[Federal Register Volume 67, Number 31 (Thursday, February 14, 2002)]
[Notices]
[Pages 6947-6949]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 02-3618]


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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-289]


Amergen Energy Company, LLC Three Mile Island Nuclear Station, 
Unit 1; Exemption

1.0  Background

    The AmerGen Energy Company, LLC (AmerGen, the licensee) is the 
holder of Facility Operating License No. DPR-50, which authorizes 
operation of the Three Mile Island Nuclear Station, Unit 1 (TMI-1). The 
license provides, among other things, that the facility is 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 a pressurized water reactor (PWR) located 
in Dauphin County in Pennsylvania.

2.0  Request/Action

    Title 10 of the Code of Federal Regulations (10 CFR), part 50, 
Sec. 50.44, ``Standards for combustible gas control system in light-
water-cooled power reactors,'' and 10 CFR part 50, Appendix A, General 
Design Criterion (GDC) 41, ``Containment atmosphere cleanup,'' 
establish requirements for controlling the amount of hydrogen inside 
the reactor containment following a postulated loss-of-coolant accident 
(LOCA). These requirements provide specific assumptions and methods to 
define the amount of hydrogen generated, the rate at which hydrogen is 
generated, and the requirements of a combustible gas control system to 
control the concentration of hydrogen in the containment following a 
design-basis LOCA to below flammability limits. Appendix E to 10 CFR 
part 50, Section VI, ``Emergency Response Data System [ERDS],'' 
contains requirements to provide information on the concentration of 
hydrogen inside the containment following accidents as part of the 
ERDS. Section 50.44(a) to 10 CFR part 50 requires a means for control 
of hydrogen that may be generated following a postulated LOCA by (1) a 
metal-water reaction involving the fuel cladding and the reactor 
coolant, (2) radiolytic decomposition of the reactor coolant, and (3) 
corrosion of metals. Section 50.44(b) of 10 CFR and 10 CFR part 50, 
Appendix E, Section VI.2.a.(i).4 require that the hydrogen control 
measures must be capable of measuring the hydrogen concentration in the 
containment, ensuring a mixed atmosphere in the containment and 
controlling combustible gas concentrations in the containment following 
a LOCA. Section 50.44(c)(1) of 10 CFR part 50 requires that it must be 
shown that following a LOCA, but prior to effective operation of the 
combustible gas control system, either an uncontrolled hydrogen-oxygen 
recombination would not take place in containment, or the plant could 
withstand the consequences of uncontrolled hydrogen-oxygen 
recombination without loss of safety function. Section 50.44(h)(2) 
requires a combustible gas control system to maintain the concentration 
of combustible gases following a LOCA to below flammability limits. 
These systems can be of two types: Those allowing controlled release 
from containment such as a purge system, or those that do not result in 
a significant release from the containment such as recombiners. GDC 41 
of Appendix A to 10 CFR part 50 requires that the hydrogen control 
system described above must control hydrogen as necessary following a 
LOCA to assure that containment integrity is maintained, and must meet 
redundancy and single failure requirements. Additional NRC staff 
guidance is provided in Regulatory Guide (RG) 1.7. NRC staff review and 
acceptance criteria are specified in Section 6.2.5 of the Standard 
Review Plan (NUREG-0800, July 1981). By letter dated September 20, 
2000, as supplemented by letters dated August 2 and September 28, 2001, 
the licensee requested an exemption to the above requirements in order 
to remove requirements for a hydrogen control system from the TMI-1 
design basis. The proposed request for exemption included a related 
license amendment application which would remove the hydrogen control 
system from the plant's operating license Technical Specifications and 
the Updated Final Safety Analysis Report.

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. These circumstances include 
the special circumstances as stated in 10 CFR 50.12(a)(2)(ii), 
``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.'' The underlying

[[Page 6948]]

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 an uncontrolled 
hydrogen-oxygen recombination without loss of safety function.
    In its request, AmerGen asserts that the TMI-1 containment has 
sufficient safety margin against hydrogen burn following design-basis 
and severe accidents without use of the hydrogen monitoring or 
concentration control systems. The TMI-1 Probabilistic Risk Assessment 
(PRA) indicates that none of the accident sequences addressed that 
could realistically threaten containment due to hydrogen combustion are 
impacted by the hydrogen monitoring or concentration control systems. 
The TMI-1 Individual Plant Examination (IPE) concluded containment 
survival is almost certain following hydrogen combustion when the 
reactor building cooling units and the reactor building spray system 
are operating. The licensee's plant-specific containment integrity 
analysis for TMI-1 indicates that the ultimate pressure capacity of the 
containment is between 137 and 147 psig (TMI-1 PRA, Level 2, Appendix 
1). This estimate is reasonable when compared to Table 6.1 of NUREG/CR-
6475, ``Resolution of the Direct Containment Heating Issue for 
Combustion Engineering Plants and Babcock & Wilcox Plants.'' A safety 
margin exists for containment integrity even for conservative hydrogen 
concentration levels. The NRC 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. NRC sponsored studies, such as NUREG-1150, ``Severe Accident 
Risks: An Assessment For Five U.S. Nuclear Power Plants,'' December 
1990, and NUREG/CR-5662, ``Hydrogen Combustion, Control And Value 
Impact Analysis For PWR [pressurized water reactor] Dry Containments,'' 
June 1991, have found hydrogen combustion to be a small contributor to 
containment failure for large, dry containment designs due to the 
robustness of these containment types and the likelihood of a spurious 
ignition source. Additionally, 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 TMI-1's. This is true despite the fact 
that the hydrogen quantities produced in these events is substantially 
larger than the hydrogen production postulated by 10 CFR 50.44(d) and 
RG 1.7, Revision 2, ``Control of Combustible Gas Concentrations in 
Containment Following a Loss-of-Coolant Accident (LOCA),'' November 
1978. 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, 
Revision 1, Volume 7, Part 1, ``Evaluation of Severe Accident Risks: 
Zion Unit 1,'' March 1993, 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 LOCA. 
Additionally, 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 TMI-1 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.
    Although hydrogen igniter systems would provide some added margin 
that containment integrity can be maintained during hydrogen burns, 
Generic Issue (GI)-121, ``Hydrogen Control for PWR Dry Containments,'' 
found that hydrogen combustion was not a significant threat to dry 
containments, and concluded that there was no basis for new generic 
hydrogen control measures (i.e., igniters). Equipment survivability in 
concentrations greater than 6 percent was addressed as part of GI-121, 
which references NUREG/CR-5662, which assessed the benefits of hydrogen 
igniters. NUREG/CR-5662 concluded that simulated equipment can 
withstand a LOCA and single burn resulting from a 75-percent metal-
water reaction in a large, dry containment. However, the multiple 
containment burns due to the operation of ignition systems could pose a 
serious threat to safety-related equipment located in the source 
compartment. The multiple burn environment was found potentially to be 
a threat because the source compartment temperature remains elevated 
from the previous burn. However, for TMI-1, this is not a concern for 
the above radiolysis and corrosion case because there is ample time 
between burns to reduce elevated containment temperatures via 
containment heat removal systems. Therefore, an additional burn in the 
long term due to radiolysis and corrosion would not have a similar 
impact on equipment survivability at TMI-1.
    In a postulated LOCA, the TMI-1 emergency operating instructions 
(EOIs) direct the control room operators to monitor and control the 
hydrogen concentration inside the containment after they have carried 
out the steps to maintain and control the higher priority critical 
safety functions. Key operator actions associated with the control of 
hydrogen include placing the hydrogen recombiners or hydrogen purge 
system in operation at very low hydrogen concentration levels. These 
hydrogen control activities could distract operators from more 
important tasks in the early phases of accident mitigation and could 
have a negative impact on the higher priority critical operator 
actions. An exemption from hydrogen recombiner and purge-
repressurization system requirements will eliminate the need for these 
systems in the EOIs and hence simplify the EOIs. The NRC staff still 
expects the licensee's severe accident management guidelines to address 
combustible gas control. The NRC staff has determined that the

[[Page 6949]]

simplification of the EOIs would be a safety benefit.
    As stated previously, 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 licensee's analysis, the NRC 
staff's evaluation of the risk from hydrogen combustion, resolution of 
GI-121, and the TMI-1 IPE, the NRC staff has determined that 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 also for 
the more limiting severe accident with up to 100 percent metal-water 
reaction. Therefore, the requirements for hydrogen recombiners as part 
of the TMI-1 design basis are unnecessary, and their removal from the 
design basis is acceptable. Additionally, elimination of the hydrogen 
recombiners from the EOIs 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),'' 
and the elimination of any commitments made in regard to NUREG-0737, 
Item II.F.1, Attachment 6, ``Containment Hydrogen Monitor.'' However, 
in the Statement of Considerations for Appendix E to Part 50, the 
Commission stated that the ERDS data (which include data from the 
continuous hydrogen monitors) provide the data required by the NRC to 
perform its role during an emergency. This conclusion is still valid 
for not only the NRC staff, but also for licensees. The major vendors' 
core damage assessment methodologies continue to include continuous 
hydrogen monitoring. Core damage assessment methodologies were reviewed 
by the NRC 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 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.
    The NRC staff has determined that for the requested exemptions 
related to the hydrogen recombiners and backup hydrogen purge system, 
pursuant to 10 CFR 50.12(a)(2)(ii), special circumstances are present, 
in that application of the regulations in the particular circumstances 
is not necessary to achieve the underlying purpose of the rule.

4.0  Conclusion

    Accordingly, the Commission has determined that, pursuant to 10 CFR 
50.12(a), the exemption from the hydrogen recombiner and hydrogen purge 
system requirements is authorized by law, will not present an undue 
risk to the public health and safety, and is consistent with the common 
defense and security. Also, special circumstances are present. 
Therefore, the Commission hereby grants AmerGen Energy Company, LLC, an 
exemption from the requirements for hydrogen recombiners and the 
hydrogen purge system of 10 CFR 50.44, and 10 CFR part 50, Appendix A, 
General Design Criterion 41, for the TMI-1.
    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 (67 FR 1788).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 8th day of February 2002.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
 Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 02-3618 Filed 2-13-02; 8:45 am]
BILLING CODE 7590-01-P