[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]
BILLING CODE 7590-01-P
