ML031110402
| ML031110402 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  (NPF-003) |
| Issue date: | 09/03/2002 |
| From: | Demoss G Office of Nuclear Regulatory Research |
| To: | Michael Cheok, Matthew Kirk, Marksberry D Office of Nuclear Regulatory Research |
| References | |
| FOIA/PA-2003-0018 | |
| Download: ML031110402 (22) | |
Text
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Gary Demoss /-
Don Marksberry; Mark Kirk; Michael Cheok 9/3/02 8:25AM first cut at slides for Ashok For Dop and Mike, I have attached Mark Kirk's slides, which contain the pictures and graphics necessary to really describe the event Gary 415-6225
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Probabilistic Analysis Issues Associated the Davis Besse CRDM Nozzle Cracking and RPV Head Wastage Gary DeMoss, Don Marksberry, Mike Cheok RES/DRAA/OERAB PREDECISIONAL[NFORMATION thZEedecasional Information
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-Paae2-11 OVERVIEW PERFORMANCE DEFICIENCY Licensee failed to properly implement a boric acid wastage program Licensee failed to properly implement an inspection program for the detection of reactor coolant leakage THREE POTENTIAL FAILURE MECHANISMS LEADING TO LOCA As-Found Cavity Rupture - Cladding exposed in as-found condition fails at normal or transient conditions, pressure below SRV setpoints Enlarged Cavity Rupture - Cavity grows larger and fails at operating pressure CRDM Ejection - Circumferential crack grows and causes CRDM nozzle to fail and rod ejection AS-FOUND CAVITY RUPTURE - MLOCA Pre ca i ~oe aft rr
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- davs bessebri-wo Initiating event analysis - based on the rupture probability distribution multiplied by the frequency of pressure transients between operating pressure (2165 psi) and SRV set point (2500 psi) expected values of rupture pressure are around 6000 psi.
Probability of failure *1 x 10-3 Conditional Core Damage Probability given initiating event - SPAR Model and licensee provided values are about 3 x 10-3 for MLOCA ACDF- *3 x 106 Issues/uncertainties - Operating on the lower tail of the rupture probability distribution; uncertainty in strength of cladding material PredacseenMI0lTft& ton
nMarksberry - Davis bessebrief.wpdP CRDM NOZZLE EJECTION-MLOCA Initiating event - Have a probability of rupture vs. time curve from ANL. Assumed nozzle 3 wetted for 8 years. Then assumed nozzle 2 wetted for 4 years and nozzle 5 wetted for 2 years. They added 64% and 4% respectively. Probability of ejection is 2.7 x 1 -2.
Conditional Core Damage Probability given initiating event - SPAR Model and licensee provided values are about 3 x 1 0-3 for MLOCA ACDF- *8 x 10-5 Issues/uncertainties - Crack propagation rates of wetted nozzles.
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ENLARGED CAVITY RUPTURE - LLOCA Initiating event frequency - calculated from the probability that DB operated longer, thus had more time for cavity growth Probability that DB was one of the five early B&W plants - 5/7=0.71 Probability that DB had a better availability record - 2/7=0.2 Probability that DB had 7 additional years - 5/7 *2/7 = 0.2 Probabilities based on B&W reactor availability factors:
Probability that DB operated for additional 1.44 years before Oconee 3 cracking discovered - 0.57 Probability that DB operated for additional 2.0 years before Oconee 3 cracking discovered - 0.14 Initiating event frequency range - 0.2 to 0.57 / yr.
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Pagie 6 gq Don arkserrydavsbesebrif~wp Ie6 ENLARGED CAVITY RUPTURE - LLOCA (Continued)
Conditional Core Damage Probability given initiating event - SPAR Model estimate is 2 x 1 -2, licensee provided value is about 1 x 1 -2 for LLOCA ACDF-2 x 10-3to 6.2 x 10-3 Issues/Uncertainties Bounding corrosion rates 2"/yr - 4.5 to 7.5 years until rupture 7"/yr. - 1 to 2 years until rupture Likely ASP approach - At the time of discovery, the probability of LLOCA was not significantly greater than nominal.
Pradeqislonal Information
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I Uon MarKSDerry - Clavis besse briet.wpd Page 7 9 CONDITIONAL CORE DAMAGE PROBABILITIES Soure LOCAICCDPs,~K j
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L CA, s DB IPE not 6.87x1 0-3 1.08x1 0-2 provided DB Recalculated for size N/A 2.91x1 0-3 N/A of the exposed clad area SPAR 2QA 6.46x1 04 Not Analyzed SPAR 3i 3.47x104 2.54x10-3 2.1x10-2 (MLOCA range is 2" to 5")
SUMMARY
OF RESULTS - VERY PRELIMINARY Ewdectsional lnformation
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<2.9x104 cavity enlarged large 0.2 to 0.57 1.08x1 0.22.Ox1O-3 cavity to 6.2x10-3 nozzle medium 2.7x1 0-2 2.91 x10-3 8x10-5 e je c tio n_
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v 5.5 MEB Inputs to NRR Probabilistic Assessment of Davis Besse Head Degradation
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- RFQ, Mark Kirk, Bill Cullen, Wally Norris, Jim Davis, Nilesh Chokshi Office of Nuclear Regulatory Research, Division of Engineering Technology, Materials Engineering Branch, United States Nuclear Regulatory Commission VG
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- Photos / depictions of corrosion cavity at Nozzle #3
- Contents of NRR Phase 3 RSD
- Stress analysis As-found condition Potential future states v/ Corrosion rate
- CRDM circumferential cracking VG
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Contents of NRR Phase 3 RSD
- The Phase 3 Risk Significance Determination that has been drafted by Steve Long analyzes three accident sequences that could contribute to risk Vessel failure caused by rupture of the un-backed cladding
/ For the corrosion cavity as it existed when Davis Besse stopped operations in February 2002 Vessel failure caused by rupture of the un-backed cladding
/ For the corrosion cavity in a larger condition that might have existed.... ?????
Vessel failure caused by the development of a 3600 circumferential crack, followed by ejection of the nozzle from the RPV head VG
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Philosophy Driving MEB Stress Analysis of Cladding Rupture
- Preliminary analyses (in 3/02 81 4/02) showed that the probability of cladding rupture caused by pressures up to the set-point pressure (2500 psi) is very low
- Most "uncertainties" in this analysis are knowable, at least in principal Cavity geometry Cladding tensile properties Cladding thickness & geometry Because of the low rupture proba ii lese "uncertainties ave been bounded in our calculations (pending availability of better Information).
The one uncertainty that we have retained in our calculation is the uncertainty in how accurately our finite element model can predict cladding rupture VG
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Page6 6 Basis of Finite Element Model Cladding pTensile iRropertiesill (flower-bounded from avaW/abed data,)>
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Finite Element Model Reinforcement provided by J-groove weld was not modeled.
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.Estimated Median Clad Thickness [in.]
Instability [psi]
Failure Pressure [psi]
Minimum Measured 0.24 6650 7355 Spec. Nominal 0.1825 5180 5732 Failure Probabilities Distribution at Operating Pressure at Set Point Pressure (2165 psi)
(2500 psi)
Log-Laplace 4.14E-07 2.15E-06 Beta 0
0 Gamma 8.17E-19 1.50E-15 Normal 8.44E-1 0 8.90E-09 Random Walk 0
0 Inverse Gaussian 4.01 E-29 1.79E-22 Averages 6.91 E-08 3.60E-07 VG
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Analysis of Larger Cavities
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- 1 Observations from Analyses of Larger Cavities
-* Cavity shape has only a minor effect on the predicted burst pressure, especially at pressures near the set-point and operating pressures
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Completing this Work
- Reporting 2 ORNL letter reports
/ 1 is done
/ 1 is in review Provide comments to NRR on DRAFT RSD dated 8 02
- Maywish to Compare exposed cladding geometry modeled to that measured from dental mold Compare tensile properties used in analysis to those measured from Davis Besse cladding VG