[Federal Register Volume 63, Number 214 (Thursday, November 5, 1998)]
[Notices]
[Pages 59811-59813]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-29644]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-302]
Florida Power Corporation et al. (Crystal River Unit 3);
Exemption
I
The Florida Power Corporation et al. (FPC or the licensee) is the
holder of Facility Operating License No. DPR-72, which authorizes the
operation of Crystal River Unit 3. The license states that the licensee
is subject to all rules, regulations, and orders of the Nuclear
Regulatory Commission (NRC or the Commission) now or hereafter in
effect.
The facility consists of a pressurized-water reactor at the
licensee's site located in Citrus County, Florida.
II
The Code of Federal Regulations at 10 CFR Part 50, Appendix K,
Section I.D.1, ``Single Failure Criterion,'' requires that accident
evaluations use the combination of emergency core cooling system (ECCS)
subsystems assumed to be operative ``after the most damaging single-
failure of ECCS equipment has taken place.'' The proposed action would
exempt the licensee from the single-failure requirement for very-low-
probability scenarios under certain circumstances. The exemption is
limited to the systems required for preventing boron precipitation
during the long-term cooling phase of a loss-of-coolant accident
(LOCA). 10 CFR 50.46(b)(5) requires that the ECCS be capable of
providing long-term core cooling. Post-accident boron precipitation is
a potential, but unlikely, challenge to maintaining long-term core
cooling.
By letter dated October 31, 1997, as supplemented by letters dated
December 13, 1997, February 27, 1998, and April 24, 1998, FPC requested
an amendment to its operating license for Crystal River Unit 3. The FPC
amendment request addressed prevention of boron precipitation following
a LOCA that involved the following:
(1) Reactor vessel vent valves (RVVVs) that are effective when
needed for all LOCA conditions except for (a) some LOCAs between the
reactor coolant pumps and the reactor vessel (RV) at an elevation below
the cold-leg mid-pipe at the junction with the RV and (b) decay heat
generation rate comparable to approximately a month following extended
operation at full power for some LOCAs.
(2) If the RVVVs are not effective, then, according to the
licensee's
[[Page 59812]]
calculations, Motor Control Center (MCC) 3AB is needed to provide power
to open valves within 8 hours for the worst-case LOCA to (a) initiate
water injection via auxiliary pressurizer spray (APS) or (b) initiate
the dump-to-sump (DTS) method of moving water from a hot leg to the
reactor building sump.
Should MCC 3AB fail before the APS or DTS initiates, both of these
systems will fail to initiate in these licensing scenarios. In a June
4, 1998, submittal, FPC requested an exemption from the single-failure
requirement with respect to this failure. FPC justified its request by
stating that the proposed exemption meets the underlying purpose of the
rule in that there are conservatisms in the calculations that cause
underprediction of available repair time, so that, using realistic
assumptions, sufficient time would be available to perform repairs to
restore MCC 3AB if needed. As a result, the licensee stated that there
was reasonable assurance of the availability of an active boron
precipitation method (APS or DTS) if one were needed. FPC states that
timely recognition of boron precipitation is assured by compliance with
plant procedures and further states that prompt operator actions will
be taken to restore an active method in the event of MCC 3AB failure.
One element of the licensee's justification was to credit flow
through the hot-leg nozzle gaps. According to FPC's calculations, APS
is not fully effective until 21 hours after LOCA initiation, but it may
be needed within 8 hours if a single failure other than the failure of
MCC 3AB makes DTS unavailable. FPC addressed this problem by crediting
flow through hot-leg nozzle gaps to provide a boron dilution means for
the first 21 hours. However, the NRC does not accept credit for hot-leg
nozzle gap flow because FPC has not established that the nozzle gaps
will remain functional after a LOCA. Therefore, during this time
period, a failure to meet the Appendix K Item I.D.1 single-failure
criterion remains. However, the NRC has determined that the licensee
has given adequate justification in its submittal to extend the
exemption to this scenario.
III
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 (1) when 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. Special circumstances are
present whenever, according to 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 purpose of the single-failure criterion requirement
is to assure long-term cooling performance of the ECCS in the event of
the most damaging single-failure of ECCS equipment. As a licensing
review tool, the single-failure criterion helps assure reliable systems
as an element of defense in depth. As a design and analysis tool, it
promotes reliability through enforced redundancy. Since only those
systems or components that are judged to have a credible chance of
failure are assumed to fail, the criterion is applied to such responses
as valve movement on demand, emergency diesel generator start, short
circuit in an electrical bus, and fluid leakage caused by gross failure
of a pump or valve seal during long-term cooling. Reactor vessels or
certain types of structural elements within systems, when combined with
other unlikely events, are not assumed to fail because the
probabilities of the resulting scenarios have been deemed sufficiently
small that they need not be considered. Certain passive failures 24
hours or more after initiation of a LOCA, such as pipe breaks, are not
addressed as single failures because the compounded probabilities were
judged sufficiently small that they could be discounted without
affecting overall systems reliability.
The single-failure criterion was developed without the benefit of
numerical failure assessments. Regulatory requirements and guidance
consequently were based upon categories of equipment and examples that
must be covered or that are exempt, and do not allow a probabilistic
consideration during routine implementation. Hence, a single failure,
whether or not there is a substantial impact upon overall system
reliability, would not meet the regulatory requirements. A non-
beneficial result is inconsistent with the objective of the single-
failure criterion, which was not intended to force changes if
essentially no benefit would accrue. This is the case with the
potential MCC 3AB failure.
FPC estimated that the combined probability of the LOCA of concern
and failure of MCC 3AB is 10-10/reactor-year. (The
probability of the LOCA of concern is 10-7/reactor-year and
the failure probability of MCC 3AB given the LOCA of concern is
10-3/reactor-year.) If MCC 3AB were to fail, FPC would
initiate its Emergency Plan Implementing Procedure to re-power MCC 3AB
from an alternate electric power source. FPC stated that sufficient
time will be available and that radiological conditions should permit
such activities.
In addition, there are other conservatisms in the licensee's
analyses. These include:
Presence of buffer compounds may increase solubility limit
margins. FPC concluded that solutes in the sump water will increase
boron solubility, but did not credit the effect in its calculations.
This is a conservatism when considering MCC 3AB repair and APS
unavailability time.
Decay heat was calculated using Appendix K methods. FPC's
calculations, in accordance with its licensing basis, use a decay heat
generation rate that is roughly 25 percent too high. A realistic decay
heat would increase the time available before boron precipitation
became a concern. This is a significant conservatism when considering
MCC 3AB repair and APS unavailability time.
Boron solubility. FPC used a boron solubility decreased by
4 weight percent from the published values, consistent with previously
accepted evaluation models. This is a conservatism when considering MCC
3AB repair and APS unavailability time.
Boron precipitation. The approved evaluation models are
based upon preventing precipitation. Should precipitation occur,
significant boron would have to precipitate to prevent core cooling.
This unquantified conservatism is significant when considering MCC 3AB
repair and APS unavailability time.
Despite the licensee's determination that there is no safety-
significant vulnerability associated with the two particular instances
of failing to meet the single-failure criterion, FPC has developed and
implemented procedures to address the conditions should they occur. It
has shown that there is essentially no benefit to be achieved by
investing in additional equipment to eliminate the single-failure
aspects since the combined probability of the LOCA of concern with the
failure is very low. With regard to the availability of APS during the
first 21 hours following a LOCA should DTS be unavailable, realistic
calculations without the conservative assumptions discussed above
predict that APS would be available.
These calculations, along with the low estimate of core damage
probability resulting from this scenario, result in a
[[Page 59813]]
conclusion that essentially no benefit would be achieved by requiring
modifications to meet the single-failure criteria for the specific
scenario during this time period.
IV
For these foregoing reasons, the NRC staff has concluded that it is
not necessary to meet the single-failure requirement of Appendix K,
Section I.D.1, with respect to (1) failure of Motor Control Center 3AB
and the resulting inability to initiate an active means of controlling
core boron concentration and (2) the active methods not meeting the
single-failure criterion for the period when approved licensing methods
predict that APS is not effective following certain LOCAs to adequately
ensure that boron precipitation does not interfere with long-term
cooling. The NRC staff has determined that there are special
circumstances present, as specified in 10 CFR 50.12.(a)(2)(ii), in that
application of 10 CFR Part 50, Appendix K, Section I.D.1, is not
necessary in order to achieve the underlying purpose of this
regulation, which is to provide adequate assurance that boron
precipitation will not interfere with the capability of the ECCS to
provide long-term core cooling.
Accordingly, the Commission has determined that, pursuant to 10 CFR
50.12(a), this exemption is authorized by law, will not endanger life
or property or the common defense and security, and is otherwise in the
public interest. Therefore, the Commission hereby grants the following
exemption:
The Florida Power Corporation, et al., is exempt from the
single-failure criterion requirement of 10 CFR Part 50, Appendix K,
Section I.D.1, with respect to (1) failure of Motor Control Center
3AB and the resulting inability to initiate an active means of
controlling core boron concentration and (2) failure of the active
means to meet the single-failure criterion for the period when
approved licensing methods predict that APS is not effective
following reactor coolant pump discharge breaks provided that:
procedural guidance shall be maintained that describes the actions
necessary to restore an active method of boron precipitation
mitigation in the event of a failure of Motor Control Center 3AB.
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of this exemption will have no significant impact on the
quality of the human environment (63 FR 54162).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 29th day of October, 1998.
For the Nuclear Regulatory Commission.
Frank J. Miraglia,
Acting Director, Office of Nuclear Reactor Regulation.
[FR Doc. 98-29644 Filed 11-4-98; 8:45 am]
BILLING CODE 7590-01-P
