[Federal Register Volume 61, Number 111 (Friday, June 7, 1996)]
[Notices]
[Pages 29142-29144]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-14395]



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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-244]


Exemption

    In the Matter of Rochester Gas and Electric Corporation, R.E. 
Ginna Nuclear Power Plant)

I

    On December 10, 1984, the Nuclear Regulatory Commission issued 
Facility Operating License No. DPR-18 to Rochester Gas and Electric 
Corporation (RG&E) for the R.E. Ginna Nuclear Power Plant (Ginna). The 
license stipulated, among other things, that the facility is subject to 
all rules, regulations, and orders of the Commission.

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 
Reactors,'' of Appendix K to 10 CFR Part 50 requires that for (1) 
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) 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.
    License Condition 2.D provided an exemption from 10 CFR 50.46(a)(1) 
that the emergency core cooling system (ECCS) performance be calculated 
in accordance with an acceptable calculational model which conforms to 
the provisions of Appendix K (SER dated April 18, 1978). The exemption 
will expire upon receipt and approval of revised ECCS calculations.
    By letter dated November 5, 1992, as supplemented on June 19, 1995, 
RG&E (the licensee) requested an exemption from 10 CFR Part 50, 
Appendix K, Paragraphs I.D.3 and I.D.5 based on revised ECCS 
calculations.
    The November 5, 1992, exemption request was supported first by a 
plant specific ECCS evaluation model (EM) using a methodology not yet 
approved by NRC (WCAP-10924-P, Volume 2, Revision 2, Addendum 3). The 
proposed EM would have supported the May 1993, 1994, and 1995 core 
reloads. However, the WCAP-10924-P, Revision 2, Volume 2, Addendum 3 
methodology has not yet been approved by NRC. On June 19, 1995, the 
licensee supported the November 5, 1992, exemption request by an 
updated plant specific EM using a methodology approved by NRC (WCAP-
10924-P, Volume 1, Revision 1, Addendum 4). The proposed June 19, 1995, 
EM includes larger peaking factors necessary to support conversion to 
an 18-month fuel-cycle reload to begin in May 1996.
    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 Ginna 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. This EM described in WCAP 10924-
P, Volume 1, Revision 2, Addendum 4, ``Westinghouse UPI Model 
Improvements,'' dated August 1990, which has been generically approved 
in a staff SER of February 8, 1991, 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 
with its technical improvements for Ginna, it will not comply with 
Paragraph I.D.3. The staff SER of February 8, 1991, finds 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 this 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, is 
to assure that the calculation of core exit flow is performed using an 
EM code model 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

[[Page 29143]]

Description and Validation, WCAP-10924-P, April 1986, and (2) a 
Westinghouse Large-Break LOCA Best Estimate Methodology, Volume 2: 
Application to Two-Loop PWRs Equipped with Upper Plenum Injection, 
WCAP-10924, Volume 2, Revision 1, April 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 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 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 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 Ginna 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, 17 x 17 Refill Heat 
Transfer Tests and Analysis, WCAP-8793, August 1976).
    These experiments were performed as a full length 17 x 17 
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 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 WCAP/TRAC calculate flow 
diversion as a result of the blockage and take into account of 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 and the application of 
the regulation to calculate core reflood rates and heat transfer during 
a LB 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

[[Page 29144]]

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 Ginna 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 Ginna UPI design for 
calculations of core reflood rates and heat transfer during a LB LOCA, 
and that the approved EM satisfies the purpose of the exempted 
requirements.
    Thus, using the best-estimate thermal-hydraulic approved LBLOCA 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.
    Accordingly, 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 LB LOCA EM described in any approved version of WCAP-10924-P 
incorporated in the Ginna Technical Specifications may be used in core 
operating report, and 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 13891).
    This exemption is effective upon issuance.

    For the Nuclear Regulatory Commission.

    Dated at Rockville, Maryland, this 31st day of May 1996.

Steven A. Varga,
Director, Division of Reactor Projects--I/II 1Office of Nuclear Reactor 
Regulation.
[FR Doc. 96-14395 Filed 6-6-96; 8:45 am]
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