ML070890463
| ML070890463 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 03/29/2007 |
| From: | Tam P NRC/NRR/ADRO/DORL/LPLIII-1 |
| To: | Scarpello M, Simpson S, Vasey B American Electric Power Co |
| References | |
| TAC MD3137, TAC MD3138 | |
| Download: ML070890463 (5) | |
Text
From: Peter Tam To: Scarpello, Michael; Simpson, Sue; Vasey, Bob Date: 03/29/2007 8:32:06 AM
Subject:
Draft RAI on D.C. Cook Risk-Informed ISI Program (TAC MD3137, 8)
Bob:
We are reviewing your 9/29/06 submittal regarding the risk-informed inservice inspection program for Class 1 and 2 piping. Our PRA reviewer Stephen Dinsmore developed the following draft RAI questions that we would like to discuss with you in a conference call. Please contact me to set up such a call. [Please note that the NRC staff is addressing a similar set of draft RAI with Grand Gulf.]
(1) The licensee requests authorization to implement a risk-informed inservice inspection (ISI) program based on American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Case N-716 (N-716). There appears to be, however, some differences between the methodology in N-716 and the method applied by the licensee:
(a) Table 3 of N-716 discusses high, medium, and low failure potential, and pairs these potentials with degradation categories large break, small leak, and none respectively. It does not appear that this table was used in the submittal. Was this table used in the submittal? If not, what was used in lieu of Table 3?
(b) Section 5(c) of N-716 does not appear to provide a with probability of detection (POD) and without POD option in the calculation, but the submittal includes one set of estimates for with POD and another w/o POD in Table 3.4-1. Please clarify how the with POD and w/o POD columns in Table 3.4-1 are consistent section 5(c) of N-716.
(c) The estimates in the w/o POD column in Table 3.4-1 all seem to include a standard POD of 0.5. Is this correct? If not, please provide some examples using the conditional core damage probability (CCDP) values from page 11 of 35 to produce the entries in Table 3.4-1 and 3.4-2.
(d) Section 7 of N-716, Program Updates, includes several steps that make up a program update. Page 15 of 35 in the licensee's submittal states that, [u]pon approval of the RIS-B Program, procedures that comply with the guidelines described in Reference 2
[ Electric Power Research Institute (EPRI) TR-112657 (EPRI Topical)] will be prepared to implement and monitor the program. Please identify the Sections in the EPRI topical that describe the update program that the licensee intends to implement. Please describe and compare the update program that the licensee intends to implement against the characteristics of such a program as described in Section 7 of N-716.
(2) Regulatory Guide (RG) 1.178, An Approach for Plant-Specific Risk-Informed Decisionmaking for Inservice Inspection of Piping, describes one acceptable process for developing a RI-ISI program. Please explain how:
(a) The approach used to analyze piping system failures for the plant-specific PRA of pressure boundary failures compares to the approach described in Section 2.1.4 of RG 1.178; (b) The process used to assess piping failure potential for the plant-specific PRA of pressure boundary failures compares to the process outlines in section 2.1.5 of RG 1.178; (c) The quantitative results of the pipe failure frequency that resulted from the failure potential assessment compares to the weld failure frequencies proposed in Section 5(a) of N-716 that are eventually used in your change in risk estimates; (d) The consequence evaluation performed as part of the plant-specific PRA of pressure boundary failures compares with the process outlined under Section 2.1.6 of RG
1.178.
(3) Please fully define the population of welds to which the 10% guideline is applied and what inspections are counted.
(a) Is the guideline to examine a minimum 10% of all high-safety-significant (HSS) welds, 10% of all HSS butt welds, 10% of all HSS butt welds >= 4 NPS, or something else?
(b) What type of inspections can be counted, e.g., can visual examinations or wall thickness exams be counted in the 10%?
(c) What percentage of Class 1 butt welds (regardless of normal pipe size (NPS)) will be inspected in the proposed risk-informed program?
(4) Section 5(c) in N-716 does not clearly specify what population of welds should be included in the change of risk estimates and what welds may be excluded. The description of the parameters in the equations in Section 5(c) indicates that any weld that was inspected under Section XI or that will be inspected under the RI-ISI program will be included in the change in risk estimate.
(a) Is the population of welds that should be included in the N-716 change in risk estimate all welds that were inspected under Section XI, and that will be inspected under the RI-ISI program? If not, where in code Case N-716 is the guidance that reduces the population of welds that should be included in the change in risk estimate (b) If all welds that were or will be inspected are included in the change in risk estimates in Table 3.4-1 and 3.4-2 in your submittal, how are the CCDP, CLERP, and the failure frequency estimated for low-safety-significant (LSS) welds?
(5) Under Section 3.3 on page 8, your submittal identifies 4 primary guidelines on selecting inspection locations, or 6 guidelines if each sub-bullet in (1) is counted as a guideline. Please describe briefly how each of these six guidelines was applied (e.g., how many inspections were influenced by the guideline and if application of the guideline resulted in changes to the original locations) when you were selecting inspection locations. Also, were any inspections added due to change in risk considerations?
(6) The relationship between N-716's guideline that any piping segment whose contribution to core damage frequency (CDF) is greater than 1E-6/year is a high safety significant (HSS) segment, and the EPRI topical guidelines for safety significant categorization is unclear. For example, a low consequence segment in the EPRI Topical methodology has a CCDP less than 1E-6, an identical numerical value but a different metric than the 1E-6/year guideline in N-716.
Page 3-8 in the EPRI Topical provides an explanation that the CCDP and conditional large early release probability (CLERP) ranges were selected to guarantee that all pipe locations ranked in the low consequence category do not have a potential CDF impact higher than 1E-8 per year or a potential large early release frequency (LERF) impact higher than 1E-9 per year.
Inspection of Table 3.1-1 and 3.1-2 in your submittal also indicates that there are no entries in the CDF > 1E-6" column indicating that no segments in the CNP Units 1 and 2 flooding PRA exceeded this guideline.
(a) The N-716 code case section 2(5) does not include a LERF guideline analogous to the CDF guideline, and Table 3-1 and 3-2 in your submittal includes a column for CDF but not for LERF. Please explain why a LERF guideline is not included as a guideline in parallel with CDF.
(b) Please provide a discussion justifying the guideline value for CDF selected in section 2(5) in N-716 (i.e., 1E-6/year).
(c) Please provide a list of all the piping segments that were compared to the
>1E-6/year criteria along with the CDF and LERF estimates, the pipe failure frequency, and the CCDP and conditional large early release probability for each segment, (d) Please provide any observations made during any independent reviews of the CNP flooding PRA or observations from the internal events review that are also applicable to the flooding analysis. Please describe how these observations have been resolved such that there is confidence that segments that have a CDF greater than the guideline value have been identified.
(e) Page 3 of your submittal states that [i]nternal flooding was recently addressed (2006) to complete the effort to address all Westington Owners Group certification Level A and B F&Os. To the extent not discussed in the response to RAI 6(d), please explain what addressed means. Were changes made to a flooding analysis? If changes were not made, how are the F&Os addressed? How were the changes that were made, or the explanation for not requiring changes, reviewed for technical adequacy?
(7) Page 12 describes how the CCDP and CLERP of different types of HSS pipe breaks are estimated in support of the change in risk estimates. Some values appear to be derived from representative sequences from the PRA models while others are directly estimated. For example, bounding values for pipe breaks that result in isolable loss-of-coolant accidents (LOCAs) are directly estimated as the product of the CCDP from unisolable LOCAs and the probability of a motor operated valve failing to close on demand. Direct estimation can be very analyst-specific and essentially bypasses the PRA peer review process upon which the NRC relies to minimize the staff review of the plant-specific PRA for each risk-informed submittal.
(a) Please identify events modeled in the CNP PRA that are similar to the directly estimated values on page 12 of your submittal or further clarify why the your PRA can not be used to develop the required estimates (these appear to be ILOCA, PLOCA, PILOCA-OC, and PILOCA-IC). If applicable events in the PRA can be identified, please provide a description of these events and the bounding CCDP and CLERP values for these types of breaks derived from the PRA.
(b) In the Table on page 12, please describe the difference between row 2, isolable LOCA (assumed to be inside containment), and row 5, potentially isolable LOCA inside containment. In what category would a pipe break that relied on a MOV that does not close automatically but that could be closed remotely by a manual action be placed?
(c) Row 6, Class 2 SDC - IC states that the CCDP and CLERP are [e]stimated based on a loss of shutdown cooling during mid-loop operation. Are these values intended to develop the safety significance of these segments during shutdown, or as surrogates for power operation. If these values are intended as surrogates for power operation, please explain why these values are reasonable surrogates. If not intended as surrogates for power operation, how was the safety significance of these segments during power operation addressed.
(d) The last row in the Table on page 12 includes an entry labeled Class 2 LSS". What characteristics results in a Class 2 LSS" designation? The same entry further states that the CCDPs and CLERPs of pipe ruptures associated with these welds are [e]stimated based on upper bound for Medium Consequence. Please provide a discussion explaining why selecting these values is appropriate.
(e) The ASME standard RA-Sa-2003, element IE-C12 discusses the evaluation of the likelihood of a interfacing system LOCA. In which category does the interfacing system LOCA belong in your Table on page 12?
(8) Under Section 2.2 on Page 4, you state that, [t]he requirements of MRP-139 will be used for inspection and management of primary water stress corrosion cracking (PWSCC) susceptible welds and will supplement the RIS_B Program selection process. Please describe what is meant by supplement. How will the PWSCC degradation mechanism be addressed,
as any other mechanism or differently? How will any inspections that might be required by MRP-139 be credited in the RIS_B program?
(9) Is the guideline to examine a minimum 10% of all HSS welds, or 10% of all HSS butt welds, or 10% of all HSS butt welds >= 4 NPS?
(10) Under Section 3.4 on Page 11, your submittal states the risk from implementation of this program is expected to remain neutral or decrease when compared to that estimated from current requirements. Consistent with this, the total change in risk in the two tables on page 14 is always negative. Is Cook committing to ensuring that these total risk numbers will be maintained at or below 0 as it monitors the program overtime as described in Section 4 of your submittal?
CC: Dinsmore, Stephen; Keim, Andrea Mail Envelope Properties (460BB1C6.D18 : 12 : 35330)
Subject:
Draft RAI on D.C. Cook Risk-Informed ISI Program (TAC MD3137, 8)
Creation Date 03/29/2007 8:32:06 AM From: Peter Tam Created By: PST@nrc.gov Recipients Action Date & Time aep.com Transferred 03/29/2007 8:32:34 AM mkscarpello (Michael Scarpello) rgvasey (Bob Vasey) sdsimpson (Sue Simpson) nrc.gov OWGWPO03.HQGWDO01 Delivered 03/29/2007 8:32:13 AM SCD1 CC (Stephen Dinsmore) Opened 03/29/2007 8:54:00 AM nrc.gov OWGWPO04.HQGWDO01 Delivered 03/29/2007 8:32:19 AM ATK CC (Andrea Keim) Opened 03/29/2007 9:34:01 AM Post Office Delivered Route aep.com OWGWPO03.HQGWDO01 03/29/2007 8:32:13 AM nrc.gov OWGWPO04.HQGWDO01 03/29/2007 8:32:19 AM nrc.gov Files Size Date & Time MESSAGE 16106 03/29/2007 8:32:06 AM Options Auto Delete: No Expiration Date: None
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