Part 21 Report - 1998-180
ACCESSION #: 9801280254
SIEMENS
January 15, 1998
NRC:98:001
Document Control Desk
ATTN: Chief, Planning, Program and Management Support Branch
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555
Interim Report of Evaluation of a Deviation Pursuant to 10 CFR
21.21(a)(2)
The following information is provided pursuant to the requirements of 10
CFR 21 to submit an interim report on issues that will not be completed
within 60 days of discovery.
An interim report for an issue under evaluation by Siemens Power
Corporation is enclosed:
Interim Report No. 98-001 "RELAP4 Excessive Variability"
Those SPC customers potentially impacted by this issue will be provided a
copy of this interim report.
If you have any questions or if I can be of further assistance, please
call me at (509) 375-8142.
Very truly yours,
J. S. Holm, Manager
Product Licensing
/arn
Attachment
cc: Mr. E. Y. Wang (USNRC)
Project No. 702
Siemens Power Corporation
Nuclear Division 2101 Horn Rapids Road
Engineering & Manufacturing P.O. Box 130
Richland, WA 99352-0130
Tel: (509) 375-8100
Fax: (509) 375-8402
Interim Report (98-001)
Subject:
Interim report of evaluation of a deviation pursuant to 10 CFR
21.21(a)(2)
Title:
RELAP4 Excessive Variability
Identification of Basic Activity:
PWR Large Break LOCA Analysis
Basic Activity Supplied by:
Siemens Power Corporation - Nuclear Division
Nature of Deviation:
Discontinuities in the modeling approximations used in the RELAP4 portion
of the PWR LBLOCA analysis may result in excessive variability in the
peak cladding temperature. Excessive variability means that a large
change in the result produced by the code can be caused by small changes
in the input. These small changes in input would not be expected to
physically cause significantly large changes in the results. This
excessive variability can be triggered by the time to CHF lockout as
reported to the NRC in Reference 1 in December 1996. The influence of
the CHF lockout time upon RELAP4 results is further explained in
Reference 2, March 1997. The identified RELAP4 excessive variability is
related to the volume average flow model and enthalpy transport models in
RELAP4. Other models in RELAP4 may also contribute to the excessive
variability.
An evaluation of whether this excessive variability represents a
reportable defect under 10 CFR 21 has been initiated. The impact cannot
be fully quantified until changes to the RELAP4 code are completed; thus
the evaluation cannot be completed in 60 days. A schedule for completing
the evaluation is provided below.
Discovery Date:
December 2, 1997
Corrective Actions to Date:
Nonconformance Report (NCR) 6169 was issued in June 1997 as a result of
an unexpected calculational result. A plant blowdown calculation was
performed to provide boundary conditions for a RELAP4 gadolinia rod hot
channel calculation being performed as part of another NCR evaluation
(NCR 6141). The analyst reran the limiting UO sub 2 rod calculation with
insignificant changes in input from that used in the Analysis of Record
calculation and this resulted in an unexpectedly large increase in the
peak cladding temperature.
The cause was initially determined to be an unrealistically short time to
CHF in the RELAP4 hot channel calculation. This issue had been
previously addressed in a potential error evaluation where the
unrealistically short time to CHF was classified as an invalid
calculation. It was concluded at that time (June 1997) that no potential
safety issue existed because the calculation was classified [illegible]
invalid.
Two corrective actions were planned in response to NCR 6169. First,
revise the LBLOCA guideline to indicate that the analyst should make
plots from the RELAP4 hot channel runs to observe time to CHF in the core
volumes to assure that there was no unrealistic calculation of time to
CHF. Second, complete the previously planned model changes to address
the RELAP4 problem.
SPC has a model development program in progress to address the RELAP4
excessive variability and remove the Dougall-Rohsenow correlation known
non-conservatism from RELAP4 The date for completion of this program was
stated in Reference 2 to be December 1997, but the schedule has been
delayed to August 1998. The delay is due to the complexity of the
changes being made.
Further consideration has led to the conclusion that the first corrective
action, revise the LBLOCA guideline, was not an adequate approach. The
first corrective action was not adequate since the excessive variability
is not directly related to early CHF lockout but is primarily related to
the volume average flow and enthalpy transport models in RELAP4. NCR
6169, Revision 1 was issued on December 2, 1997 classifying this as a
deviation potentially reportable under 10 CFR 21.
A developmental code version has been created which modifies the models
which cause the RELAP4 excessive variability. A calculation has been run
on the case which prompted the issuance of NCR 6169 with the result that
the fuel and cladding temperatures at the end of blowdown were reduced
relative to the analysis of record. This indicates that the impact of the
excessive variability for this case is a conservative increase in the
calculated peak cladding temperature and that the Analysis of Record
results are conservative. Further evaluations are underway to confirm a
similar trend for other plants.
If during this evaluation the changes to the evaluation model are
determined to be significant, this information will be provided to the
affected licensees and the NRC.
There is no indication at this time that the RELAP4 excessive variability
problem, after correction, will result in peak cladding temperatures
which violate 10 CFR 50.46 limits.
Evaluation Completion Schedule Date:
April 3, 1998
References:
1. Letter, H. D. Curet (SPC) to NRC Document Control Desk, "Compiler
and Spurious CHF Lockout," HDC:96:067, December 12, 1996.
2. Letter, H. D. Curet (SPC) to NRC Document Control Desk, "RELAP4 -
Spurious CHF Lockout," HDC:97:022, March 10, 1997.
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