[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