[Federal Register Volume 64, Number 90 (Tuesday, May 11, 1999)]
[Pages 25383-25384]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-11817]



[Docket No. 50-482]

Wolf Creek Nuclear Operating Corporation (Wolf Creek Generating 
Station); Exemption


    Wolf Creek Nuclear Operating Corporation (WCNOC or licensee) is the 
holder of Facility Operating License No. NPF-42, which authorizes 
operation of the Wolf Creek Generating Station (WCGS). The license 
provides, among other things, that the licensee is subject to all 
rules, regulations, and orders of the Commission now and hereafter in 
    The facility is a pressurized water reactor located at the 
licensee's site in Coffey County, Kansas.


    Section 50.60(a) to 10 CFR part 50 requires that except as provided 
in Sec. 50.60(b), all light-water nuclear power reactors, other than 
reactor facilities for which the certifications required under section 
50.82(a)(1) have been submitted, must meet the fracture toughness and 
material surveillance program requirements for the reactor coolant 
pressure boundary set forth in appendices G and H of 10 CFR part 50. 
Section 50.60(b) of 10 CFR part 50 states that proposed alternatives to 
the described requirements of appendices G and H of part 50 or portions 
thereof may be used when an exemption is granted by the Commission 
under 10 CFR 50.12.


    By letter dated December 29, 1998, WCNOC requested that the NRC 
exempt WCGS from the application of specific requirements of 10 CFR 
50.60 and appendix G to 10 CFR Part 50. Specifically, WCNOC proposes to 
use American Society for Mechanical Engineers (ASME) Code Case N-514 to 
permit setting the pressure setpoint of WCGS's cold overpressure 
mitigation system (COMS) such that the pressure-temperature (P-T) 
limits required by appendix G of 10 CFR part 50 could be exceeded by 
ten percent during a low temperature pressure transient.
    The Commission has established requirements in 10 CFR part 50 to 
protect the integrity of the reactor coolant system pressure boundary. 
As a part of these, appendix G of 10 CFR part 50 requires that P-T 
limits be established for reactor pressure vessels (RPVs) during normal 
operation and vessel hydrostatic testing. As stated in appendix G, 
``The appropriate requirements on . . . the pressure-temperature limits 
. . . must be met for all conditions.'' In order to avoid approaching 
these P-T limit curves and provide pressure relief during low 
temperature overpressurization events, pressurized water reactor 
licensees have installed protection systems (COMS) as part of the 
reactor coolant system pressure boundary. WCNOC is required as part of 
the WCGS Technical Specifications (TS) to develop, update,

[[Page 25384]]

and submit reactor vessel P-T limits and COMS setpoints for NRC review 
and approval.
    WCNOC determined that the exemption request from the provisions of 
10 CFR 50.60 and appendix G was necessary since these regulations 
require, as noted above, that reactor vessel conditions not exceed the 
P-T limits established by appendix G. In referring to 10 CFR 50.12 on 
specific exemptions, WCNOC cited special circumstances regarding 
achievement of the underlying purpose of the regulations as their basis 
for requesting this exemption (10 CFR 50.12(a)(2)(ii)).
    WCNOC noted in support of the 10 CFR 50.12(a)(2)(ii) criteria that 
the underlying purpose of the subject regulation is to establish 
fracture toughness requirements for ferritic materials of pressure 
retaining components of the reactor coolant pressure boundary. To 
accomplish this, appendix G of Section XI of the ASME Code requires the 
P-T limits be calculated: (a) Using a safety factor of 2 on the 
principal membrane (pressure) stresses, (b) with margin added to the 
reactor vessel nil ductility reference temperature (RTNDT) 
in accordance with Regulatory Guide 1.99, Rev. 2, ``Radiation 
Embrittlement of Reactor Vessel Materials,'' (c) assuming a flaw at the 
surface with a depth of \1/4\ of the vessel wall thickness and a length 
of 6 times its depth, and (d) 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 reactor 
vessel material. Code Case N-514 provides for normal operation within 
the P-T limits determined in accordance with ASME Section XI, appendix 
G, but allows determination of setpoints for COMS events such that the 
maximum pressure in the vessel would not exceed 110 percent of the 
appendix G limits.
    WCNOC proposed that establishing the COMS pressure setpoint in 
accordance with the N-514 provisions, such that the vessel pressure 
would not exceed 110 percent of the P-T limit allowances, would still 
provide an acceptable level of safety and mitigate the potential for an 
inadvertent actuation of the COMS. The safety margins provided by 
application of Code Case N-514 result in a safety factor of 1.8 on the 
principal membrane (pressure) stresses, with all other factors, 
including assumed flaw size and fracture toughness, remaining the same 
as ASME Section XI, appendix G, methodology. Due to the isothermal 
nature of the COMS events, the margin with respect to toughness for a 
COMS transient is within the range provided by ASME Section XI, 
appendix G, for normal heatup and cooldown in the low temperature 
range. Thus, applying Code Case N-514 will satisfy the intent of 10 CFR 
50.60 for fracture toughness requirements. Further, application of Code 
Case N-514 will relieve operational restrictions for WCGS; it will 
reduce the potential for inadvertent RCS pressure relief events, 
thereby improving plant safety, and will reduce unnecessary burdens on 
operators during important plant evolutions.
    The Commission has determined that application of 10 CFR 50.60 in 
these particular circumstances is not necessary to achieve the 
underlying purpose of that rule and that the use of Code Case N-514 
would meet the underlying intent of the regulation. Based upon a 
consideration of the conservatisms which are explicitly defined in the 
appendix G methodology, it was concluded that permitting the COMS 
setpoint to be established such that the vessel pressure would not 
exceed 110 percent of the limit defined by P-T limit curves would 
provide an adequate margin of safety against brittle failure of the 
reactor vessel. This is also consistent with the determination that has 
been reached for other licensees under similar conditions based on the 
same considerations. Therefore, the exemption requested under the 
special circumstances of 10 CFR 50.12(a)(2)(ii) was found to be 
acceptable. The staff also agrees that limiting the potential for 
inadvertent COMS actuation may improve plant safety.


    The Commission has determined that, pursuant to 10 CFR 50.12, this 
exemption is authorized by law, will not present an undue risk to the 
public health and safety, is consistent with the common defense and 
security, and is otherwise in the public interest. Therefore, the 
Commission hereby grants Wolf Creek Nuclear Operating Corporation an 
exemption from the requirements of 10 CFR 50.60 in order to apply ASME 
Code Case N-514 for determining the WCGS cold overpressurization 
mitigation system pressure setpoint.
    Pursuant to 10 CFR 51.32, the Commission has determined that the 
granting of this exemption will have no significant impact on the 
environment (64 FR 23136).
    This exemption is effective upon issuance.

    Dated at Rockville, Md., this 30th day of April 1999.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 99-11817 Filed 5-10-99; 8:45 am]