Part 21 Report - 1997-790

ACCESSION #: 9710080070 Holtec Center, 555 Lincoln Drive West, Marlton, NJ 08053 Telephone (609) 797-0900 HOLTEC Fax (609) 797-0909 INTERNATIONAL June 27, 1997 Mr. Thomas Greene, NRR/PECB U.S. Nuclear Regulatory Commission Mail Stop: 0-11E4 Washington, DC 20555-0001 Reference: Accession 9601190322 Subject: 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: On October 16, 1995, Holtec International had issued a 10CFR21 notice to Region I of the USNRC advising the Commission of our company's discovery of a serious deficiency in the general purpose finite element Code ANSYS. Our analysts found that ANSYS produced non-unique results (different results on different computers) when a dynamic model of a free-standing submerged fuel rack for a DOE facility was subjected to 3-D seismic excitations. Inasmuch as free-standing spent fuel racks in spent fuel pools are stored at numerous commercial and military installations, this discovery of numerical instability in ANSYS warranted that we inform the Commission immediately. ANSYS is part of Holtec's library of QA validated codes, but we have administratively prohibited the use of this Code over the past ten years in dynamic analysis of free-standing underwater structures. Our initial reason was the lack of suitable elements in ANSYS to model fluid coupling effects in multi-body problems (such as array of fuel racks in a pool). All versions of the Code to date have continued to suffer from the above deficiency with respect to modeling of fluid coupling in an array of racks. The 1995 analysis was conducted at the behest of the DOE, which led to the discovery of the numerical discrepancy in solutions obtained on different computers. Holtec Center, 555 Lincoln Drive West, Marlton, NJ 08053 Telephone (609) 797-0900 HOLTEC Fax (609) 797-0909 INTERNATIONAL Mr. Thomas Greene, NRR/PECB U.S. Nuclear Regulatory Commission June 27, 1997 Page 2 As the attached Holtec internal memo states, verbal interactions between Holtec and NRC personnel may have obfuscated the true nature of the deficiency in ANSYS. Therefore, I request that the Commision review the information provided by our Dr. Soler in the attached memo, and act on our Part 21 notification accordingly. We have no knowledge of whether the problem still exists in more recent updates of the ANSYS Code. ANSYS is a robust finite element Code which has served as a reliable workhorse in our day-to-day structural analyses. However, we continue to treat this Code as unsuitable for simulating complex nonlinear transient problems (such as dynamic modeling of spent fuel racks) which may be of safety consequence to the nuclear industry. By our notification of October 16, 1995, our company has discharged its obligations per 10CFR21. However, Holtec QA feels an obligation to provide to your attention the contents of the attached inter-office memo. We would be pleased to provide additional technical information if required by the Commission. Sincerely, Vikram Gupta Quality Assurance Manager VG:nlm cc: Mr. Gautam Bagchi, Structural/Geophysical Branch cc: Chief, Region I, King of Prussia, Pennsylvania Attachment: Inter-Office Memo from A.I. Soler to V. Gupta INTEROFFICE MEMORANDUM TO: Vik Gupta, Q.A. Manager FROM: Alan I. Soler, V.P. of Engineering, Holtec International DATE: June 23, 1997 SUBJECT: Problems with use of ANSYS in Fuel Rack Analyses On Wednesday, June 18, 1997, a meeting was held concerning spent fuel rack licensing for the TVA, Watts Bar plant. During the course of that meeting, I had occasion to reference the 10CFR21 issue on ANSYS 1_/ raised by Holtec (which eventually resulted in a phone conversation between Holtec and the NRC subsequent to ANSYS Inc. presenting some additional material in June, 1996). The Branch Chief of Structures and Geosciences, Dr. Bagchi, questioned me on the subject and quite emphatically asked why Holtec withdrew the 10CFR21 notification. I replied to him that to my knowledge and recollection Holtec did no such thing, and that while I did acknowledge over the phone and in a subsequent fax transmission that had ANSYS provided to me the information that they provided to the NRC, I might have been able to reconsider the issue, that verbal and taxed statement in no way implied a "withdrawal of the notification". It is not even clear to me that Holtec is permitted to withdraw such notification and even if we are, such notification would come from the QA manager at Holtec. At the time, It was my understanding that NRC was going to make a decision internally concerning the issuance of the notification, and it is my recollection of the phone conversation that Dr. Kim (structural branch) thought it should be issued even if only to warn the industry that despite all of the disclaimers in the ANSYS manual, non-linearities are not easy things to deal with and that problems may occur with even the most experienced users. There appears to be the incorrect perception at the NRC that we caused NRC to expend resources to evaluate a potential problem and then retracted the query at the last minute. This perception should be dispelled. Subsequent to the June 18, 1997 meeting concerning Watts Bar (which does not involve any use by Holtec of ANSYS for non-linear dynamic analyses), I have given more thought to the 10CFR21 notice issue observations: I have the following: _________________________ 1_/ Holtec Part 21 letter dated October 16, 1995 to Region 1, USNRC. _________________________ 1. ANSYS is the general purpose finite element code that has been used in the past and continues to be used at Holtec for a wide variety of linear and non-linear analyses. In accordance with our own internal QA requirements, we independently confirm that the example problems provided by ANSYS are reproducible on the PC computer platforms in use at Holtec. Each version of ANSYS is subject to a comprehensive Q. A. validation process before it is accepted for use within the company's Q.A. system. 2. As a result, ANSYS is qualified for use in a wide variety of problems, except the specific class of nonlinear problems involving underwater response of free standing structures. Holtec, with a single exception of a DOE project, has not used ANSYS to simulate the non-linear, time dependent behavior of an array of spent fuel racks in a spent fuel pool subject to a 3-D seismic event. Over 20 years ago, we developed and validated our own computer code which is not a general purpose code, but is directly focused on the particular aspects of spent fuel rack analysis which are not amenable for modeling by a general purpose code. The special treatment of near field and far field fluid coupling of a generally rectangular array of closely spaced bodies, the fact that the physical problem involves a multitude of free standing bodies which may make and break contact with adjacent bodies, the fact that frictional effects must be considered at some of the same locations, and the fact that the solution requires a time history analysis with appropriate time stepping algorithms taken in aggregate make this simulation of the behavior of an array of spent fuel racks a complex problem. ANSYS, in some of their responses, admit to the inabilities of the code to analyses such complex problems. Further, in my opinion, the existing fluid element in ANSYS, developed for two concentric cylindrical bodies with a small fluid filled annulus between the bodies, is not capable of correctly modeling the near and far field effects of an array of adjacent rectangular bodies. As I noted to Dr. Bagchi during the Watts Bar June 18,1997 meeting, Holtec: has used ANSYS in only a single spent fuel rack application to the Department of Energy at the specific request of DOE. It was this single application of ANSYS to the spent fuel rack simulation that led Holtec to issue the 10CFR21 notification which raised an issue of multiple solutions occurring on different computer platforms. 3. The complete record of the various submittals to ANSYS by Holtec and the responses from ANSYS to Holtec was given to the NRC by Holtec as part of the 10CFR21 package submittal. It is not necessary here to rehash the details of those documents here. However, among other items, the responses by ANSYS acknowledged that they felt that the problem was highly complex, involved severe non-linearities, and suggested that additional damping would possibly eliminate the multiple solutions. The lack of any more specific root cause evaluation on ANSYS's part left me with no recourse save to comply with federal regulations and issue the 10CFR notification in a timely manner. 4. Subsequent to the NRC issuance of a draft notice for comments to both Holtec and to ANSYS, ANSYS finally provided a simple 2-D solution which did not simulate behavior under a 3-D load input, but simulated the vertical drop of a box onto a fixed surface subject only to a gravity field. The problem focused on the two gap elements that simulated the contact. The response to be expected was that of a "pogo stick" with the rigid box bouncing in the vertical plane with the uni-dimensional motion preserved. ANSYS demonstrated that this "perfect" or "idealized" non-linear simulation problem did lead to multiple solutions on two different platforms although it did not address whether a multiplicity of platforms would give only the same two differing solutions in the more complex fuel rack problem. The conclusion suggested by the ANSYS simulation, prompted by the draft NRC notification, was that the inference that some of the gaps in the Holtec problem had the same initial value and that this created a "perfect" or "idealized" configuration which would be prone to multiple solutions. ANSYS demonstrated their thesis by slightly perturbing the initial gap on one of the two elements in their simple model at time 0.0, and demonstrated that this eliminated the "perfect symmetry" and led to a single solution on both platforms. I concurred with their conclusions that a perfect" or "idealized" solution could certainly lead to multiple solutions. However, I did not concede that the fuel rack problems wherein the non-unique solutions were produced by ANSYS had the perfect symmetry of the 2-D problem. Neither did I offer that the draft notice should be withdrawn. It was my understanding that serious discussions were to be undertaken within NRC as to the necessity or advisability of a notice or suggesting additional clarifications by ANSYS as to the limitations of this code within their manuals. 5. In subsequent conversations by phone with Dr. Bagchi on June 19, 1997, I received the impression, perhaps erroneously, that the final resolution by the NRC not to issue the notice or to request some additional discussion by ANSYS was based on an internal NRC conclusion that ANSYS had sufficient general warning in their documentation about the potential pitfalls of non-linear simulations, and that Holtec had most likely erred by failing to recognize that they had a number of gap elements with the same initial gap which would potentially simulate a "non-realizable or perfect" problem. Upon serious reflection subsequent to the June 18, 1997 meeting, I felt that this issue needed to be addressed and upon further consideration, I do not believe that the ANSYS response really closes the issue. 6. ANSYS makes much of the fact that the Holtec spent fuel rack solution using ANSYS involved a number of non-linear elements, a transient solution, no damping at the gap elements, and Holtec's choice of a "perfect" problem. In reality, while the first two assertions are correct, the latter two are not. The appropriate structural damping, mandated by the DOE specification, was present in the solution, and I had previously noted this fact to ANSYS during our initial queries on the problem. Further, the fuel rack problem was not "perfect". While it is true that some of the various gap elements in the model had initial gaps that were either 0.0 or were chosen to reflect the reality of the simulation (i.e. a spent fuel rack initially at rest on a supposedly flat slab), the fact of the matter is that the model was real and truly represented an achievable initial state. That is, there was no artificial construct in an attempt to simplify the solution. Of most importance is the fact that the loading in the Holtec model was not symmetric and by itself served to provide the initial perturbation needed to drive the simulation to a unique solution. The input seismic loading had three components; therefore, no matter what "perfection" existed at time=0.0, at every time greater than zero, the gap elements experience different forces at all subsequent time instants. This is markedly different from the ANSYS test solution where the problem should always respond in a completely symmetric manner as a single degree of freedom model throughout the time of the event. On the basis of the above, my feeling is that the response given by ANSYS which tends to absolve the code at the expense of the user, (sophisticated or unsophisticated) is not so clear cut. Either ANSYS should stable the model request for a spent fuel rack dynamic analysis simulation is too complex for implementation on its program, or take steps to ferret out the still hidden interaction between gap elements and the solution algorithm which makes their solution suspect. I believe that there considerations should be passed on to the USNRC. *** END OF DOCUMENT ***

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