ML22166A350
| ML22166A350 | |
| Person / Time | |
|---|---|
| Issue date: | 06/14/2022 |
| From: | David Rudland NRC/NRR/DNRL |
| To: | |
| References | |
| Download: ML22166A350 (13) | |
Text
Predicting Pipe Rupture Frequencies Using xLPR David L. Rudland Senior Technical Advisor for Materials Division of New and Renewed Licenses Office of Nuclear Reactor Regulation Nuclear Regulatory Commission June 14, 2022 xLPR Public Meeting
Purpose
- This presentation discusses an initial study conducted in 2019 focused on whether xLPR can be used to estimate pipe rupture frequencies
- The focus of xLPR Version 2.1 has been investigating leak-before-break behavior of reactor coolant piping
- Can xLPR be used to confirm pipe rupture frequencies?
- NUREG-1829, Estimating Loss-of-Coolant Accident (LOCA)
Frequencies Through the Elicitation Process
- This presentation will not address or discuss any regulatory issues or requirements 2
Base Input - Per xLPR Inputs Group OD=34 inch (864mm), t=2.6 inch (66mm)
Alloy 82/182, Dissimilar metal weld 605F (318C), 2,250 psi (15.5MPa)
Operating load - 20ksi (138MPa) bending, membrane 0.5ksi (3.4MPa)
Earthquake loading considered - 0.0002/yr Typical transients considered - impact to fatigue trivial 10-yr inservice inspection (ISI) with no mitigation 1 GPM leak detection (LD) 3 Primary water stress-corrosion cracking (PWSCC) initiation - Direct Model 1 1.5mm deep, 4.8mm long Circumferential cracks All other inputs per xLPR documentation Weld residual stress (WRS)
Initial Runs
- Focused on probability of pipe rupture with 1 GPM LD and 10-year ISI
- Even with many realizations and importance sampling on WRS and PWSCC growth parameters, no ruptures predicted with LD and ISI
- To aid in calculation, initiation model was turned off and five initial flaws were used, each with a depth of 1.5 mm and a length of 171 mm, which covers 37% of the inner circumference 4
- WRS was also modified Initial Runs 5
Failures caused by surface crack rupture (5%) or transition crack ruptures (95%)
Only very long surface cracks can cause rupture with LD and ISI.
Crack Initiation
- The previous results were conditional on the existence of 5 initial cracks
- Can use the initiation model to estimate the probability of multiple cracks 6
Assumed 400MPa (58ksi) stress and a temperature of 318C (605F)
Single Crack Analyses
- Same inputs but a single pre-existing crack was assumed
- Varied the initial crack length, kept the initial depth at 1.5mm
- Failures driven by transition crack ruptures 7
Wall thickness differences cause trends - R/t constant!
Multiple Crack Analyses 8
- Same inputs as before, but two and three initial cracks were assumed With crack growth and flaw coalescence a leaking flaw has a long enough crack length such that the transition crack fails before leaking greater than the leak detection limit Transition crack ruptures
Multiple Crack Analyses 9
4-inch diameter Same trends as large diameter case Surface crack ruptures
Estimating Rupture Frequency
- Use trends to estimate rupture probability for small flaw
- Combine this with probability of initiating 2 flaws
- Assume 50 welds/pipe system
- Convert to annual frequency 10 Contribution of non-rupture breaks are NOT included here
Known Conservatisms/Assumptions
- Linear WRS assumption
- No axial defects
- Assumed PWSCC controls all joints in pipe system
- All flaws initiate at time 0 with some probability
- Constant R/t, constant loads 11
Summary
- These results suggest that the pipe break frequencies in NUREG-1829 were conservative as compared to those calculated in this effort
- However, additional analyses are needed for a more robust comparison with the piping break frequencies developed through the expert elicitation effort of NUREG-1829
- Conducting xLPR analyses with pre-existing defects may be useful for efficiently estimating pipe rupture frequencies with active degradation mechanisms 12
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