ML20206B169

Topical Rept Evaluation of General Nuclear Sys,Inc Castor V/21 Borated Stainless Steel Basket Evaluation. Borated Stainless Steel Basket Design Acceptable for Types of Spent Fuel Previously Identified in Rev 1 to 850930 Rept

ML20206B169
Person / Time
Issue date:	04/03/1987
From:	NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
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ML20206A843	List: ML20206A839 ML20206B169
References
REF-PROJ-M-37 NUDOCS 8704080362
Download: ML20206B169 (6)

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REVIEW OF CASTOR V/2180 RATED STAINLESS STEEL BASKET' EVALUATION SUBMITTED BY GNSI JANUARY, 1987

1. Backaround On September 30, 1985, examination of a Castor V/21 cask revealed cracks ct some of the welded joints on the basket following a series of demonstration t2sts conducted at Idaho National Emergency Laboratory. This event immediately raised doubts about the adequacy of the basket design. Initial. speculation regarding the cause of the cracks pointed to the possibility of interfere'nce due to the gap between the basket and the cask body being smaller tiian the thermal expansion of the basket. On the other hand, the possibility that the borated stainless steel, from which the basket was fabricated, did not have sufficient ductility to accommodate the etrains that might be expected was also cause for concern. This concern was heightened when it was discovered that the elevated temperature material properties of the borated stainless steel reported in the TSAR were not based upon test but reflected ASME code data for stainless steel without boron. The large spread between yield strength and ultimate strength values indicated a high level of ductility that might not be realized with borated stainless steel.

The preliminary report submitted by GNSI in November of 1985 did not adequately explain the cause of the cracking or enhance our confidence in the ability of the material to resist brittle fracture. At a meeting on December 13, 1985, we expressed our concerns in great detail and formulated acceptance criteria for qualifying the basket design. The main items to be addressed by GNSI were documented and appear in summary form in a letter from John Roberts to GNSI dated December 20, 1985. The report submitted by GNSI by letter dated August 15, 1986, contained a credible explanation for the.we.l.d cracking and detailed steps to be taken to avoid this't'ype of fracture. However, the data to qualify the material contained too many uncertainties and anomalies e704080362 870403 1

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to justNy the minimum' mech'aMcal properi.ies stipulated in the material speci-fications. As a consequence, the allowable stresses became-subject to the same degree of uncertainty. Neither did the material evaluations satisfactorily address the brittle fracture concerns. A further meeting with GNSI took place en October 7,1986, wherein specific acceptance criteria were agreed upon with regarding to qualifying the borated stainless steel. A revised report was subsequently submitted .by GNSI by letter dated February 5,1987.

2. Cause of Basket Weld Failures To assure proper alignment of the themocouple lances with the installed fuel assemblies during the tests at INEL, GNSI limited the lateral displacement of the basket relative to the primary lid from 0.8 to 1.5 mm (.031 to

.059 inches). These dimensions were determined by separate measurements of the basket outside diameter and the inside diameter of the cask body. Under the test conditions the radial expansion of the basket was calculated to be 4 mm l

(.159 inches). The difference has to be absorbed by compression of the basket f plate elements. Assuming, conservatively, that the gap in the cold condition w:s 0.059 inches, the basket is free to expand this distance with no other constraint than the reaction of the barrel. When the basket just touches the cask wall the load due to barrel constraint is -2584 pounds and stresses in all members are elastic. As the expansion continues the load in the basket increases sharply reflecting now the stiffness of the basket plates. Elastic deformation

can only be sustained for a compression of about 0.01' inches. Beyond this the force on the 10 mm plate is high enough to compress it plastica 11y for the l

remaining expansion of 0.099 inches. On cooldown the load on the 10 mm plate

' is relieved elastica 11y until it just loses contact with the cask body. At this point the barrel of _the basket is still exerting a compressive force of

-2584 pounds on the plate. On cooling to room tesiperature, the force on the plates reverses from compression to a maximum tension load of 4336 pounds.

! Based upon a finite element model, GNSI computed a residual tensile force of 4284 pounds. Since there is uncertainty about the temperature at which the l

tensile stress became equivalent to the ultimate tensile strength of the l

material, we could assume, conservatively, room temperature values of 81.2, ksi.

2 Even so, with an effective weld area of 0.039 in /in, the load required to 2

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fracture the weld would be' 3160' pounds 'which is: substantially lower than the e

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4336 pounds applied. Therefore,we agree with the GNSI conclusions that the

' cause of weld cracking was plastic yielding and subsequent relaxation of the basket plates due to thermal expansion greater than the as-built clearances between the basket and the cask body. Given the magnitude of the loads, this failure would have occurred even if the material of the basket had been unborated stainless steel. .

It is obvious, then, that to avoid this type of failure, adequate clearances need to be specified for the design and adequate inspection procedures adopted to assure that these clearances are achieved during fabrication. This issue is addressed in the section designated Appendix 8: Summary of Basket Dimension Measurement Technique. We believe that adoption of these procedures will elin-inate weld failures due to constrained thermal expansion. .

3. Material Evaluation Borated stainless steel is not described by any authoritative standard or specification. Since it does not have the same properties as unborated stain-less steel, it is incumbent upon the applicant to fully characterize this material so that its response to all the cask loading conditions may be evalu-ated. This includes furnishing minimum, mechanical properties data that can be used to establish allowable stresses for normal and accident conditions. In S

, addition, the ductility and toughness characteristics must be defined for both Furthermore, the base metal and welded structures made from this material.

j assurance of the quality of the material must be provided by a specification sufficiently detailed to guarantee the uniformity and reproducibility of the l

material properties important for the evaluation of safety.

i In lieu of an authoritative standard GNSI has submitted a detailed specification in ASTM format for borated stainless steel intended for use as a f

i basket material. This specification, designated BS 05 Rev. 2 meets the require-ments of a quality standard since it defines the material with sufficient i specificity with respect to composition, heat treatment, and mechanical; proper-ties and describes adequate procedures for testing and inspection. While the i

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specification provides as'suran'e c that the material used ior fabr~icating baskets -

will be of uniform and consistent quality, it does not completely define the properties that it might possess to accommodate'all the loading' conditions.

l I The characterization of the borated stainless' steel was established by.a comprehensive test program designed to demonstrate the strength, ductility and toughnes_s, of both the base metal and welded assemblies. The sampling proce-dure incorporated provisions for preparing test specimens from a large popula-tion of projuction plates and in sufficient quantity to establish valid statis-tical parameters for the. properties evaluated. Quality testing standards were used for tensile tests (DIN 50 145, ASTM method E8)'and the toughness test (ASTM method E 604). Testing was perfomed at temperatures bracketing a range that was not expected to be exceeded over the design life of the cask.

The strength and ductility properties reported were "A" basis, reflecting a probability of 99% that the values would be exceeded at a confidence level of 95%. Analyses were performed demonstrating the validity of the assumed normal

distributions. The dynamic tear test results showed that the material was not subjected to brittle fracture behavior over the temperature range from -60*C to 350 C for the thickness of the plates used to fabricate the basket. It should be noted that the 100% shear fracture demonstrated by this material exceeds the requirements for Category I fracture toughness recommended in NUREG/CR-1815 for ferritic steel in this thickness range. The implication is that the borated i

steel base material in the thickness range used for the basket should be

capable of fracture arrest behavior.

In conclusion, we find that the test program reported by GNSI has adequately characterized the borated stainless steel, made in accordance with specification

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BS 05 Rev. 2, for the purpose of basket design.

4. Structural Evaluation i

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4.1 Nomal Handling Accelerations The analysis of the basket for normal handling koa'ds is adequately covered in the paragraph designated "4.2.1.6 Fuel Basket Analysis" in the subject report.

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The summary of fueli basket stresses and limits in Table 4.2-5 shows that all -

stresses are below the allowable values and no failures under normal loading ccnditions for the' structures analyzed are expected. However, most of the welds-were not addressed and, while our confirmative analysis indicates that they are low enough to be of no concern, for completeness they should be reported in the revised TSAR. We also believe that the loading model for the chevron shown in Fig. 4.2-21 is incorrect. The length should be 159.45/16 = 9.97 inches.

However, we conclude that this would not significantly affect the results of the analysis.

4.2 Thermal Stress Analysis f

I The analysis of the thermal stress in the basket is adequately covered in the paragraph designated "4.2.1.4.2 Thermal Stress Analysis" in the subject

report. The thermal stresses and strains in the unconstrained basket show large margins of safety. Stresses resulting from en off-center basket have bzen addressed and shown to be of no concern.

4.3 Accident Analysis 1

The structural adequacy of the basket was investigated by staff by performing a confirmatory analysis for the 5-foot 2-inch side drop and tip over accident conditions. The analysis utilized a finite element model of the basket and assumed that the deceleration loads could be repre'sented as uniform pressure

loads on the basket plates supporting the fuel assemblies. Our previous analysis determined that the deceleration of the cask on the concrete slab resulted in a

! load equivalent to 150 g's. However, since the fuel assemblies and basket are significantly more flexible than the cask body, deamplification of the decelera-i tion loading was considered likely. To determine the degree of deamplification,

! the fuel rod natural frequency was compared with the vibration frequency of the cask dropping onto the slab.

It was determined that the dynamic loading on the basket can be expected not to exceed the 52 g's assumed by GNSI in their analysis. Our confirmatory cnalysisshowsthat52g'sproducesamaximumstressiNtensitybelowtheallow-able limits for the materials.

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U Based on' these analyses and our preyious review efforts .in particular ,

with respect to criticality, shielding and! thermal analyses, we find that the-GNSI-borated stainless steel basket design is acceptable for those types of PWR

• spent fuel previously identified in enclosure 1 of the NRC staff's ' letter of approval' dated September 30, 1985, to GNSI for Revision 1 of the TSAR.

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