United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9801280254


January 15, 1998


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


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



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)


Interim report of evaluation of a deviation pursuant to 10 CFR



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


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]


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


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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