NRC Generic Letter 88-11, NRC Position on Radiation Embrittlement of Reactor Vessel Materials and Its Impact on Plant Operations
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July 12, 1988
TO ALL LICENSEES OF OPERATING REACTORS AND HOLDERS OF CONSTRUCTION PERMITS
SUBJECT: NRC POSITION ON RADIATION EMBRITTLEMENT OF REACTOR VESSEL MATERIALS AND ITS IMPACT ON PLANT OPERATIONS (GENERIC LETTER 88-11)
The purpose of this letter is to call your attention to the attached copy of Revision 2 to Regulatory Guide 1.99, "Radiation Embrittlement of Reactor Vessel Materials," which became effective May 1988. It will be used by the NRC in reviewing submittals regarding pressure-temperature (P-T) limits and for analyses other than pressurized thermal shock (PTS) that require an estimate of the embrittlement of reactor vessel beltline materials.
Licensees and permittees should use the methods described in Revision 2 to Regulatory Guide 1.99 to predict the effect of neutron radiation on reactor vessel materials as required by Paragraph V.A. of 10 CFR Part 50 Appendix G, unless they can justify the use of different methods. The use of the Revision 2 methodology may result in a modification of the pressure-temperature limits contained in Technical Specifications in order to continue to satisfy the requirements of Sec. V of 10 CFR Part 50, Appendix G. Within 180 days of the effective date of Revision 2, licensees should submit the results of their technical analysis and a proposed schedule for whatever actions they propose to take. In the event that such actions are necessary, their schedule is negotiable provided that all actions (hardware, procedures, and/or staff modifications) are completed (fully implemented and operational) within 2 plant outages (approximately 3 years) after the effective date of Revision 2 to Regulatory Guide 1.99.
PWR licensees should note that the Low-Temperature-Overpressure Protection (Ltop) set points and enable temperatures, which are determined from the P-T limits, may also have to be revised as a result of Revision 2. Since Revision 2, in general, results in a lowering of the Appendix G pressure curves and a shift to higher enable temperatures, the resulting narrowing of the operating window may restrict flexibility on heatup and cooldown operations.
Standard Review Plan 5.2.2, "Overpressure Protection," and the associated Branch Position RSB 5-2 is being changed to provide some relief from this impact. Paragraph II.B, which requires protection "at low temperature," is being amended to define the required enable temperature for the Ltop system based on a fracture criterion. Automatic, or passive, protection of the upper end of the P-T limits will not be required but administratively controlled. At the lower end of the P-T limits, for example during startup, automatic protection of the Appendix G P-T limits is still required for anticipated operational occurrences.
As plants age, it is expected that the operating window will continue to narrow and startup operations will become more difficult. Revision 2 accelerates this narrowing of the operating window. Licensees are encouraged to
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review system hardware and operating procedures to determine what changes could be made to reduce the likelihood of Ltop challenges. If changes can be implemented to demonstrate that the frequency of an Ltop event that would exceed Appendix G limits is expected to be much less than one per reactor lifetime, then the staff would consider alternatives to Appendix G Ltop set points with appropriate justification of adequate safety from the standpoint of fracture prevention.
BWR licensees should note that the use of Revision 2 as the basis for P-T limits for BWR pressure tests will require higher pressure test temperatures in many cases. The NRC does not accept the BWR Owners Group position that the margins given by following the procedures of Appendix G, 10 CFR Part 50 can safely be reduced.
With regard to the pressurized thermal shock issue in PWRs, the staff is presently considering an amendment to the PTS Rule, 10 CFR 50.61, that will replace the equations for RTPTS given in paragraph (b)(2) with the calculation procedure given in Section C.1 Revision 2 to Reg. Guide 1.99, but will not change the screening criterion.
Based on calculations reported in the Regulatory Analysis, a number of reactor vessels will reach the screening criterion sooner, using Revision 2, and in a few cases that date will precede the end of license. To see if their plant falls in this category, licensees may wish to repeat the calculation of RTPTS values submitted to the NRC in response to the PTS Rule (January 23, 1986 submittal) for the critical materials in the vessel beltline, using Section C.1 of Revision 2 to Regulatory Guide 1.99. The purpose of this suggestion is simply to provide early warning that further flux reduction should be considered in some plants.
This request for information is covered by the Office of Management and Budget under Clearance Number 3150-0011, which expires December 31, 1989. Comments on burden and duplication may be directed to the Office of Management and Budget, Reports Management, Room 3208, New Executive Office Building, Washington, D.C. 20503.
Sincerely,
Frank J. Miraglia, Jr. Associate Director for Projects Office of Nuclear Reactor Regulation
Enclosure:
Revision 2 to R.G. 1.99