Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
ACCESSION #: 9606210210
ANSYS ANSYS, Inc.
201 Johnson Road Telephone 412.746.3304
Houston, PA 15342-1300 Facsimile 412.746.9494
June 19, 1996
Thomas Greene, NRR/PECB
U.S. Nuclear Regulatory Commission
Mail Stop: 0-11E4
Washington, DC 20555-0001
Ref: Accession 9601190322
Subj: Request for a Technical Review of a Draft Information Notice
Regarding Erroneous Results from ANSYS 5.0A and ANSYS 5.1
Computer Codes
Dear Mr. Greene:
ANSYS, Inc. takes issue with the Draft Information Notice; we
believe that it contains correct and misleading technical information.
We do not believe this condition is reportable as described since the
ANSYS numerical algorithms used in its solution are correct, The model
which was developed by Holtec is an idealization which cannot be
constructed in the physical world, and the results achieved by ANSYS are
completely consistent with the engineering idealization and numerical
methods employed. With reference to the two points described to the
draft notice:
1. An explanation of this behavior was identified after Holtec's
filing with the NRC, and is the agreed-upon explanation for the
system-dependent solutions. Computer inaccuracy due to the
idealized model and not ANSYS algorithms were the cause of the
system-dependent solutions (see Holtec's fax dated June 19,
1996).
2. To date, we have not received the model which was used to
generate the results you describe as item 2 (i.e. disparate
results for forces and displacements are obtained depending
upon the techniques employed to launch tho analysis runs).
Your draft information notice was the first time we became
aware of this claimed error. Therefore, we cannot review and
comment on the results until an input file is made available to
us. We have requested this information from Holtec and Mr.
Y.S. Kim, NRR.
In discussions with Holtec personnel, this behavior was never
substantiated and may have been due to an aspect of their in-
house submission procedure. Until we review the ANSYS inputs
and techniques employed, we cannot rule out user error.
illegible print
Thomas Greene
Nuclear Regulatory Commission
Page Two
June 19, 1996
Background
A Holtec model was supplied to ANSYS, Inc. on August 7, 1995. The
model had little damping in the gaps, severe nonlinearities, a path
dependent solution, and a large number of small time steps, Analyses were
performed using the Holtec model on two different Unix workstations which
resulted in similar solutions. However, using Personal Computers the
solutions were different. Recommendations were made in our October 18,
1995 letter to Holtec to modify their model so that these differences
would be minimized, Those modifications resulted in a less than 5%
difference in results from various Personal Computers.
Further investigation of Holtec's model resulted in our realization
that the reported condition was due to Holtec's model relying on perfect
symmetry, The model was inherently unstable and prone to differences due
to machine precision, The model's perfect symmetry resulted in symmetric
and simultaneous contact with its enclosure, but only if the
displacements were perfectly symmetric. A slight change in the models
dimensions resolves the problem immediately and results in repeatable
solutions on all platforms.
Holtec now agrees that this is the phenomena which produced the
differing results and that the structure they modeled could never
physically exist.
The way Holtec chose to model their problem made it inherently
numerically unstable and prone to differences due to machine precision.
The various hardware platforms on which we offer the ANSYS program have
differences in the fundamental precision at which the computations are
performed. Various supported hardware and operating system environments
can provide as many as 80 bits of precision to 64 bits of precision. Any
numerical representation with this many bits is more than sufficient for
engineering accuracy in a simple linear static analysis without numerical
complications. However, when hundreds of thousands, or millions of
operations are repetitively performed on such numbers, the differences in
final results can begin to creep up into significant digits that may be
of interest to the engineer.
For these reasons it is recommended that numerical sensitivity
studies be performed to determine the solution sensitivity to small
changes in the input. If, for instance, small changes in geometric
locations (tolerances), material properties, convergence criteria or
damping result in large differences in the solution output, then it is
likely that tho modal or idealization is suspect. On the other hand, if
small numerical changes in such input quantities have little effect on
the end result, then the sensitivity of the solution to numerical
precision can generally be eliminated as a concern. The Holtec problem
is extremely sensitive.
Thomas Greene
Nuclear Regulatory Commission
Page Three
June 19, 1996
Detailed Description of the Holtec Model
The structure being analyzed is a very stiff cross-braced space
frame, (approximately 70 in x 56 in x. 200 in high) represented in the
transient analysis by a superelement. A superelement is a stiffness
matrix representation of a linear portion of the model.
In the transient, the superelement (which has no constraints of its
own) is supported entirely by contact elements. At the top, 4 sets of
contacts support +/- lateral motion with a gap of exactly 1.595 inches
each. In reality, the structure is arbitrarily situated inside its
enclosure, but the easiest way to input the 1.595 clearance is to assume
a perfectly symmetric installation,
The transient excitation is applied as a time-varying acceleration
by using the ACEL command in the x, y and z directions. Since the
structure is symmetric and the gaps are identical, the ux and uy
displacements at the 4 top corner nodes are identical (for the displayed
number of digits) until some small numerical difference causes one of the
gaps to close at a fraction of a second before the others, The time at
which this occurs is a function of the machine accuracy. It is only the
high accuracy of double precision arithmetic which allows the 4 sets of
contacts to close simultaneously for the first few collisions.
This phenomenon has been demonstrated in a simpler model as a 2
dimensional block bouncing on a flat structure. This model has been used
by ANSYS to demonstrate differences caused by computer precision.
We request that this information notice he put on hold until these
technical issues have been resolved. We are confident that the behavior
observed and reported is not an error in the ANSYS program.
Sincerely,
ANSYS, Inc.
William J. Bryan
Quality Assurance Manager
cc: Holtec
A. Soler
ANSYS, Inc,
D. Looman
M. Imgrund
J. Bittner
D. Conover
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