ML16342C965
| ML16342C965 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 06/13/1995 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML16342C964 | List: |
| References | |
| NUDOCS 9506280762 | |
| Download: ML16342C965 (12) | |
Text
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAF TY EVALUATION BY TH OFFICE OF NUCLEAR R ACTOR R
GU T
NS RVIC INSPECT ON R
U ST FOR RE PACIFIC GAS AND E ECTRIC COHPANY DIAB 0 CANYON NUC POWER P ANT UNIT OS.
1 A
2 DOCKET NOS. 50-2 5 AND 50-323 1.0 J5TR OUCTltlll The Technical Specifications for Diablo Canyon Nuclear Power Plant, Unit Nos.
1 and 2
(DCPP) state that the inservice inspection and testing of the American Society of Hechanical Engineers (ASHE) Code Class 1, 2, and 3
components shall be performed in accordance with Section XI of the ASHE Boiler and Pressure Vessel Code and applicable Addenda as required by 10 CFR 50.55a(g),
except where specific written relief has been granted by the Commission pursuant to 10 CFR 50.55a(g)(6)(i).
Section 50.55a(a)(3) of Title 10 of the Code of Federal Regulations states that alternatives to the requirements of paragraph (g) may be used, when authorized by the NRC, if (i) the proposed alternatives would provide an ac'ceptable level of quality and safety or (ii) compliance with the specified requirements would result in hardship or unusual difficulties without a compensating increase in the level of quality and safety.
Pursuant to 10 CFR 50.55a(g)(4),
ASHE Code Class 1, 2, and 3 components (including supports) shall meet the requirements that become effective subsequent to editions specified in 10 CFR 50.55a(g)(2) and (g)(3), except the design and access provisions and the preservice examination requirements, set forth in the ASHE Code,Section XI, "Rules for Inservice Inspection of Nuclear Power Plant Components,"
to the extent practical within the limitations of
- design, geometry, and materials of construction of the components.
The regulations require that inservice examination of components and system pressure tests conducted during the first 10-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASHE Code incorporated by reference in 10 CFR 50.55a(b) on the date 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein.
The applicable edition of Section XI of the ASHE Code for the DCPP second 10-year inservice inspection (ISI) interval is the 1989 Edition.
The components (including supports) may meet the requirements set forth in subsequent editions and addenda of the ASHE Code incorporated by reference in 10 CFR 50;55a(b) subject to the limitations and modifications listed therein and subject to Commission approval.
Pursuant to 10 CFR 50.55a(g)(5)(iii), if the licensee determines the conformance with an examination requirement of Section XI of the ASHE Code is impractical for its facility, information shall be submitted to the Commission 950b2807b2 950bl3 PDR ADQCK 05000275 8
in support of that determination.
After evaluation of the determination, pursuant to 10 CFR 50.55a(g)(6)(i),
the Commission may grant relief and may impose alternative requirements that are determined to be authorized by law; will not endanger life, property, or the common defense and security; and are, otherwise in the public interest, giving due consideration to the burden upon the licensee that could result if the requirements were imposed.
By letter dated April 18,
- 1995, PGKE (the licensee) requested approval for the implementation of 4he alternative rules of ASHE Section XI Code Case N-416-1 dated February 15, 1994, entitled "Alternative Pressure Test Requirement for Welded Repairs or Installation of Replacement Items by Welding Class 1, 2, and 3,Section XI, Division 1," pursuant to 10 CFR 50.55a(a)(3) to be applied to the ISI program for DCPP.
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~EVA UATI N
C de C se N-416-Alte tive Pressu e Tes Re uirement or We ed e
irs or Installation o
Re lacement Items b
Weldin Class 1
2 and 3 Section XI Division 1
Com onent Identification ASIDE Class 1, 2, and 3 piping systems AS Code Sectio XI Second Interval Re uirements The 1989 Edition,Section XI, IWA-4700(a) requires that a system hydrostatic test be performed in accordance with IWA-5000 after repair s by welding on the pressure retaining boundary.
icensee's Basis or Re uest:
Hydrostatic pressure testing of welded repairs are required to meet the Code.
As discussed
- below, PG&E believes that hydrostatic tests impose significant costs, including potentially increased outage duration, while adding marginal (if any) value to the total repair or replacement quality.
Hardships are generally encountered with hydrostatic testing performed in accordance with the, Code.
For example, since the hydrostatic test pressure would be higher than nominal operating
- pressure, hydrostatic pressure testing frequently requires significant effort to set up and perform.
The need to use special equipment, such as the temporary attachment of test pumps and
- gages, and the need for individual valve lineups, can cause the testing to be on the outage critical path.
Piping components are designed for a number of loadings that are postulated to occur during the various modes of plant operation.
Code hydrostatic testing subjects the piping components to a small increase in pressure over the nominal operating pressure and is
Q7 not intended to present a significant (potentially destructive) challenge to pressure boundary integrity.
Accordingly, hydrostatic pressure testing is primarily regarded as a means to enhance leakage detection during the examination of components under pressure, rather than solely as a measure to determine the structural integrity of the components.
Industry experience has demonstrated that leaks are not discovered as a result if hydrostatic test pressures propagating as pre-existing flaw through a pipe wall.
In most cases, leaks are found when the system is at normal operating pressure.
At Diablo Canyon, hydrostatic pressure testing is required only upon installation and then once every 10-year inspection interval for Class 1, 3, and portions of the Class 2 boundary, while system leakage tests at nominal operating pressures are conducted a
minimum of once each refueling outage for Class 1 systems, and once each 40-month inspection period for Class 3 and the remainder of Class 2 systems.
In addition, leaks may be identified during routine system walkdowns by plant operators.
Hydrotests have the added potential to initiate leak paths at mechanical connections (valve packing glands, flange joints),
which are acceptable during the test but could continue to leak after return to service.
Pro osed Alternative Examination The licensee proposes to apply Code Case N-416-1 as an alternative for welded repairs or installation of replacement items by welding in Class 1, 2, and 3 piping.
In addition, the licensee proposes to perform additional surface examinations on the root pass layer of butt and socket welds on the pressure-retaining boundary of Class 3 components exceeding 2" nominal pipe size.
The licensee also proposes to perform VT-2 visual examinations in accordance with the requirements of the 1989 Edition of Section XI instead of the 1992 Edition specified by Code Case N-416-1.
Eval atio In lieu of hydrostatic pressure testing for welded repairs or installation of replacement items by welding, Code Case N-416-1 requires a visual examination (VT-2) be performed in conjunction with a system leakage testing using the 1992 Edition of Section XI, in accordance with paragraph IWA-5000, at nominal operating pressure and temperature.
This Code case also specifies that NDE of the welds be performed in accordance with the applicable Subsection of the 1992 Edition of Section III.
The 1989 Edition of Sections XI and III is the latest edition referenced in 10 CFR 50.55a.
The staff has compared the system pressure test requirements of the 1992 Edition of Section XI to the requirements of IWA-5000 of the 1989 Edition of Section XI.
In summary, the 1992 Edition imposes a more uniform
set of system pressure test requirements for Code Class I, 2, and 3 systems.
The terminology associated with the system pressure test requirements for all three Code classes has been clarified and streamlined.
The test frequency and test pressure conditions associated with these tests have not been changed.
The hold times for these tests have either remained unchanged or increased.
The corrective actions with respect to removal of bolts from leaking bolted connections have been relaxed in the 1992 Edition.
This relaxation has been accepted by the staff in previous safety evaluations.
The post-welded repair NDE requirements oi'he 1992 Edition of Section III remain the same as the requirements of the 1989 Edition of Section III. Therefore, the staff finds this aspect of Code Case N-416-1 to be acceptable.
Hardships are generally encountered with the performance of hydrostatic testing performed in accordance with the Code.
For example, since hydrostatic test pressure would be higher than nominal operating pressure, hydrostatic pressure testing frequently requires significant effort to set up and perform.
The need to use special equipment, such as temporary attachment of test pumps and gages, and the need for individual valve lineups can cause the testing to be on critical path.
Piping components are designed for a number of loadings that would be postulated to occur under the various modes of plant operation.
Hydrostatic testing only subjects the piping components to a small increase in pressure over the design pressure and, therefore, does not present a significant challenge to pressure boundary integrity.
Accordingly, hydrostatic pressure
'esting is primarily regarded as a means to enhance leakage detection during the examination of components under pressure, rather than solely as a measure to determine the structural integrity of the components.
The industry experience has demonstrated that leaks are not being discovered as a result of hydrostatic test pressures propagating a preexisting flaw through wall.
This experience indicates that leaks in most cases are being found when the system is at normal operating pressure.
This is largely due to the fact that hydrostatic pressure testing is required only upon installation and then once every 10-year inspection interval, while system leakage tests at nominal operating pressures are conducted a minimum of once each refueling outage for Class I systems and each 40-month inspection period for Class 2 and 3 systems.
In addition, leaks may be identified by plant operators during system walkdowns which may be conducted as often as once a shift.
Following the performance of welding, the Code requires volumetric examination of repairs or replacements in Code Class I and 2, but requires only a surface examination of the final weld pass in Code Class 3 piping components.
There are no ongoing NDE requirements for Code Class 3 components except for visual examination for leaks in conjunction with the 10-year hydrostatic tests and the periodic pressure tests.
Considering the NDE performed on Code Class I and 2 systems and considering that the hydrostatic pressure tests rarely result in pressure boundary leaks that would not occur during system leakage tests, the staff believes that increased assurance of the integrity of Class I and 2 welds is not
I
commensurate with the burden of performing hydrostatic testing.
- However, considering the nature of NOE requirements for Code Class 3 components, the staff does not believe that the requirement in Code Case N-416-1 of only performing system pressure testing is an acceptable alternative unless additional surface examinations on the root pass layer of butt and socket welds on the pressure retaining boundary of Class 3 components are performed.
In its April 18, 1995, letter, the licensee committed to perform this'dditional examination and to use the surface examination method in accordance with ASNE Section III.
The staff finds this commitment acceptable.
For clarification, it should be noted that, consistent with the Code Case requiring performance of NDE in accordance with the methods and acceptance criteria of the 1992 Edition of Section III, the scope of examination should also be in accordance with the 1992 Edition of Section III.
The additional surface examination of the root layer of Class 3 pressure retaining welds should be performed only when those pressure retaining welds are required to have a surface examination performed in accordance with the 1992 Edition of Section III.
For those Class 3 welds receiving radiography in lieu of a surface examination in accordance with Section III, no additional surface examination of the root layer needs to be performed.
In addition, the licensee proposes to comply with the requirements of Code Case N-416-1 with one exception.
Code Case N-416-1 contains a requirement that a visual examination (VT-2) shall be performed in conjunction with a system leakage test using the 1992 Edition of Section XI, in accordance with paragraph IMA-5000, at nominal operating pressure and temperature.
The licensee proposes to perform the VT-2 examination using the requirement of the 1989 Edition of Section XI, instead of the 1992 Edition specified by Code Case N-416-1.
The licensee indicates that the VT-2 examination requirements specified in the 1989 Edition are currently approved by NRC and are used in other existing VT-2 examination programs of the licensee.
The licensee further indicates that the existing program has been proven effective in maintaining leak tight integrity of the pressure boundary and that maintaining a separate VT-2 program using the 1992 Edition for Code Case N-416-1 is not cost effective.
The staff finds the proposed exception acceptable through the second ISI program plan interval (using the 1989 Edition of the Code) on the bases (1) that the 1989 Edition VT-2 visual examination requirements are currently approved by the staff and (2) that the difference between these requirements and those of the 1992 Edition specified in Code Case N-416-1 are not significant in nature and should not impact the effectiveness of the visual examination.
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The staff concludes that compliance with the Code hydrostatic testing requirements for welded repairs or replacements of Code Class 1, 2, and 3
components would result in hardships without a compensating increase in the level of quality and safety.
Accordingly the licensee's proposed alternative to use Code Case N-416-1 is authorized for DCPP, pursuant to 10 CFR 50.55a(a)(3)(ii).
Use of Code Case N-416-1 is authorized until such time as this Code case is published in a future revision of Regulatory Guide 1.147.
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At that time if the licensee intends to continue to implement this Code case, the licensee is to do so by incorporating any limitations issued in Regulatory Guide 1. 147.
The granted exception to Code Case N-416-1 will remain effective through the second ISI program plan after which time the entire DCPP VT-2 examination program should conform to the applicable Code edition for the third ISI interval.
Principal Contributor:
M. A. Hiller Date:
June 13, 1995
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