ML20197K234
| ML20197K234 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 12/12/1997 |
| From: | Miraglia F NRC (Affiliation Not Assigned) |
| To: | COMMONWEALTH EDISON CO. |
| Shared Package | |
| ML20197K239 | List: |
| References | |
| NUDOCS 9801050266 | |
| Download: ML20197K234 (7) | |
Text
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7590-01-P i
UNITED STATES OF AMERICA HUCLEAR REGULATORY ColttIS$10N In the Matter of i
Com0 WEALTH EDISON COMPANY 00cket No. STN 50-457 (BraidwoodNuclearStation.
Unit No. 2) 1 EXEMPTION
.1 I
I' Commonwealth Edison Company (ConEtt, the licensee) is the holder of Facility Operating License No. NPF-77, which authorizes operation of the Braidwood Nuclear Station, Unit 2.
The license provides, among other thin 2s, that the licensee is subject to all rules, regulations, and orders of the Commission now or hereafter in effect.
II.
In its letter dated November 30, 1994, as supplemented on May 11, 1995, the licensee requested an exemption from the Commission's regulations. Title 10 of the Code o# Federal Regulations, Part 50, Section 60 (10 CFR 50.60),
' Acceptance Criteria for Fracture Prevention Nessures for Lightwater Nuclear Power Reactors for Normal Operation,' states that all lightwater nuclear pcnr reactors must comply with the fracture toughness and material surveillance program requir,ssents for the reactor coolant pressure boundary as stated in Appendices G and'H to 10 CFR Part 50.,Appendtx G to 10 CFR Part 50 defines pressure-temperature (P-T) limits during any condition of normal operation, including anticipated operational occurrences and system hydrostatic tests to which the pressure boundary may be subjected over its service lifetime, and 9901050266 971212 PDR ADOCK 05000457 P
l that are obtained by conforming to the methods of analysis and the margins of j
safety in the American Society of Mechanical Engineers (ASME) Boiler and PressureVesselCode(Code),SectionXI,AppendixG.
It is required in 10 CFR l
i 50.55a that any reference to the ASME Code,Section XI, in 10 CFR Part 50 refers to the addenda through the 1988 Addenda and editions through the 198g f
Edition of the Code unless otherwise noted.
It is specified in 10 CFR 50.60(b)thatalternativestothedescribedrequirementsinAppendixGto 10 CFR Part 50 may be used when an exemption is granted by the Commission under 10 CFR 50.12.
To mitigato low-temperature overpressure transients that would produce pressure excursions exceeding the required limits while the reactor is operating at low temperatures, the licensee installed a low-temperature o9erpressure protection (LTOP) system. The system contains pressur relieving devices called power-operated relief valves (PORVs). The PORVs are set at a low er.ough pressure so that if an LTOP transient occurred, the mitigation system would prevent the pressure in the reactor vessel from exceeding the required limits. To prevent the PORVs from lifting as a result of normal operating pressure surges, some margin is needed between the PORV setpoint and i
the nomal operating pressure.
In addition, normal operating pressure must be high enough to prevent damage to reactor coolant pumps that may result from cavitation or inadequate differentiel pressure across the pump seals. Hence, the licensee must operate the plant within a pressure window that is defined as the difference between the minimum pressure required for reactor coolant Pumps and the operating margin to keep the PORVs from lifting. When
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1 instrument uncertainty is considered, the operating window is small and I
presents difficulties for plant operation.
I The licensee has requested the use of the ASME Code Case N-514, ' Low
. Temperature Overpressure Protection,' for detemining the LTOP system setpoint. Code Case N-514 allows use of an LTOP system setpoint so that
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system pressure does not exceed 110 percent ef the P-T limits during an LT0P event. Code Case N-514 is consistent with guidelines developed by the ASME Working Group on Operating Plant Criteria to define pressure limits during LTOP events that avoid certain unnecessary operational restrictions, provide adequate margins against failure of the reactor pressure vessel, and reduce the potential for unnecessary activation of pressure-relieving devices used for LTOP. The content of this code case has been incorporated into the ASME Code,Section XI. Appendix G, and was published in the 1993 Addenda to Section XI.
Pursuant to 10 CFR 50.12, the Comission may, upon application by any interested person or upon its own initiative, grant exemptions from the requirements of 10 CFR Part 50 (1) when the exemptions are authorized by law, will not present an undue risk to public health or safety, and are consistent with the common defense and security; and (t) when special circumstances are present. Special circumstances are present whenever, according to 10 CFR 50.12(a)(t)(11), ' Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule....'
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The underlying purpose of to CFR 50.60 and 10 CFR Part 50, Appendix G, is to establish fracture toughness requirements for ferritic materials of pressure-retaining components of the reactor coolant pressure boundary to provide adequate margins of safety during any condition of nonsal operation, including anticipated operational occurrences, to which the pressure boundary may be subjected over its service lifetime. Section !Y.A.2 of Appendix G to 10 CFR Part 50 requires that the reactor vessel be operated with P-T limits at least as conservative as those obtained from following the methods of analysis and the required margins of safety of Appendix G of Section XI of the ASME Code.
Appendix G of the ASME Code requires that the P-T limits be calculated (1) using a safety factor of 2 on the principal membrane (pressure) stresses, (2) assuming a flaw at the surface with a depth of one-fourth of the vessel wall thickness and a length of 6 times its depth, and (3) using a conservative fracture toughness curve that is based on the lower bound of static, dynamic, and crack arrest fracture toughness tests on material similar to the Braidwood reactor vessel material.
For detemining the LTOP system setpoint, the licensee proposed to use safety margins based on an alteriente methodology consistent with ASME Code Case N-514. The code case allows the setpoint for mitigating LTOP events to be so determined that the maximum pressure in the vessel would not exceed 110 percent of the Appendix G P-T limits. 'This results in a safety factor of 1.8 on the principal membrane stresses. All other factors, including assumed flaw size and fracture toughness, remain the same. Although this methodology would reduce the safety factor on the principal membrane stresses, the proposed
.5-criteria will produce adequate makins of safety for the reactor vessel during LTOP transients and, thus, will satisfy the underlwing purpose of 10 CFR 50.60 for fracture toughness requirements. Further, by relieving the operational restrictions, the potential for undesirable lifting of the PORVs would be j
reduced, thereby making the plant safer.
IV.
For the foregoing reasons, the NRC staff has concluded that the licensee's proposed use of the alternate methodology in detemining the acceptable setpoint for LTOP events will not present an undue risk to public health and safety and is consistent with the common defense and security. The NRC staff has detemined that there are special circumstances present, as specified in 10 CFR 50.12(a)(2)(ii), in that application of 10 CFR 50.60 is j
not necessary in order to achieve the underlying purpose of this regulation I
which is to provide adequate fracture toughness of the reactor pressure boundary.
Accordingly, the Connission has determined that, pursuant to 10 CFR 50.12(a), an exemption is authorized by law, will not endanger life or property or the common defense and security, and is, othemise, in the rblic interest.
Therefore, the Commission hereby grants an exemption from the i
requirements of 10 CFR 50.60; in accordance with ASME Code Case N-514, the LTOP system setpoint may be detemined so that system pressure does not exceed 110 percent of the Appendix G P-T limits in order to be in complia..te with '
these regulations. This exemption is applicable only to LTOP conditions during normal operation.
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Pursuant to 10 CFR 51.32, the Commission has detemined that granting this exemption will not have a significant effect on the quality of the htaen envirtmment (62 FR 59008).
This exemption is effective upon issuance, e
FOR THE NOCLEAR REGULATORY COPMISSION M AueA ((1.f Frank J. Mirgdia, d ng Director Office of Nuclear Reactor Regulation Dated at Rockville, Meryland, this 12th day of December 1997 e
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Pursuant to 10 CFR 51.32, the Commission has determined that granting this exemption will not have a significant effect on the attality of the human environment (62 FR 59008).
This exemption is effective upon issuance.
FOR THE NUCLEAR REGULATORY COMMISSION OriginalSigned & */
Fra.. J. Mira911a Acting Director Office of Nuclear Reactor Regulation Dated at Rockville Maryland, this 12th day of Ik cember 1997
- see previous page for concurrence DOClHENT NAME:
BR99600.EXE Te receive e copy of this document Indeste in the boa;
'C' = Copy without en '.ssures *E' = Copy with enclosures *N' = No copy 0FFICE PM:PD3 2 LA:PD3 2 PM:PD3 2 TECHEDITOR NRR:EMCB NAME
- GDICK
- CM00RE
- SBAILEY
- RSANDERS
- TSULLIVAN DATE 09/19/97 11/15/97 11/18/97 09/26/97 10/1/97 mummmmmumummmmmmmmme mamme mumma mammmmmmmmmemamma summm ummmmmmmumscumm 0FFICE OGC 0:PD3 2 (A)D:DRPW NRR:ADPR NRR:(A)D NAME
- RCAPRA IADENSAM
- RZIMMERMAN "FMIRAGLIA DATE 11/12/97 12/05/97 12/8/97 12/10/97 12/12/97 0FF.'CIAL RECORD COPY
.