[Federal Register Volume 66, Number 141 (Monday, July 23, 2001)]
[Notices]
[Pages 38327-38328]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-18326]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-269, 50-270, and 50-287]
Duke Energy Corporation; Oconee Nuclear Station, Units 1, 2, and
3; Exemption
1.0 Background
The Duke Energy Corporation (the licensee) is the holder of
Facility Operating License Nos. DPR-38, DPR-47, and DPR-55, which
authorize operation of the Oconee Nuclear Station, Units 1, 2, and 3
(ONS). The licenses provide, among other things, that the facilities
are subject to all rules, regulations, and orders of the U.S. Nuclear
Regulatory Commission (NRC, the Commission) now or hereafter in effect.
The facility consists of three pressurized water reactors located
in Seneca County in South Carolina.
2.0 Request/Action
By letter dated July 26, 2000, Duke Energy Corporation, licensee
for the ONS, requested an exemption from certain requirements of 10 CFR
50.44, 10 CFR part 50, Appendix A, General Design Criterion 41, and 10
CFR part 50, Appendix E, Section VI pertaining to the hydrogen control
system requirements (i.e., recombiners and containment post-accident
hydrogen monitors) and the removal of these requirements from the ONS
design basis.
Regulatory requirements for the hydrogen control system are
specified in 10 CFR 50.44 and 10 CFR part 50, Appendix A, (General
Design Criteria 41, 42, and 43). Additional staff guidance is provided
in Regulatory Guide (RG) 1.7. Staff review and acceptance criteria are
specified in Section 6.2.5 of the Standard Review Plan. With regard to
combustible gas control system requirements, ONS is subject to the
requirements of 10 CFR 50.44(g).
3.0 Discussion
Pursuant to 10 CFR 50.12, the Commission may, upon application by
any interested person or upon its own initiative, grant exemptions from
the requirements of 10 CFR part 50, when (1) 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
(2) when special circumstances are present.
For this exemption, these special circumstances include
consideration that the quantity of hydrogen prescribed by 10 CFR
50.44(d) and RG 1.7 which necessitated the need for hydrogen
recombiners would be bounded by the hydrogen generated during a severe
accident. As shown in the attached safety evaluation, the staff has
found that the relative importance of hydrogen combustion for large,
dry containments with respect to containment failure is quite low. This
finding supports the argument that the hydrogen recombiners are not
risk significant from a containment integrity perspective and that the
risk associated with hydrogen combustion is not from design basis
accidents but from severe accidents. Studies have shown that the
majority of risk to the public is from accident sequences that lead to
containment failure or bypass, and that the contribution to risk from
accident sequences involving hydrogen combustion is actually quite
small for large, dry containments such as Oconee's. This is true
despite the fact that the hydrogen produced in these events is
substantially larger than the hydrogen production postulated by 10 CFR
50.44(d) and RG 1.7. Hydrogen combustion sequences that could lead to
early containment failure typically involve up to 75 percent core
metal-water reaction. Hydrogen combustion sequences that could lead to
late containment failure involve additional sources of hydrogen due to
the interaction of corium and the concrete basemat after vessel breach.
Although the recombiners are effective in maintaining the RG 1.7
hydrogen concentration below the lower flammability limit of 4 volume
percent, they are overwhelmed by the larger quantities of hydrogen
associated with severe accidents that would typically be released over
a much shorter time period (e.g., 2 hours). However, NUREG/CR-4551
states that hydrogen combustion in the period before containment
failure is considered to present no threat to large, dry containments.
Table A.4-5 of NUREG/CR-4551 shows that the contribution of hydrogen
combustion to late containment failure is also very small. Therefore,
the relative importance of hydrogen combustion for large, dry
containments with respect to containment failure has been shown to be
quite low.
The recombiners can, however, prevent a subsequent hydrogen burn if
needed due to radiolytic decomposition of water and corrosion in the
long term. Analysis performed in accordance with the methodology of RG
1.7 shows that the hydrogen concentration will not reach 4 volume
percent for 15 days after initiation of a design basis Loss of Coolant
Accident (LOCA). Additionally, as described in the attached safety
evaluation, hydrogen concentrations on the order of 6 percent or less
are bounded by hydrogen generated during a severe accident and would
not be a threat to containment integrity since there is ample time
between burns to reduce elevated containment temperatures using the
installed containment heat removal systems. The ONS Individual Plant
Examination (IPE) concluded that containment survival is almost certain
following hydrogen combustion when the Reactor Building Cooling Units
and the Reactor Building Spray System are operating.
The underlying purpose of 10 CFR 50.44 is to show that, following a
LOCA, an uncontrolled hydrogen-oxygen recombination would not take
place, or that the plant could withstand the consequences of
uncontrolled hydrogen-oxygen recombination without loss of safety
function. Based on the analysis, which includes the staff's evaluation
of the risk from hydrogen combustion, resolution of Generic Issue 121,
``Hydrogen Control for PWR Dry Containments,'' and the ONS IPE, the
plant could withstand the consequences of uncontrolled hydrogen-oxygen
recombination without loss of safety function without credit for the
hydrogen recombiners for not only the design basis case, but the more
limiting severe accident with up to 100 percent metal-water reaction.
Therefore, the requirements for hydrogen recombiners as part of the ONS
design basis are unnecessary and their removal from the design basis is
justified. Additionally, elimination of the hydrogen recombiners from
the Emergency Operating Instructions would simplify operator actions in
the event of an accident and, therefore, would be a safety benefit.
Consequently, pursuant to 10 CFR 50.12(a)(2)(ii), application of the
regulation is not necessary to achieve the underlying purpose of the
rule.
In the submittal, the licensee also requested an exemption from the
functional requirement for hydrogen monitoring as promulgated in part
50, Appendix E, Section VI, ``Emergency Response Data System (ERDS),''
or any commitments made in regard to NUREG-0737, Item II.F.1,
Attachment 6, ``Containment Hydrogen Monitor.'' In the Statement of
Considerations for Appendix E to part 50, the Commission stated that
the ERDS data (which includes the continuous hydrogen monitors)
provides the data required by the NRC to perform its role during an
emergency. This conclusion is still valid
[[Page 38328]]
for not only the staff but licensees. The major vendors' core damage
assessment methodologies continue to include continuous hydrogen
monitoring. Core damage assessment methodologies were reviewed by the
staff in response to NUREG-0737, Item II.B.3(2)(a). Continuous hydrogen
monitoring is needed to support a plant's emergency plan as described
in 50.47(b)(9). Implementing documents such as Regulatory Guide (RG)
1.101, Revision 2, which endorsed NUREG-0654, and RG 1.101, Revision 3,
which endorsed NEI-NESP-007, Revision 2 define the highest Emergency
Action Level, a General Emergency, as a loss of any two barriers and
potential loss of the third barrier. Potential loss of a third barrier
depends on whether or not an explosive mixture exists inside
containment. The continuous hydrogen monitors are used for determining
whether an explosive mixture exists inside containment. Therefore, the
licensee's request for exemption from the functional requirements for
hydrogen monitoring is not approved.
4.0 Conclusion
Accordingly, the Commission has determined that, pursuant to 10 CFR
50.12(a), the exemption pertaining to the recombiners is authorized by
law, will not endanger life or property or common defense and security,
and is, otherwise, in the public interest. Also, pursuant to 10 CFR
50.12(a)(2)(ii), special circumstances are present. Therefore, the
Commission hereby grants Duke Energy Corporation an exemption from the
recombiner requirements of 10 CFR 50.44 and 10 CFR part 50, appendix A,
General Design Criterion 41 for the Oconee Nuclear Station, Units 1, 2,
and 3.
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of this exemption will not have a significant effect on the
quality of the human environment (66 FR 37073).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 17th day of July 2001.
For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear
Reactor Regulation.
[FR Doc. 01-18326 Filed 7-20-01; 8:45 am]
BILLING CODE 7590-01-P