NRC Position on Radiation Embrittlement of Reactor Vessel Materials and its Impact on Plant Operations (Generic Letter No. 88-10)
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
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
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
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
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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.
Frank J. Miraglia, Jr.
Associate Director for Projects
Office of Nuclear Reactor Regulation
Revision 2 to R.G. 1.99
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