[Federal Register Volume 61, Number 229 (Tuesday, November 26, 1996)]
[Notices]
[Pages 60122-60124]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-30154]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-305]
Wisconsin Public Service Corporation, Wisconsin Power & Light
Co., Madison Gas & Electric Co. (Kewaunee Nuclear Power Plant);
Exemption
I
The Wisconsin Public Service Corporation, Wisconsin Power and Light
Company, and Madison Gas and Electric Company (the licensee), are the
holders of Facility Operating License No. DPR-43 which authorizes
operation of the Kewaunee Nuclear Power Plant (KNPP). The license
provides, among other things, that it is subject to all rules,
regulations, and orders of the Nuclear Regulatory Commission (the
Commission) now and hereafter in effect.
The facility consists of a pressurized water reactor located at the
licensee's site in Kewaunee County, Wisconsin.
II
The Code of Federal Regulations, paragraph I.D.3, ``Calculation of
Reflood Rate for Pressurized Water Reactors [PWRs],'' of Appendix K to
Part 50 of Title 10 of the Code of Federal Regulations (10 CFR)
requires that the refilling of the reactor vessel and the time and rate
of reflooding of the core be calculated by an acceptable model that
considers the thermal and hydraulic characteristics of the core and of
the reactor system. In particular, paragraph I.D.3 requires, in part,
that, ``The ratio of the total fluid flow at the core exit plane to the
total flow at the core inlet plane (carryover fraction) shall be used
to determine the core exit flow and shall be determined in accordance
with applicable experimental data.'' The purpose of this requirement is
to assure that the core exit flow during the post-loss-of-coolant
accident (LOCA) refill/reflood phase is determined using a model that
accounts for appropriate experimental data.
Paragraph I.D.5, ``Refill and Reflood Heat Transfer for Pressurized
Water Reactors,'' of Appendix K to 10 CFR Part 50 requires that: (1)
for reflood rates of 1 inch per second or higher, the reflood heat
transfer coefficients be based on applicable experimental data for
unblocked cores, and (2) for reflood rates less than 1 inch per second
during refill and reflood, heat transfer calculations be based on the
assumption that cooling is only by steam.
By letter dated July 23, 1996, the licensee requested an exemption
from the requirements of 10 CFR Part 50, Appendix K, paragraphs I.D.3
and I.D.5, as they apply to an evaluation model (EM) for the LOCA
analysis for two-loop Westinghouse plants such as Kewaunee (WCAP-10924-
P, Revision 1, Volume 1, Addendum 4).
The specific provision of paragraph I.D.3 from which the licensee
requested an exemption, is the calculation of core exit flow based on
carryover fraction. The licensee stated that the prescriptions for this
calculation given in paragraph I.D.3 were based on data for a bottom-
flooding configuration design. The Kewaunee design relies on upper
plenum injection (UPI) for the ECCS injection during the reflood phase
of a large-break LOCA. UPI is not a ``lower flooding design;'' its ECCS
flow patterns, flow magnitudes, core cooling mechanisms, and, in fact,
the meanings and impacts of the terms ``inlet'' and ``exit'' are
different than those of bottom flooding plants. The EM is described in
WCAP 10924-P, Revision 1, ``Westinghouse Large-Break LOCA Best-Estimate
Methodology, Volume 1: Model Description and Validation, Addendum 4:
Model Revisions,'' dated August 1990, which was generically approved in
a staff SER dated February 8, 1991. The EM determines core flow,
including flow ``exiting'' the core, flow ``entering'' the core, and
flow within the core and elsewhere within the reactor coolant system
(RCS) in accordance with applicable experimental data. The data are
different than that referenced in paragraph I.D.3, however, they were
found acceptable because they are specifically applicable to UPI
designs. Because of the differences between UPI design considerations
and those for bottom flooding designs mentioned above, the ``carryover
fraction'' as defined in paragraph I.D.3 is not calculated in the
approved EM and would not have the same technical significance if it
were. The licensee, therefore, concludes that, in using the approved
UPI model for Kewaunee, it will not comply with paragraph I.D.3. The
staff SER of February 8, 1991, finds that the WCAP-10924-P EM contains
an empirically verified model, more directly applicable to top flooding
situations, to calculate core exit flow, which satisfies the technical
purpose of the Appendix K, paragraph I.D.3 requirement to determine the
core exit flow, but does not comply with the letter of the requirement.
In more detail, the intent of the Appendix K, paragraph I.D.3,
requirement is to assure that the calculation of core exit flow is
performed using an EM which has been verified against appropriate
experimental data for LOCA accident analyses. The Westinghouse COBRA/
TRAC code (WCOBRA/TRAC) consists of: (1) Westinghouse Large-Break LOCA
Best Estimate Methodology, Volume 1: Model Description and Validation,
WCAP-10924-P-A, Rev. 1, and Addenda 1, 2, and 3, December 1988, and (2)
a Westinghouse Large-Break LOCA Best-Estimate Methodology, Volume 2:
Application to Two-Loop PWRs Equipped with Upper Plenum Injection,
WCAP-10924-P-A, Rev. 2, December 1988.
To assess WCOBRA/TRAC's capability for predicting the correct
thermal-hydraulic behavior for upper plenum injection situations,
WCOBRA/TRAC has been compared to the Japanese Cylindrical Core Test
Facility data which models the interaction effects of upper plenum
injection in a large scale test facility. WCOBRA/TRAC predicts the
thermal-hydraulic effects of the upper plenum injection such that the
carryover of steam and water into the hot legs is more realistically
calculated.
The staff finds that the exemption from the paragraph I.D.3
requirement is acceptable because the licensee has provided an
acceptable method to satisfy the underlying purpose of the requirement
that appropriately models heat transfer mechanisms in UPI designs, and
application of the regulation is not necessary to achieve the
underlying purpose of the rule.
Paragraph I.D.5, dealing with refill and reflood heat transfer for
PWRs, provides heat transfer prescriptions for refill, reflood with a
flooding rate of less than 1 inch per second, and reflood with a
flooding rate of more than 1 inch per second for bottom-flooding PWRs.
The purpose of the paragraph is to assure that heat transfer in the
core is appropriately calculated in the refill and reflood phases of
post-LOCA recovery.
Paragraph I.D.5.a requires that ``New correlations or modifications
to the FLECHT [full length emergency cooling heat transfer] heat
transfer correlations are acceptable only after they are demonstrated
to be conservative, by comparison with FLECHT data, for a range of
parameters consistent with the
[[Page 60123]]
transient to which they are applied.'' The licensee requested an
exemption from the prescriptions of this paragraph because the FLECHT
data do not portray UPI core heat transfer mechanisms as realistically
as the more recent data upon which the models in WCAP-10924 were based.
The licensee also indicates that the Kewaunee design is not lower
flooding, and that technical considerations are different between
bottom flooding designs and UPI design similar to those discussed above
for paragraph I.D.3. The licensee identified that the WCAP-10924-P EM
contains an empirically verified model which accounts for refill and
reflood heat transfer, which satisfies the purpose of the paragraph
I.D.5.a requirement. The heat transfer models in the approved UPI EM
are based on comparisons to data other than the FLECHT data cited in
paragraph I.D.5.a, and comparisons to the applicable data demonstrate
acceptable conservatism (as identified in the staff SER of February 8,
1991). Because of the differences in bases, it is not clear that the
licensee can demonstrate monotonic conservatism with respect to FLECHT
data.
Further, to meet the intent of Appendix K, paragraph I.D.5, which
is to use the most applicable data for LOCA accident analyses to
appropriately calculate heat transfer during the refill and reflood
phases; the WCOBRA/TRAC code has been verified against two independent
sets of experimental data which model the upper plenum injection flow
and heat transfer situation.
The first series of tests which have been modeled by WCOBRA/TRAC
are the Westinghouse G-2 refill downflow and counterflow rod bundle
film boiling experiments (Westinghouse G-2, 17x17 Refill Heat Transfer
Tests and Analysis, WCAP-8793, August 1976).
These experiments were performed as a full length 17x17
Westinghouse rod bundle array which had a total of 336 heated rods. The
injection flow was from the top of the bundle and is scalable to the
UPI injection flows. The pressures varied between 20-100 psia which is
the typical range for UPI top flooding situations. Both concurrent
downflow film boiling and countercurrent film boiling experiments were
modeled using WCOBRA/TRAC. Both of these flow situations are found in
the calculated core response for a PWR with UPI.
In addition to modeling these separate effects tests, WCOBRA/TRAC
has been used to model the Japanese Cylindrical Core Test Facility
experiments with upper plenum injection. The tests which have been
modeled included (1) a symmetrical UPI injection with maximum injection
flow, (2) minimum injection flows with a nearly symmetrical injection
pattern, (3) a minimum UPI injection flow with a skewed UPI injection,
and (4) a cold leg injection reference test for the UPI tests.
The results of these comparisons are documented and show that
WCOBRA/TRAC does predict heat transfer behavior for these complex film
boiling situations as well as the system response for upper plenum
injection situations.
The effect of flow blockage due to cladding burst is explicitly
accounted for in WCOBRA/TRAC with models which calculate cladding
swelling, burst, and area reduction due to blockage. These models are
based on previously approved models used in current evaluation models
and on flow blockage models determined to be acceptable by the staff.
The effect of flow blockage is accounted for from the time burst is
calculated to occur. The fluid models in WCOBRA/TRAC calculate flow
diversion as a result of the blockage and take into account the
blockage from the time the cladding burst is calculated to occur. Thus,
the heat transfer behavior is predicted for these complex film boiling
situations and, thus, the intent of Appendix K, paragraph I.D.5, which
requires flow blockage effects be taken into account, is met.
The staff finds that the exemption from the paragraph I.D.5.a
requirement is acceptable based on the provision of an acceptable
method to satisfy the purpose of the paragraph that requires
appropriate calculation of core reflood rates and heat transfer during
a large break LOCA.
Paragraph I.D.5.b requires that ``During refill and during reflood
when reflood rates are less than one inch per second, heat transfer
calculations shall be based on the assumption that cooling is only by
steam, and shall take into account any flow blockage calculated to
occur as a result of cladding swelling or rupture as such blockage
might affect both local steam flow and heat transfer.'' The EM approved
for UPI plants which the licensee proposes to reference does base heat
transfer on cooling other than steam if other regimes are calculated to
occur. The bases of acceptability, including data comparisons, for this
are discussed in the generic SER for the EM. By using this methodology,
the licensee does not comply with this requirement, since the
methodology recognizes that for a top flooding design, the
preponderance of cooling water falls down into the core from above and
may or may not be vaporized. Because the licensee's model does not meet
the ``steam cooling only'' requirement of I.D.5.b, but provides an
approved alternate methodology (which does consider the thermal and
hydraulic effects of cladding swelling and rupture, as also required in
paragraph I.D.5.b) for calculating heat transfer, the staff finds the
exemption from the requirement of I.D.5.b acceptable, as compliance is
demonstrated not to be necessary to achieve the underlying purpose of
the rule.
III
Section 50.12 of 10 CFR permits the granting of an exemption from
the regulations under special circumstances. According to 10 CFR
50.12(a)(2)(ii), special circumstances are present whenever application
of the regulation in question is not necessary to achieve the
underlying purpose of the rule.
The staff finds that the requested exemptions for Kewaunee are
acceptable, since compliance with the literal requirements of the
paragraphs cited is not necessary given that the approved EM is based
upon appropriate experimental data, the approved EM satisfactorily
accounts for the cooling mechanisms in the Kewaunee UPI design for
calculations of core reflood rates and heat transfer during a large
break LOCA, and that the approved EM satisfies the purpose of the
exempted requirements.
Thus, using the best-estimate thermal-hydraulic approved large
break LOCA EM, the underlying purpose of the Appendix K, paragraphs
I.D.3 and I.D.5 requirements can be achieved.
IV
Accordingly, 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, and is consistent with the common
defense and security.
Therefore, the Commission hereby grants an exemption from 10 CFR
Part 50, Appendix K, paragraphs I.D.3 and I.D.5. The staff also finds
that the large break LOCA EM described in any approved version of WCAP-
10924-P incorporated by Kewaunee may be used in licensing analyses, and
that further exemptions will not be necessary unless the updated
approved versions of the EM do not meet other requirements of 10 CFR
50.46 and/or Appendix K.
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of the exemption will have no significant impact on the
quality of the human environment (61 FR 42447).
[[Page 60124]]
This exemption is effective upon issuance.
Dated at Rockville, Maryland this 19th day of November 1996.
For the Nuclear Regulatory Commission.
Frank J. Miraglia,
Acting Director, Office of Nuclear Reactor Regulation.
[FR Doc. 96-30154 Filed 11-25-96; 8:45 am]
BILLING CODE 7590-01-P
