior non-through wall crack frequencies.

System

Pipe Size [NPS]

Weld Configuration

Failure Rate Uncertainty Distribution Parameters [(<TS Leak)/Weld-yr]

Mean

5 %-tile

50%-tile

95%-tile

RR

12

Elbow-to-pipe

4.32E-05

8.48E-06

3.17E-05

1.16E-04

Nozzle-to-safe-end

4.38E-05

5.52E-06

2.72E-05

1.36E-04

Pipe-to-safe-end

2.99E-05

2.98E-06

1.70E-05

9.64E-05

Pipe-to-sweepolet

3.14E-05

2.80E-06

1.71E-05

1.06E-04

Pipe-to-reducer

7.82E-05

5.71E-06

3.97E-05

2.77E-04

RR

22

Pipe-to-end-cap

1.54E-04

2.28E-05

1.01E-04

4.52E-04

Pipe-to-cross

4.24E-05

4.38E-06

2.47E-05

1.37E-04

Pipe-to-sweepolet

7.37E-05

7.02E-06

4.09E-05

2.40E-04

RR

28

Pipe-to-elbow

8.52E-05

1.59E-05

6.07E-05

2.33E-04

Nozzle-to-safe-end

6.55E-05

5.95E-06

3.61E-05

2.15E-04

Pipe-to-safe-end

1.44E-04

2.11E-05

9.36E-05

4.28E-04

Pipe-to-valve

5.96E-05

7.68E-06

3.75E-05

1.84E-04

Pipe-to-pump

8.36E-05

8.68E-06

4.85E-05

2.71E-04

Pipe-to-tee

5.78E-05

5.06E-06

3.13E-05

1.96E-04

Pipe-to-pipe

1.29E-05

5.74E-07

5.25E-06

4.78E-05

Pipe-to-cross

3.86E-05

7.89E-07

1.08E-05

1.50E-04

Reducer-to-cross

3.86E-05

7.89E-07

1.08E-05

1.50E-04

FW

12

Nozzle-to-safe-end

2.29E-06

8.61E-10

6.88E-08

5.29E-06

Elbow-to-pipe

1.75E-07

4.61E-11

4.28E-09

3.74E-07

Pipe-to-pipe

2.78E-07

7.39E-11

6.89E-09

6.33E-07

Pipe-to-safe-end

2.43E-07

6.20E-11

5.97E-09

5.50E-07

Pipe-to-reducer

9.73E-07

2.38E-10

2.24E-08

2.12E-06

FW

12

Elbow-to-reducing-tee

3.33E-07

7.46E-11

7.11E-09

6.94E-07

FW

20

Pipe-to-elbow

1.62E-06

5.57E-10

4.61E-08

3.71E-06

Pipe-to-pipe

4.10E-07

9.57E-11

8.77E-09

8.00E-07

Pipe-to-valve

3.38E-07

7.09E-11

6.78E-09

6.53E-07

Elbow-to-valve

3.54E-07

9.22E-11

8.80E-09

8.39E-07

Pipe-to-tee

4.50E-07

7.33E-11

7.20E-09

7.25E-07

Pipe-to-reducer

5.68E-07

1.17E-10

1.14E-08

1.14E-06

Tee-to-valve

2.18E-07

4.06E-11

4.12E-09

4.06E-07

Table D.13 Posterior RC and HPI/NMU Weld Failure Rate Distributions — PWR Base Cases

System

Pipe Size [NPS]

Weld Configuration

Failure Rate Uncertainty Distribution Parameters [(<TS Leak)/Weld-yr]

Mean

5 %-tile

50%-tile

95%-tile

RC

(Hot Leg)

30

Nozzle-to-safe-end

7.64E-05

2.12E-07

7.34E-06

2.61E-04

Elbow-to-pump

1.96E-05

7.59E-09

5.36E-07

4.02E-05

Pipe-to-pump

1.24E-05

5.78E-09

4.47E-07

3.17E-05

Elbow-to-pipe

1.05E-06

5.38E-10

3.94E-08

3.04E-06

RC

(Surge

Line)

14

Branch-to-CL

1.14E-06

2.90E-10

2.69E-08

2.42E-06

Nozzle-to-safe-end

2.95E-06

9.94E-10

7.99E-08

6.49E-06

Pipe-to-safe-end

1.75E-06

4.72E-10

4.00E-08

3.64E-06

Branch-to-pipe

4.76E-07

1.04E-10

1.04E-08

9.87E-07

Elbow-to-pipe

4.60E-08

1.06E-11

1.04E-09

9.66E-08

HPI/

NMU

3-%

Pipe-to-safe-end

6.56E-04

1.45E-05

1.99E-04

2.53E-03

Elbow-to-pipe

1.58E-06

3.30E-10

3.43E-08

3.39E-06

Pipe-to-valve

1.96E-06

2.36E-10

2.39E-08

2.35E-06

Pipe-to-pipe

4.55E-06

1.13E-09

1.13E-07

1.11E-05

1.00E-02

 

□ NPS1 2

□ NPS22

Подпись:Подпись:Подпись: □ NPS28image0441.00E-03

1.00E-06

> 10% > 20% > 30% > 40% > 50% > 60% > 70% > 80% > 90%

a/t-Ratio

Figure D.19 Posterior IGSCC Frequency (Non-Through Wall)

Подпись:Подпись: 1.00E-05image0471.00E-02

Подпись: о S

1.00E-06

> 10% > 20% > 30% > 40% > 50% > 60% > 70% > 80% > 90%
a/t-Ratio

image049 image050
image051

Figure D.20 Prior and Posterior IGSCC Frequency (Non-Through Wall) for NPS12 Welds

> 10% > 20% > 30% > 40% > 50% > 60% > 70% > 80% > 90%
a/t-ratio

Figure D.21 Prior and Posterior IGSCC Frequency (Non-Through Wall) for NPS22 Welds

1.00E-02

Подпись: NPS28 - Prior NPS28 - Posterior

1.00E-03

1.00E-04 1.00E-05 1.00E-06

> 10% > 20% > 30% > 40% > 50% > 60% > 70% > 80% > 90%
a/t-Ratio

Figure D.22 Prior and Posterior IGSCC Frequency (Non-Through Wall) for NPS28 Welds