ML16342C952
| ML16342C952 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 06/12/1995 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML16342C953 | List: |
| References | |
| NUDOCS 9506280397 | |
| Download: ML16342C952 (10) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON> D.C. 2055&0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION NSERVICE INSPECTION RE UES S
FOR RELIEF CIFIC GAS AND ELECTRIC COMPANY DIABLO CANYON NUCLEAR POWER PLANT UNI S
1 AND 2 DOCKET NOS. 50-275 AND 50-323
- 1. 0 INTRODUCTION The Technical Specifications for Diablo Canyon Nuclear Power Plant (DCPP),
Unit Nos.
1 and 2, state that the inservice inspection and testing of the American Society of Mechanical Engineers (ASHE)
Code Class 1, 2, and 3
components shall be performed in accordance with Section XI of the ASME 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 acceptable 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 ASME 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 ten-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR 50.55a(b) on the date twelve months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein.
The 1977 Edition, through Summer 1978 Addenda, of Section XI is the applicable edition of the ASHE Code for DCPP 10-year inservice inspection (ISE) interval.
The components (including supports) may meet the requirements set forth in subsequent editions and addenda of the ASME 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 that conformance with an examination requirement of Section XI of the ASHE Code is not practical for its facility, information shall be submitted to the 9506280397 950hi2 PDR ADQCK 05000275 i
Commission in support of that determination and a request made for relief from the ASHE Code requirement.
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.
In a letter dated April 18, 1995,.the licensee, PG&E, proposed an alternative examination to the requirements of the ASHE Boiler and Pressure
- Code,Section XI.
PGLE requested approval for the implementation of the alternative rules of ASHE Section XI Code Case N-498-1, dated Hay ll, 1994, "Alternative Rules for 10-Year System Hydrostatic Testing for Class 1, 2, and 3 Systems" pursuant to 10 CFR 50.55a(a)(3) for 10-year hydrostatic testing on Class 3
systems.
2.0 EVALUATION
- 2. 1 Licensee's Re vest The licensee has requested relief from pressurizing the Class 3 systems to hydrostatic pressure before performance of the VT-2 examination.
1 2.1.1 ASHE Code,Section XI, RequirementsSection XI, Table IWD-2500-1, Categories D-A, D-B, and D-C, contains the requirements for Class 3 system hydrostatic and leakage testing.
The Code requires system hydrostatic testing once per 10-year interval at or near the end of the interval.
- 2. 1.2 Licensee's Proposed Alternative Testing The licensee proposed to use the alternative contained in Code Case N-498-1, a
system leakage test and visual examination (VT-2) at nominal system pressure, in lieu of hydrostatic testing, for Class 3 Systems.
- 2. 1.3 Licensee's Basis for Relief The licensee's April 18, 1995, letter provided the following basis for use of Code Case N-498-1:
Code Case N-498 is generically approved and modifies the requirement for hydrostatic testing (ref. Reg.
Guide 1. 147, Rev.
9, April 1992) for Code Class 1 and 2 systems.
This requirement is contained in the latest approved 1989 Edition of the Code.
The extension of this concept to Code Class 3 systems, as approved in Code Case N-498-1, is based on the potential to damage system components during hydrostatic tests and the increased safety risk to personnel performing tests.
The special pumps, test preparation, and system breaches that are required for hydrostatic
test pressurization increase the effort required for hydrostatic testing compared to nominal operating pressure testing.
Based on industry experience, the increase in system pressure during a
hydrostatic test is no more conducive to detection of leaks than pressurization to nominal operating pressure.
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 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 of hydrostatic test pressures propagating a 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 I, 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 I systems.
In
- addition, leaks may be identified during routine system walkdowns by plant operators.
Although Section XI hydrostatic testing would not impair the structural integrity of the pressure boundary, it has the potential to initiate leak sites at mechanical connections (valve packing glands, flange joints), which are acceptable during the test but could continue to leak after return to service.
Such leaks may have minimal safety significance but may result in additional effort for containment,
- cleanup, and disposal of the leakage.
Also, the potential for spills, contamination, and longer personnel exposure time in radiation areas are not justified when compared to testing performed at normal operating conditions.
2.1.4 Evaluation Information prepared in conjunction with ASME Code Case N-498-1 notes that the system hydrostatic test is not solely a test of the structural integrity of the system but also provides a means to enhance leakage detection.
That this was the original intent is indicated in a paper by S.H.
Bush and R.R. Maccary, "Development of In-Service Inspection Safety Philosophy for U.S.A. Nuclear Power Plants,"
ASME, 1971.
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 since piping dead weight, thermal expansion, and seismic loads, which may present far greater challenge to the structural integrity of a system than fluid pressure, are not part of the loading imposed during a hydrostatic test.
Water is used as a test medium in the hydrostatic test.
Because water is highly incompressible, any small leak from a high pressurized water solid system can be readily detected by a sharp decline in system pressure, or by continuing pumping to maintain the system pressure.
As such, hydrostatic pressure testing is primarily regarded as a means to enhance leakage detection during the examination of components under pressure, since such a test provides a good indication for evidence of "any" system leakages, especially those that might originate from small through-wall cracks of the pressure boundary.
Consequently, this in-service hydrostatic pressure test required by the Code enhances the possibility of timely discovery of small through-wall flaws which, because of tiny leak size, might not be readily detected by any other means such as system walkdowns or installed leak-detection systems.
PG&E requested approval for the implementation of the alternative rules of ASHE Section XI Code Case N-498-1, dated Hay 11, 1994, "Alternative Rules for 10-Year System Hydrostatic Testing for Class 1, 2, and 3 Systems,"
in lieu of 10-year hydrostatic testing of Class 3 systems.
The licensee may already use N-498, "Alternative Rules for 10-Year System Hydrostatic Testing for Class 1,
and 2 Systems,"
since use of Code Case N-498 for Class 1 and 2 systems is approved by the NRC in Regulatory Guide 1. 147, Rev.
11.
The rules for Code Class 1 and 2 in N-498-1 are unchanged from N-498.
The staff found N-498 acceptable because the alternative provided adequate assurance and because compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Revision N-498-1 encompasses Class 3 components and specifies requirements for Class 3 that are identical to those for Class 2 components.
In lieu of 10-year hydrostatic pressure testing at or near the end of the 10-year
- interval, Code Case N-498-1 requires a visual examination (VT-2) be performed in conjunction with a system leakage test in accordance with paragraph IWA-5000.
A system leakage test may be conducted to demonstrate that unacceptable leaks from through-wall flaws do not exist.
This would meet the intent of the hydrostatic test as noted above.
Currently, licensees expend considerable time, radiation dose, and dollar resources carrying out hydrostatic test requirements.
A significant amount of effort may be necessary (depending on system, plant configuration, Code class, etc.) to temporarily remove or disable Code safety and/or relief valves to meet test pressure requirements.
The safety assurance. provided by the enhanced leakage gained from a slight increase in system pressure during a
hydrostatic test are offset or negated by the following factors:
having to gag or remove Code safety and/or relief valves',
placing the system in an off-normal state, erecting temporary supports in steam lines, possible extension of refueling outages, and resource requirements to set up testing with special equipment and gages.
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Class 3 systems do not normally receive the amount and/or type of Non-Destructive Examinations that Class 1 and 2 systems receive.
While Class 1
and 2 system failures are relatively uncommon, Class 3 system leaks occur more frequently, and the failure mode typically differs.
Based on a review of Class 3 system failures requiring repair for the last five years in Licensee Event Reports and the Nuclear Plant Reliability Data System databases, the most common causes of failures are erosion-corrosion (EC), microbiologically induced corrosion (HIC), and general corrosion.
Licensees generally have programs in place for prevention, detection, and evaluation of EC and HIC.
Leakage from general corrosion fs readily apparent to inspectors when performing a VT-2 examination during system pressure tests.
Giving consideration to the minimal amount of increased assurance provided by the increased pressure associated with a hydrostatic test versus the pressure for the system leakage test and the hardship associated with performing the ASHE Code-required hydrostatic test, the staff finds that compliance with the Section XI hydrostatic testing requirements results in hardship and/or unusual difficulty for the licensees without a compensating increase in the level of quality and safety.
Accordingly, the licensee's proposed alternative, use of Code Case N-498-1 for Code Class 3 systems, is authorized for DCPP, Unit Nos.
1 and 2, pursuant to 10 CFR 50.55a(a)(3)(ii).
- However, Code Case N-498-1 is currently being considered for endorsement through Regulatory Guide (RG)
- 1. 147.
The licensee is authorized to use this code case until its publication in a future revision of the RG.
At that time if the licensee intends to continue to implement this code case, the licensee is to follow all provisions in Code Case N-498-1, with limitations issued in Regulatory Guide
- 1. 147, if any, or request relief.
- 3. 0 CONC LUS ION The staff evaluated the information provided by PG&E in support of its request for relief.
Based on the information submitted, the alternative for hydrostatic testing contained in the licensee's proposal is authorized pursuant to 10 CFR 50.55a(a)(3)(ii) for Class 3 systems as compliance with the specified hydrostatic testing requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Principal Contributors:
C. K. Battige H. A. Hiller Date:
0une 12, 1995
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