ML20238F868
| ML20238F868 | |
| Person / Time | |
|---|---|
| Issue date: | 09/02/1998 |
| From: | Wen P NRC (Affiliation Not Assigned) |
| To: | Drake A WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| References | |
| PROJECT-694 NUDOCS 9809080033 | |
| Download: ML20238F868 (8) | |
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September 2, 1998 Mr. Andrew Drake Westinghouse Owners Group Westinghouse Electric Corporation Mail Stop ECE 5-16 P.O. Box 355 Pittsburgh, PA 15320-0355 l
SUBJECT:
OPEN ITEMS RELATED TO V/ WESTINGHOUSE OWNERS GROUP l
APPLICATION OF RISK-INFORMED METHODS TO PIPING INSERVICE l
INSPECTION (WCAP-14572, REVISION 1)
I
Dear Mr. Drake :
]
As you know, the staff has arranged a meeting with the Westinghouse Owners Group (WOG) on September 22,1998 to discuss the subject issues. Enclosed is a list of open items the staff would like to discuss during the meeting.
4 If you have any questions, please contact me at 301/415-2832 (email, pxw@nrc. gov) or Syed Ali at 301/415-2776 (email, saa3@nrc. gov).
1 Sincerely, Original Signed By]
Peter C. Wen, Project Manager Generic issues and Environmental Projects Branch Office of Nuclear Reactor Regulation l
l
Enclosure:
Open items Related to WOG Application of Risk-Informed Methods to Piping Inservice inspection (WCAP-14572, Rev 1)
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4*****s September 2, 1998 Mr. Andrew Drake Westinghouse Owners Group Westinghouse Electric Corporation Mail Stop ECE 5-16 P.O. Box 355 Pittsburgh, PA 15320-0355
SUBJECT:
OPEN ITEMS RELATED TO WESTINGHOUSE OWNERS GROUP APPLICATION OF RISK-INFORMED METHODS TO PIPING INSERVICE INSPECTION (WCAP-14572, REVISION 1)
Dear Mr. Drake :
As you know, the staff has arranged a meeting with the Westinghouse Ownero Group (WOG) on September 22,1998 to discuss the subject issues. Enclosed is a list of open items the staff would like to discuss during the meeting.
If you have any questions, please contact me at 301/415-2832 (email, pxw@nrc. gov) or Syed Ali at 301/415-2776 (email, saa3@nrc. gov).
Sincerely, R$vc. u Peter C. Wen, Project Manager Generic issues and Environmental Projects Branch Office of Nuclear Reactor Regulation
Enclosure:
Open items Related to WOG Application of Risk-Informed Methods to Piping inservice inspection (WCAP-14572, Rev 1)
Project No. 694 cc:
See next page l
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I OPEN ITEMS RELATED TO WOG APPLICATION OF RISK-INFORMED METHODS TO PIPING INSERVICE INSPECTION (WCAP-14572, REV 1) l l
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Enclosure
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1, The WCAP topical report states that the number of welds, fittings, or lengths of pipe is not incorporated into the estimated failure probabilities for a pipe segment. Most welds, or fittings, in a pipe segment, though, would have some finite probability of a failure.
l Therefore, the probability of a failure in a pipe segment should increase with the number of welds in the segment. Justification other than assuming common mode failure is l
required for ignoring the number of welds (or fittings) in a segment (INEEL Open Item 2).
2.
The WCAP states that the median values for stresses were set equal to one-half the stress values calculated by ASME Code stress analysis. This may be appropriate for stresses due to seismic loads but seems to be too much of a reduction for stresses due to internal pressure, deadweight, and thermal expansion, since these stresses normally have much lesser uncertainty (INEEL Open item 3).
3.
The WCAP gives little specific information relative to the application of welding residual stresses in the SRRA models. WOG needs to describe the details of the residual stress distributions applied (for various pipe sizes and material) and explain the basis for these (INEEL Open item 5).
4.
The WCAP does not mention the possibility of axial cracks, which could be of concern for the case of longitudinal welds. WOG should describe the failure criteria used to evaluate axial cracks in the SRRA models (INEEL Open item 8).
5.
(a)
According to the WCAP, the program PRODIGAL is used to calculate initial flaw characteristics for the piping welds and the program CLVSQ is used to calculate leak rates as a function of crack length. WOG should clarify whether the computer programs PRODIGAL and CLVSQ been appropriately verified and validated (INEEL Open item 13).
(b)
The program PRODIGAL is used to calculate the number of flaws per weld length near the inner surface of the pipe. WOG should explain whether all such flaws calculated by PRODIGAL are treated as surface breaking flaws in the l
SRRA fracture calculations. If so, is the flaw density in the failure probability calculations adjusted to reduce the effect of flaws that are not actually surface flaws (Open Item 14, INEEL).
I (c)
The program CLVSQ uses a simplified correlation to calculate leak rates.
I Discuss the accuracy of this correlation and explain why the more detailed model of pc-PRAISE was simplified. Explain whether this simplification consistently i
errs on the conservative side (Open item 15, INEEL).
(d)
It is the staff's understanding that the existing correlation for leak rates is limited to PWR RCS conditions. WOG should address the applicability of those l
correlations to other plant conditions. Clarify whether the SRRA code can be l
applied to BWRs. If yes, justify the applicability of the correlations used to i
calculate leak rates under BWR operating conditions (RES Open Item 31).
j 6.
The WCAP states that the high-cycle fatigue stress due to mechanical vibration is specified in the SRRA input for the small pipe size of 1 inch. A correction factor is applied to this stress to obtain the fatigue stress for other pipe sizes. The report identifies what these correction factors are for the other pipe sizes, but does not identify l
the value that is actually used for the high-cycle fatigue stress for the 1-inch pipe size (INEEL Open item 16).
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7.
(a)
Figure 4-2 of the WCAP Supplement 1 presents a comparison of SRRA model predictions with industry plant data relative to the probability of violating minimum wall thickness criteria because of flow accelerated corrosion wastage. The graph indicates that the SRRA model tends to over predict the failure probability early in plant life and to under predict later in life. WOG should provide an explanation for this behavior that may be used to refine the model (INEEL Open item 17).
(b)
Supplement 1 to WCAP provides information on assumptions made in the SRRA wall thinning model. However, it appears that many users could have difficulty in relating inputs to the model with the type of information available to plant technical staff. In addition, users may not have sufficient insight into the assumptions behind the wall thinning model to perform calculations in a correct and consistent manner. WCAP should provide guidance for plant personnel executing the SRRA code for FAC that provides reasonable assurance that the calculated results for FAC failure probabilities are appropriate (RES Open item 27).
8.
(a)
One issue of this element is not fully addressed by the methodology is the process by which to identify the scope of plant systems to be included in the RI-ISI proposed program is considered, but plant safety functions that rely on the affected piping have not been identified (INEEL Open item 20).
(b)
Operator action to isolate a break and mitigate the immediate consequences of the break are credited in the ISI analysis. For example, an operator closing an MOV will stop the loss of water from a break downstream of the MOV. The methodology does not recommend as' signing a probability that the operator (s) fail to perform the action. Instead, two sets of calculations are performed, one assuming all such actions are successful and one assuming that all such actions fail. The RRW and RAW measures are calculated for these different assumptions. The WCAP states that if any of the results are greater then the RRW guidelines, they are highlighted. It is not clear if the segment is assumed to be high safety significant. The staff finds the use of these sensitivity studies to bound the possible impact of the operator's attempts to mitigate the event acceptable because a quantitative estimate of such unusual actions under such unusual conditions would not increase the precision of the results and may, in fact, obscure important results (SPSB Open item 44).
(c)
The staff finds the expert panel process and information presented in the worksheets as meeting the intent of the integrated decision-making process, as discussed in RG 1.174 and 1.178, in that traditional, PRA, deterministic, l
qualitative and quantitative insights are integrated in identification of HSS piping segments. However, no guidance to the expert panelis presented in the WCAP that provides consistency in categorization of piping segments among the various licensees who apply the WCAP method. WCAP needs to provide guidance that ensures consistency among the licensees (RES Open item 43).
(d)
The WCAP needs to define more clearly the review of internal event recovery actions while developing surrogates (SPSB Open item P3(2)).
(e)
The WCAP needs to define more clearly what analysis documentation should be
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kept in retrievable files (Surry spreadsheets still marked preliminary draft) (SPSB Open item P3(3)).
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9.
It cpperrs that the possibility of check valves failing to close and automatic isolation valves failing to close is not considered during the analysis. That is, proper operation of check and isolation valves is assumed when defining segment boundaries. The staff notes that the probability of such valves failing to close range from 1E-02/ demand to less than 1E-03/ demand. In general, the staff expects the difference in the conditional consequences on one side of a valve versus the other would not be so great that this assumption would have any impact on the results. Containment isolation valves should, l
however, be reviewed to ensure the assumption is appropriate. WOG should clarify what credit is being given for automatic closure of isolation valves and whether the 3
probability of isolation failure would be considered in determining segment boundaries (INEEL Open item 21, SPSB Open item P4).
10.
In section 3.2 of the WCAP, in the Walkdown section, it is noted that for high energy piping that has the potential to whip following a postulated failure, it should be considered that a whipping pipe that has the pctential to impact other piping will not rupture lines of equal or greater sire, and that it should be assumed that a through-wall crack will develop in a line that is impacted by a whipping pipe of the same size. WOG should provide the basis for these assumptions (INEF Open item 22)
I 11.
Although it is stated in several places that the augmented programs are not included, the detailed results provided by Surry seem to include changes to the augmented programs. Furthermore, the augmented programs are credited in the pipe failure probability calculations. The degree to which the augmented programs are incorporated i
in the RI ISI needs clarification (SPSB Open item 23).
12.
WCAP should provide guidance for the analyst on the code limitations for complex l
geometries and guidance for effective use of the code in such applications (RES Open j
ltem 28).
l 13.
WCAP should specify the level of training and qualification that the code user needs to properly execute the SRRA code (RES Open item 29).
4 14.
WOG should describe how proof testing is addressed in the SRRA calculations. If it is not addressed, clarify what impact does its neglect have on the calculated failure probabilities and categorization (RES Open item 32).
15.
(a)
The probability of detection curves used in calculations need to be justified for the material type, inspection method, component geometry, and degradation mechanism that apply to the structurallocation being addressed, in this regard, users of the Sl(RA code should seek additional NDE reliability data and insights beyond the information and limited number of examples provided by WCAP-l 14572 Revision 1 Supplement 1 (RES Open item 33).
(b)
With proper inputs, the code provides a useful tool to assist in estimating piping failure probabilities due to wall thinning. The main difficulty with the methodology is the need for a consistent basis to assign input parameters, an area where the code documentation provides little guidance. There is a potential for different users of the code to estimate significantly different values for failure probabilities as a result of different judgements and interpretations associated with the high, medium and low categories of wall thinning. WOG should expand the code documentation to provide additional guidance for selecting the input for the calculation (RES Open item 35).
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16.
Supplement one to the WCAP references an outdated assumption of limiting failure probabilities to 1E-08 if the code calculates lower numbers. That is no longer the method and the document should reflect the final methodology (RES Open item 36).
17.
WCAP needs to provide justification for the guidance provided for location of circumferential and longitudinal breaks in high energy piping runs (RES Open item 38).
18.
For the standby systems, the equations in the WCAP do not incorporate the contribution of exposure time (allowed outage times). The staff finds the arguments acceptable that if the AOT does riot significantly contribute to the calculated CDF/LERF or system unavailability, then the contribution from AOT need not be calculated. However, the WCAP should provide a checklist to ensure that such contributions are negligible when development the analytic model (RES Open item 41).
19.
The staff considers that the sensitivity study where the pipe failure probabilities are assigned range factors provides valuable insights and should be made part of the methcdalogy to be used by each licensee. WOG needs to confirm that the sensitivity calculations would be part of the WCAP methodology to ensure that the medium safety significance range is appropriately selected, e.g., that no low safety significant segments migrate to high safety significance when resonable variations in the pipe failure probabilities are considered (RES Open item 45, SPSB Open item P2).
20.
The WCAP states that implementation of an RI-ISI program for piping should be initiated at the start of a plant's 10-year inservice inspection interval consistent with the requirements of the ASME Code Section XI, Edition and Addenda committed to by an Owner in accordance with 10 CFR 50.55a. However, it is not clear if it is the intent of this section to provide the staff periodic update on the RI-ISI program. The staff requires a 10-year update! status report be provided for staff information. This enables the staff to monitor program effectiveness and industry experience. This section of WCAP should be modified to provide such guidance (RES Open item 46).
21.
WCAP needs to clarify as to why it's appropriate to reduce the segment failure potential by factor of three on page 206 (SPSB Open item PS).
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e Westinghouse Owners Group Project No. 694 cc:
Mr. Nicholas Liparuto, Manager Equipment Design and Regulatory Engineering Westinghouse Electric Corporation -
Mail Stop ECE 4-15 P.O. Box 355 Pittsburgh, PA 15230-0355 Mr. Andrew Drake, Project Manager Westinghouse Owners Group Westinghouse Electric Corporation Mail Stop ECE 5-16 P.O. Box 355 Pittsburgh, PA 15230-0355 Mr. Jack Bastin, Director Regulatory Affairs Westinghouse Electric Corporation 11921 Rockville Pike Suite 107 Rockville,' MD 20852 Mr. Hank Sepp, Manager Regulatory and Licensing Engineering Westinghouse Electric Corporation PO Box 355 Pittsburgh, PA 15230-0355 l
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