ML15275A025
| ML15275A025 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 10/23/2014 |
| From: | Wong W AREVA |
| To: | Office of Nuclear Material Safety and Safeguards |
| Shared Package | |
| ML15275A022 | List: |
| References | |
| L-MT-15-056, TAC L24939 1301415.301, Rev. 0 | |
| Download: ML15275A025 (40) | |
Text
L-MT-1 5-056 ENCLOSURE 3 STRUCTURAL INTEGRITY ASSOCIATES, INC.
CALCULATION PACKAGE 1301415.301 TITLE:
DEVELOPMENT OF AN ANALYSIS BASED STRESS ALLOWABLE REDUCTION FACTOR (SARF)
DRY SHIELDED CANISTER (DSC) TOP CLOSURE WELDMENTS REVISION 0 OCTOBER 2014 39 pages follow
iStructural Integrity Associates, Inc.
File No.: 1301415.301
~Project No.: 1301415 CALCULATION PACKAGE Quality Program Type: [] Nuclear LI Commercial PROJECT NAME:
Monticello ISFSI - DSC 11 through 16 Exemption Request CONTRACT NO.:
1005, Release 48, Amendment 6 CALCULATION TITLE:
Development of an Analysis Based Stress Allowable Reduction Factor (SARF) - Dry Shielded Canister (DSC) Top Closure Weldments Document Affected Project Manager Preparer(s) &
Reiin PgsRevision Description Approval Checker(s)
Reiio agsSignature
& Date Signatures & Date 0
1 - 30 Initial Issue Preparer:
A-i1-A-2 J,*g6 B-i B-7
~/Wilson Wong Richard Bax 10/23/14 10/23/14 Checkers:
James W. Axline 10/23/14 J. Wu 10/23/14 C. Fourcade 10/23/14 Page 1 of 30 F0306-O01R2
j7Sbvo~urul Iategrily Associates, inC.u Table of Contents 1.0 OBJECTIVE................................................................................. 4 2.0 TECHNICAL APPROACH................................................................ 5 2.1 Finite Element Model and Flaw Simulation....................................... 5 3.0 ASSUMPTIONS / DESIGN INPUTS...................................................... 6 4.0 CALCULATIONS........................................................................... 7 4.1 Pressure Loading.................................................................... 7 4.2 Side Drop Loading.................................................................. 8 5.0 RESULTS OF ANALYSIS................................................................. 8
6.0 CONCLUSION
S AND DISCUSSION................................................... 10
7.0 REFERENCES
............................................................................. 12 APPENDIX A ANSYS INPUT FILES......................................................... A-i APPENDIX B SI REPORT 1301415.405, REVISION 0, "EXPECTATIONS FOR FIELD CLOSURE WELDS ON TilE AREVA-TN NUHOMS 61BTH TYPE 1 & 2 TRANSPORTABLE CANISTER FOR BWR DRY FUEL STORAGE,"........................................................ B-i File No.: 1301415.301 Revision: 0 Page 2 of 30 F0306-0 1R2
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List of Tables Table 1: OTCP Stress Reduction Factor Results - Pressure Loading.......................... 13 Table 2: OTCP Stress Reduction Factor Results - Side Drop Loading........................ 14 Table 3: ITCP Stress Reduction Factor Results - Pressure Loading........................... 15 Table 4: ITCP Stress Reduction Factor Results - Side Drop Loading......................... 16 Table 5: OTCP and ITCP Deflection Load Cases - Pressure Load Case...................... 17 List of Figures Figure 1. Finite Element Model and OTCP and ITCP Details.................................. 18 Figure 2. OTCP Postulated Flaw Configuration - Radial #1.................................... 19 Figure 3. OTCP Postulated Flaw Configuration - Radial 12.................................. 20 Figure 4. OTCP Postulated Flaw Configuration - Laminar..................................... 21 Figure 5. OTCP Postulated Flaw Configuration - Circumferential #1......................... 22 Figure 6. OTCP Postulated Flaw Configuration - Circumferential #2......................... 23 Figure 7. OTCP Postulated Flaw Configuration - Circumferential #3......................... 24 Figure 8. OTCP Postulated Flaw Configuration - Circumferential 114....................... 25 Figure 9. ITCP Postulated Flaw Configuration - Circumferential.............................26 Figure 10. OTCP Pressure Load Case - Displaced Sh~ape (Exaggerated)...................... 27 Figure 11. JTCP Pressure Load Case - Displaced Shape (Exaggerated)......................28 Figure 12. Side Drop Model...................................................................... 29 Figure 13. OTCP and ITCP Stress Path Definitions............................................. 30 File No.: 1301415.301 Page 3 of 30 Revision: 0 F03 06-0 1R2
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1.0 OBJECTIVE The objective of this calculation is to develop a quantitative basis for a stress allowable reduction factor (SARF) to address weld quality in the inner top cover plate (ITCP) and outer top cover plate (OTCP) weldments of the NUHOMS dry shielded canister (DSC) system. This workscope is in support of the USNRC CofC Exemption submittal for DSC's 11 through 16, currently at the Monticello Nuclear Generating Plant (MNGP).
Weld quality is described as a global effect, for which a factor is used to reduce the stress allowables to account for potentially less than sound weldments. The SARF has historically been tied to the level of non-destructive examination (NDE) performed on the weldment. That is to say, the greater the degree of NDE performed (such as volumetric) the greater the SARF (less reduction in stress allowable).
The ASME Code [5, NG-3352] contains values for SARF for a range of NDE. Specifically, a VT only scope of NDE would state an SARE of 0.35 for a partial penetration weldment. However, it should be clearly noted that the ASME Code table for SARF's has no limitations/definitions/requirements on the weld size, the weld/base metal materials, the welding configuration, the welding position, and most importantly, the welding process. In addition, as this table is from NG, the level and comprehensiveness of the design analysis is less than that for an NP-type component, such as the DSC. The 0.35 SARF is a conservative factor that addresses all types of welding. In the case of the DSC weldments, these are specific joint geometries, with high quality materials, favorable welding positions, and again, most importantly, a high purity welding processes (GTAW), and therefore, strict adherence to the 0.35 SARF number for a VT only NDE examination weldment is not warranted.
The intent of this calculation, for this exemption request only, is to evaluate a series of postulated weld flaws and determine, for each configuration, the effect on the unflawed stress results. The effect of the stress results will be comparative, performed by comparing the analysis results of the flawed configuration to those from the same geometry, but in an unflawed configuration.
The determination of the impact on stress results will be performed by finite element analysis (FEA) in which selected elements of the ITCP and OTCP weldments will be "removed" to represent "flawed/suspect" weld quality.
Various distributions of flaw size (length and depth) and frequency (spacing), will be examined.
The intent of this calculation is to analytically determine the type of flaw distribution that would justify a specific SARF. A separate work scope has been performed to evaluate, for the specific DSC weldments (DSC's 11 through 16), what are the expected type and density of flaw distributions. It is the overall intent for this project workscope that it can be shown that the type of flaw distribution, which would support an acceptable SARF, will be of significantly greater magnitude than those populations that would be expected for the type of welding used for the DSC weldments.
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2.0 TECHNICAL APPROACH The determination of the impact of weld quality on stress results (SARF) will be performed by the finite element methods. Both the flawed and unflawed geometry of the top end of the DSC will be modeled.
To represent the presence of postulated flaws, selected elements within the model will be removed and analyses performed using representative load cases. By comparing the results from the unflawed and flawed FE models for these loads cases, a ratio, or stress allowable reduction factor can be determined.
A range of flaws will be analyzed to develop a range of SARF values corresponding to the range of flaw populations.
Typical types of flaws will be considered, and a range of distributions of flaw size (length and depth) and frequency (spacing), will be examined.
Three types of flaws will be addressed.
- Radial: a postulated flaw oriented in a plane radial to the DSC longitudinal axis and spanning the weldment from cover plate to shell.
- Circumferential: a planar flaw oriented in a plane parallel to the DSC axis and oriented circumferentially around the DSC.
- Laminar: a planar flaw in a plane perpendicular to the longitudinal axis of the DSC and spanning the weldment from cover plate to shell.
In the determination of what flaw types to analyze in the OTCP and the ITCP, the size/volume of the weldment was considered. The OTCP weldment is both large in size and volume absolutely, and also relative, to the weldment volume of the ITCP. Therefore, all three types of flaws are evaluated for the OTCP. The ITCP weldment, due to its reduced weldment size, is evaluated using a single flaw of significant cross-section, which represents elements of all three types. Figures showing these flaw types, location, and orientation are shown in Figures 2 through 9.
2.1 Finite Element Model and Flaw Simulation A single finite element model (FEM) is developed using the ANSYS finite element analysis software [2].
The model represents a 1800 sector of the upper end of the DSC. The model includes the outer top cover plate and weldment, the inner top cover plate and weldment, and a portion of the DSC shell.
The FEM utilizes the ANSYS 3-D structural element (SOLID45). The unflawed model contains all portions of the two weldments.
The modeling of the postulated flaw is done by "killing" the selected elements that represent the flaw size and location, using the EKILL command in ANSYS. This command deactivates the element such that it contributes near zero stiffness to the overall stiffness matrix. The result is a redistribution of loading and stresses around "killed" elements.
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The ANSYS model of the top end geometry is shown in Figure 1 which illustrates the full model and then localized sections through the OTCP and ITCP.
3.0 ASSUMPTIONS / DESIGN INPUTS The top end geometry of the DSC is defined in Reference 3. The OTCP, ITCP, and DSC shell dimensions, as well as the materials, are provided in Reference 3. A number of assumptions were made during development of the finite element model, which are listed as follows:
- The model consists of a half-symmetric portion of the inner top cover plate (ITCP), outer top cover plate (OTCP), and the top 20 inches of the outer DSC cylinder. The 20 inches equates to greater than 4.0-*Rt, thus avoiding any end affects at the free end constraint. The model is constructed of approximately 840,000 SOLID45 elements to ensure adequate mesh refinement for the ITCP and OTCP welds in the circumferential direction.
- The OTCP is modeled with the top surface set 1/8 of an inch below the end of the DSC. The J-groove weld preparation is as shown in Reference 3. The weldment is shown flush with the surface of the OTCP and not set below, as is allowed by the Reference 3 field assembly drawing.
The modeled set back weldment is considered acceptable as this is a comparative analysis and the same geometries are used in both the flawed and unflawed condition.
- The ITCP is modeled as a flat plate and the closure weldment is modeled flush with the top surface of the ITCP.
- The DSC shell, the OTCP, the ITCP, and the OTCP and ITCP weldments are modeled as SA-240, Type 304 stainless steel. Material properties are taken from Reference 4. Standard room temperature material properties for Type 304 stainless steel are used: Young's Modulus -- 28.30E6, Density = 0.283 lbs/in3, and Poisson's Ratio of 0.3.
- The analysis is performed at 70°F. This temperature is selected as this is a comparative analysis and both the unflawed and flawed runs utilize the same temperature.
- The bottom edge of the outer cylinder is fixed in the axial and circumferential directions, and symmetry boundary constraints are placed on the symmetry plane. For the side drop runs, the outer cylinder is released in the circumferential direction and is supported at the point of "impact" via radial displacement couples to a support block with reduced stiffness properties.
- The analyses are all treated as elastic.
- The localized effects of the vent and siphon block and the ITCP weldment are not modeled. This is acceptable as the weldment connection to the V/S block (1/4" groove) is similar to the majority portions of the ITCP weldment, and the intent is to determine the effects of global weld quality, not localized stress concentrations. The effect of stress discontinuity at the V/S block will be addressed by the design analysis which models this explicitly, and then uses the SARF to further modify' the stress allowables.
- The siphon/vent port cover plates are not modeled as the nominal stresses (primarily due to pressure) are sufficiently low to accommodate extremely low SARF's. Assuming a 3/16" closure groove weld [3] on a nominal 2 inch diameter cover plate results in a weld shear stress of File No.: 1301415.301 Page 6 of 30 Revision: 0 F0306-01R2
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- Dimensions for the components are taken as the nominal. This is acceptable as this is a comparative analysis.
- The evaluated paths for which the stress results are extracted and used for comparison (flawed vs unflawed) are shown in Figure 13.
4.0 CALCULATIONS The determination of the SARE, as a function of weld quality (number and density of postulated flaws),
is performed using two load cases. The pressure load is the primary normal and off normal load for these weldments and consists of internal pressure applied to the inner top cover and outer top cover. The specific definition and modeling details are described below for the pressure load case.
The drop load cases consist of a canister end drop, a canister corner drop, and a canister side drop. For this comparative analysis the canister side drop load case is utilized as it best represents the behavior of the drop event (an event that is germane to the MNGP ISFSI DSC hardware configuration) and is a more easily evaluated/modeled condition. The side drop load case develops localized stresses along a line of contact similar to the corner drop. The specific details for the side drop load case are described below.
4.1 Pressure Loading The pressure loading consists of a nominal 100 psig internal pressure applied to the top cover plates. For evaluation of the ITCP (the nominal pressure boundary) weldment quality, the pressure is applied to the inside surface of the ITCP and the DSC shell, and the contacting surfaces between the ITCP and OTCP are bonded with sliding capability using ANSYS contact elements to allow for load transfer from the JTCP to the OTCP. For the ITCP pressure analysis, CONTA174 and TARGE170 contact elements were used to prevent the ITCP from penetrating the OTCP. In these cases the OTCP acts as a non-pressure retaining structural support for the JTCP. Figure 11 shows the displaced shape for the ITCP pressure load case.
For evaluation of the OTCP weldment quality, the pressure is applied only to the inside surface of the OTCP and the inside surface of the DSC. The ITCP and the weldment to the shell are both contained within this model and are not modeled as containing flaws, nor are they loaded by pressure. The intent of applying the pressure loading to the OTCP alone is to maximize the response of the OTCP-to-DSC shell weldment, as a result of postulated flaws within the weld. Applying the pressure to the ITCP, which in turn will load the OTCP, will diminish the response of the OTCP, as there exists supplemental stiffness from the ITCP. Figure 10 shows the displaced shape for the OTCP pressure load case.
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4.2 Side Drop Loading The side drop loading case is evaluated as a static 75G load case in which the FEM of the DSC shell is oriented with the symmetry plane in the direction of the drop. For the side drop analysis, the same contact element types (CONTA 174 and TARGE 170) were used to prevent the ITCP from penetrating the DSC outer cylinder. These are not used for the OTCP weld prep-to-DSC shell potential contact region, as the area of potential contact is small relative to the OTCP weld size.
To simulate the support of the transfer cask, the lower 200 of the DSC model is supported by a material which represents the stiffness of the transfer cask given that there is a difference in diameter between the DSC and the transfer cask. In the transfer condition, the DSC is supported within the Transfer Cask on thin guide rails, and the use of a lessor stiffness support in the lower 20° degree region is representative.
Again this is a comparative analysis and the intent is to show the effect of weld quality in the weldments in the most highly stressed area of contact, which is at bottom dead center. Radial displacement couples between the DSC and support block are used. Figure 12 shows the geometry of this load case.
5.0 RESULTS OF ANALYSIS The determination of the SARF for a given postulated flaw population is performed by extracting the stress results from the unflawed geometry, and the flawed geometry for the specific load case. These stresses are extracted and linearized along identical paths to capture the change in stresses due to the missing/flawed elements.
The comparison to determine the change in stress results, as a result of the postulated flaw population, typically compares the linearized membrane (Pm) and membrane plus bending (Pm + Pb) stress intensities for a path adjacent to the postulated flaw and at other regular spacings between the postulated flaws.
These discrete ratios are then combined to produce a weighted SARF for the weld flaw pattern.
Figure 13 shows the path locations and orientations for the three types of flaws for which stresses are extracted.
In general the comparison of stress results is done by comparing linearized membrane (Pmo) and membrane plus bending (Pmo + Pb) stress intensities. However, in the case of the side drop event for the radial and laminar flaws, the high compressive stresses in all three principal stresses make the use of stress intensity not representative. In these cases, where all three principal stresses are compressive, and the resultant stress intensity is of lesser magnitude than the principal stresses, the resulting SARF's are unrealistic. In these cases the greater stress values of the three principal stresses are combined by SRSS and compared for the flawed and unflawed configuration.
An initial set of postulated flaw populations for the radial, circumferential and laminar flaw were developed and analyzed. Subsequent to initial runs, additional flaw populations for the radial and circumferential flaw cases were run. The specific geometry of the flaw populations are shown in Tables 1 through 4, along with the resulting SARF's.
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It should be noted that the intent of the calculation is to show a flaw population that is severe and thus demonstrate that large flaw populations (size, length, and density) can be tolerated, as the calculated SARF is acceptable. In the selection of the flaw population parameters, the depth of the flaws is typically set as a through-wall flaw. Obviously, such a flaw would have been unacceptable, and would have been identified by leak test examination. However, the intent of this calculation is to address structural capacity of the weldment, not confinement.1 Thus the use of the through-wall flaw allows for a conservative determination of the SARF.
Table 1 documents the calculated SARF's for the OTCP weldment subjected to pressure loading.
Table 2 documents the calculated SARF's for the OTCP weldment subjected to the side drop loading.
Table 3 documents the calculated SARF's for the ITCP weldment subjected to pressure loading.
Table 4 documents the calculated SARF's for the ITCP weldment subjected to the side drop loading.
Table 5 presents the axial deflection at the centerline of the OTCP for the various flaw configurations analyzed for the pressure load case. The intent is to show that, as expected, the stiffness of the combined OTCP and ITCP is greater (less deflection) than the OTCP alone. This is the reason that the pressure loading was applied to the OTCP alone, so as to maximize the deflection of the OTCP, and therefore challenge to the OTCP weldment. A review of the table shows that the change in deflection of the OTCP as a result of the introduction of postulated flaws, in either the OTCP or ITCP weldment, is relatively low (< 15% in the worst case). Thus the evaluation of flaws does not require the explicit evaluation of concurrent flaws in the OTCP and ITCP, as their responses (unflawed/flawed) are basically similar, and~this is a comparative evaluation.
In addition, a comparisons of the deflections of the OTCP in the unflawed and postulated flawed cases shows that for the less severe, but still significant flaw populations (Radial 2, Laminar, Circ 3, and Circ 4), the change in response (OTCP deflection) is small, typically 1% or less. It can therefore be presumed that a mix of flaw types would produce similar results as that for a single flaw type, e.g. a mix of radial, laminar, and circumferential flaws would have similar results as that for the bounding single flaw type. The worst case SARF for the selected flaw types will be utilized, thus any substitution of lesser SARF flaws (e.g. laminar) for greater SARF flaws (Circ) would be bounded.
Finally, the postulated 50% circumferential flaw for Circ 4 is positioned in the upper half of the weldment. The change in SARF values (Tables 3 and 4) between the Circ 3 and Circ 4 cases is an increase of 4% for the pressure case, and -14% for the side drop case. A 50% through-wall flaw, located in the lower portion of the weldment, would have an SARE no worse than the Circ 3 case, and the Circ 3 case SARF, for both pressure and side drop, is greater than 0.80. The placement of the 50%
through-wall flaw in the lower half of the weldment would thus not change the results to a point where the Circ 3 case would not be bounding.
SThe results demonstrate that the remaining ligaments of the DSC weidments have sufficient structural capacity, even with very severe and conservative penalties (postulated flaws) for nonconforming PT examinations, to perform their design function of restraining the OTCP and ITCP's, and additionally maintaining the confinement function during all service level load cases.
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6.0 CONCLUSION
S AND DISCUSSION The OTCP and ITCP weidments are made using both materials and processes, and in conditions which would result in high quality (very small flaw distribution). Specifically it is a stainless steel weldment made with argon cover gas in a flat position using a machine GTAW process. As such, concerns over weld porosity are minimized and the machine welding process will produce a very uniform and consistent weldment. Report 1301415.405 [1, See Appendix B] details the expected flaw distribution for this type of weldment.
A review of Tables 1 through 4 documents the calculated SARF for the selected flaw populations. The question of which flaw population to consider representative or typical, or bounding is based not on these analytical results but on the separate Reference 1 report. This report is based on the actual elements of the OTCP and ITCP welding, and considers industry experience and ISFSI Vendor experience [1, See Appendix B].
Reference 1 states in the conclusion that:
It is suggested a bounding subsurface defect condition is conservatively represented as an intermittent lack of fusion (LOF) defect evenly distributed along the canister weld. Further, the total length for LOF is conservatively estimated at 25% of the canister cover plate weld circumference. The estimated through thickness dimension is 1/8 inch, because this dimension represents a maximum weld bead thickness. One eighth inch is considered to be a conservative assumption, because it is recognized that most weld beads will be thinner especially as the weld cavity begins to fill. No credit is being taken for remelting even though remelting is normally associated with multipass welding."
Comparing this to the analyzed flaw populations:
OTCP: Both the radial and laminar flaws are not representative of the circumferentially oriented flaw described above. However, in both cases, the postulated flaws for these types are full thickness and full width, and thus would be considered more severe than a 1/8" thick, 25% total weld length flaw, with a width of one weld bead. As an example, the laminar flaw is the full width of the weld, and covers 72% of the circumferential arc. The radial Configuration 2 flaw (more limiting), shown in Figure 3, is a full height (through-wall) flaw, spanning the full weldment width, and occurring less than 2" apart.
The circumferential flaw, Configuration 3, shown in Figure 7, is a full height (through-wall) flaw, 1" long and occurring every 5". The 1" in 5" spacing is a 20% occurrence of postulated flaws, which although less than 25%, is tempered by the fact that the analyzed flaw is full height, not the expected one bead thickness dimension ('- 1/8") described above. With this consideration, the Configuration 3 circumferential flaw bounds the "conservatively assumed" flaw stated in Reference 1.
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ITCP: The 360 degree embedded flaw postulated and evaluated (Figure 9), is much more adverse than the expected flaw of Reference 1 described above.
In both the OTCP and the ITCP weidments, the weld is a multi-layer weldment, and both received multi-level VT and PT examinations. Although the PT cannot be credited, the VT can be assumed to have seen large surface breaking flaws. As a further argument that the postulated and analyzed flaws are bounding for flaws that would have not have been identified by the VT exams, the likelihood that multiple through-layer thickness flaws of the postulated percentage of arc length (e.g. the Circ 3 case flaw covers 20% of the total arc length) would occur in every layer, and would also line up with flaws below and above to create a through-wall combined flaw, and not be detected by the multiple VT's, is highly unlikely and not realistic.
Again the use of through-wall flaws is done to evaluate the structural integrity of the weldments. The validation of confinement of the weldments was separately confirmed by successful leak testing.
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7.0 REFERENCES
- 1. SI Report No. 1301415.405, Revision 0, "Expectations for Field Closure Welds on the ARE VA-TN NUHOMS 61BTH Typel1 & 2 Transportable Canister for BWR Dry Fuel Storage," October 2014, SI File No. 1301415.405. [Appendix B]
- 2. ANSYS Mechanical APDL and PrepPost, Release 14.5 (w/ Service Pack 1), ANSYS, Inc.,
September 2012.
- 3.
AREVA Design Drawings for the 61BTH, Type 1 and 2, NUH61BTH-3000, Rev 1, "NUHOMS 61BTH Type 1 DSC Main Assembly," and NUII61BTH-4008, Rev 1, NUHOMS 61BTH Type 1 & 2 Transportable Canister for BWR Fuel Field Welding, PROPRIETARY SI File No. 1301415.201P.
- 4. ASME Boiler and Pressure Vessel Code,Section II, Part D, Material Properties, 2004 Edition.
- 5. ASME Boiler and Pressure Code,Section III, Division 1, Rules for Construction of Nuclear Facility Components, 2004 Edition.
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Table 1: OTCP Stress Reduction Factor Results - Pressure Loading File No.: 130 1415.301 Revision: 0 Page 13 of 30 F0306-0 1R2
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Table 2: OTCP Stress Reduction Factor Results - Side Drop Loading SIDE DROP Radial r
Laminar Circ #I Cire #2 Circ #3 jCirc
- 4 Pm Pm++/-Pb(I)
Pm+Pb(O)
Pm Pm+Pb(I)
Pm+Pb(O)
Pm Pm+Pb(I)
Pm+Pb(O)
Pm Pm+Pb(I)
Pm+Ph(O)
Pm Pm+Ph(I)
Pm+Pb(O)
Pm Pm+Pb(I)
Pm+Pb(O)
Aege 0.976 0.921 j0.912 0.882 0.957 1.000 0.542 0.606 [0.762 0.720 0.756 0.903 0.846 0.861 (0.972 0.979 0.974 j0.974 IN0.912 0.882 0.542 0.720 0.846 0.974 Through Wall Flaw Through Wall Flaw Thr'ough Wall Flaw Through Wall Flaw Through Wall Flaw 50% Part Thr'ough Wall Flawv Pattern Arc 084 Pattern Arc 570 Pattern Arc
.14 Pattern Ar~c 584Pattern Arc 514Pattern Arc Spacing (in)
Spacing (in)
Spacing (in)
Spacing (in)
Spacing (in)
Spacing (in) 5.4 FaWit(i) 044 Flaw Arc 416 Flaw Arc Flaw Arc 206Flaw Arc 102Flaw Arc1.2 FaWih(i) 044 Length (in) 416 Length (in) 3.600 Length (in) 206 Length (in) 102Length (in)1.2 Un-Flawed Arc
.70 Un-Flawed Arc 154 Un-Flawed Arc
.84 Un-Flawed Arc 318 Un-Flawed Arce 7
Un-Flawed Arc 4.7 Spacing (in)
Spacing (in)
Spacing (in)
Spacing (in)
Spacing (in)
Spacing (in)
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Table 3: ITCP Stress Reduction Factor Results - Pressure Loading ITCP Pressure Pm Pm+Pb(I)
Pm+Pb(O) 0.964 1.000 0.954 0.954 Flaw Cross Section 0.6in Area Pattern Arc Spacing (in) 5.8 Flaw Arc Length (in)2.9 Flaw Arc Spacing (in) 2.9 File No.: 1301415.301 Revision: 0 Page 15 of 30 F0306-0 1R2
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Table 4: ITCP Stress Reduction Factor Results - Side Drop Loading ITCP Side Drop Pm Pm+Pb(I)
Pmn+Pb(O) 1.000 0.93 1 1.000 0.931 Flaw Cross 0.0 n
Section Area 0.0in Pattern Arc Spacing (in) 514 Flaw Arc Length (in) 2.0 Flaw Arc Spacing (in) 2.9 File No.: 1301415.301 Revision: 0 Page 16 of 30 F0306-0 1R2
$§j~tnwtiraI lnegriti Associates, inc; Table 5: OTCP and LTCP Deflection Load Cases - Pressure Load Case Axial Axial Deflection -
Deflection -
Ratio of Increase Component Flaw Type Unflawed Flawed (Percent change)
Configuration Configuration Flawed/Unflawed Radial 1 0.9089 0.921 1.3%
Radial 2 0.9089 0.9149 0.7%
Laminar 0.9089 0.918 1.0%
OTCP Circ 1 0.9089 1.0391 14.3%
Circ 2 0.9089 0.9507 4.6%
Circ 3 0.9089 0.9208 1.3%
Circ 4 0.9089 0.9169 0.9%
ITCP Circ 0.629 0.6314 0.4%
Note:
- 1) The deflection value was taken at the center top of each plate.
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Inc.3 OTCP WELD
-ITCP WELD
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-- Inner Top Cover Plate (ITCP)
Outer Top Cover Plate (OTCP)
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Figure 1. Finite Element Model and OTCP and ITCP Details File No.: 13014 15.301 Revision: 0 Page 18 of 30 F0306-01R2
pressure: Radialt 1 Figure 2. OTCP Postulated Flaw Configuration - Radial #1 File No.: 1301415.301 Revision: 0 Page 19 of 30 F0306-01R2
-Postulated LRadial Flaw View Pressure: Radial #2 Figure 3. OTCP Postulated Flaw Configuration - Radial #2 File No.: 1301415.301 Revision: 0 Page 20 of 30 F0306-01R2
Postulated Laminar Flaw Cutaway View Pressure: Laminar Flaw Figure 4. OTCP Postulated Flaw Configuration - Laminar File No.: 1301415.301 Revision: 0 Page 21 of 30 F0306-0 1R2
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Cutaway Pressure: Circl 1 Figure 5. OTCP Postulated Flaw Configuration - Circumferential #1 File No.: 1301415.301 Revision: 0 Page 22 of 30 F0306-01R2
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Postulated Circ Flaw
- Cutaway Pressure: Circ #2 Figure 6. OTCP Postulated Flaw Configuration - Circumferential #2 File No.: 1301415.301 Revision: 0 Page 23 of 30 F0306-01R2
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Postulated Circ Flaw Figure 8. OTCP Postulated Flaw Configuration - Circumferential #4 File No.: 1301415.301 Revision: 0 Page 25 of 30 F0306-01R2
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Figure 10. OTCP Pressure Load Case - Displaced Shape (Exaggerated)
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Pressure: No Crack Figure 11. ITCP Pressure Load Case - Displaced Shape (Exaggerated)
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Figure 12. Side Drop Model File No.: 1301415.301 Revision: 0 Page 29 of 30 F0306-01R2
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OTCP Radial Flaw OTCP Laminar Flaw Stress Path Stress Path OTCP Circumferential ITCP Flaw Stress Path Stress Path Figure 13. OTCP and ITCP Stress Path Definitions File No.: 1301415.301 Revision: 0 Page 30 of 30 F0306-01R2
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APPENDIX A ANSYS INPUT FILES File No.: 130 1415.301 Revision: 0 Page A-i1 of A-2 F0306-0 1R2
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File Name Description.
BaseMode.INPANSYS input file to construct the 3-dimensional model.
ANSYS input file to perform OTCP flawed stress analyses C*_$_%.hNP
- = 1-4 (Case Number)
$ = Side, Pressure (Loading)
% =Radial, Circ, Lam (Flaw Direction)
ANSYS input file to perform ITCP flawed stress I1_$_CIRC.INP analyses
$ : Side,_Pressure_(Loading)
Pressue.INPANSYS input file to perform OTCP non-flawed IiPressure.INPANYinufietpefrITPo-lad ANSYS input file to perform OTCP non-flawed side Ii Sie.TNPdrop stress analyses.
Genstress~mac Acroinu fl to perform lna ized patosresfxtaction Genstrss~macMacro to perform linearized path stress extraction.
Li-utmcusing the native ANSYS PRSECT command.
GETPATH.TXT Path listing for stress extraction.
Data.xlsm Excel file to compile stresses and compute ratios.
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APPENDIX B SI REPORT 1301415.405, REVISION 0, "EXPECTATIONS FOR FIELD CLOSURE WELDS ON THE ARE VA-TN NUHOMS 61BTH TYPE 1 & 2 TRANSPORTABLE CANISTER FOR BWR DRY FUEL STORAGE,"
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11515 Vrnttoi, Drive Sdte 125 Hunlersville, NC 28078 FPnne; '7D4,97-5554 Fax 704-597-80335 w*wstruclini corn rsrnith@stn.rtint corn October 23, 2014 Report No. 1301415.405.RO Quality Program: [] Nuclear [] Cocnmercial Mr. James F. Becka Xcel Energy Proj ect Supervisor -2013 DE)S Loading Campaign Monticello Nuclear Generating Plant 2807 W. Country R~oad 75 Monticello, MN 55362
Subject:
Expectations for Field Closure Welds on the ARE VA-TN NqUHOMS 612TH Type 1 & 2 Transportable Otauister for BWR Dry Fuel Storage
References:
- 1. Xcel Energy Contrauit No. 1005,*Release 48, Amendment 6.
- 2. SI Report 1301415.402 RO, "Review of TRIVIS INC Welding Procedures used for Field Welds on the Trananuclear NUTI-OMS 6IB3T-Typ~e 1 & 2 Transiportable Canister for BWR* Fuel", January 30, 2014
- 3. SI Report 130 1415.403 R2, "Assessment of Monticello Spent Fuel Canister Closure Plate Welds based on Welding VideoRecords", May 2014
- 4. "E-mail train on Questions Regarding Postulated DCS WeldingFlaw Distrubutions.pdf, from Peter Quinlan to Dick Smnith, October 10, 2014, SI File No. 1301415.205.
- 5. Repair Rates in Welded Construction -An Analysis of Industry Trends, TWI, Cambridge/UKr, Welding and Cutting, November 2012, SI Eile No.
1301415.204.
Dear Mr. Becka:
Details of the rmachineu gas tungstein arc welding (GTAW) field closure welds used ont the NTI-IQMS 61BTH transportable dry shielded caniisters (DSC) located at Xcel Energy's
'Monticello Nuclear Generating Plant QvMNGP) have been reviewed in an attempt to perform a qualitative, assessment of the likelihood that the weldsmight contain unacceptable defects. Itris known that the required 14-DE acceptance testing was not performed according to approved procedures. Sequentiql dye penetrant (PT) examinations were required on the inner top cover pl!ate weld - firtst after the weld root and hot pass(es) were Completed and again, after the final weld layet was completed. This is a relatively small weld (3/16 inch partial penetration Weld) and it was not required to perform an intermnediate inspectioin. The second weld i sa 1/2 inch
- partial penetration weld that requires a root, intermediate, and final PT inspection due to the
______________________________________Toll-Free 877-474-7693 _______________________
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- -9-539 17 File No.: 1301415.301 Page B-2 of B-7 Revision: 0 F0306-O01R2
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James P. Becka October 23, 2014 Report No. l301415.405.RO Page 2 of 6 larger size. The problem identified was that the dwell times used for both dye peneirant and devceloper Were less than required by procedure. The PT tests were performed, but procedures were not followed. This point is being emphasized because large open defects are seen very quickly with PT testing and likely would have been identified even though the dwell times were too short to meet procedure. Smaller tight defects might have been missed as the dye requires sufficient dwell time to wick and then be pulled out via the developer. This statement is in no way intended to justifyr the failure to follow approved PT procedures, but rather to apply perspective from a qualitative sense.
There are a number of reasons to believe that the field closure welds in their current condition do not contain large discontinuities that could challenge the effectiveness of the closure welds to meet their intended design function. It is the purpose of this review, performed in accordance with Reference 1, to identify valid reasons to Support this conclusion. A qualitative justification is provided that is outlined in the listing below:
Reasons to expect the subject spent fuel canister Welds are free from large discontinuities:
- 1. Use of qualified and proven welding procedures and techniques. [Reference 2]
- 2. Use of a machine OTAW process. [Reference 2]
- 3. Application of a proven and robust welding system designed specifically to support these types of field welds in these specific types of canisters. [Reference 5]
- 4. Use of ductile and easily weldable base materials (SA-240 Type 304 stainless steel).
[Reference 2]
- 5. Use 6f solid wire filler metal designed for welding these base materials and formulated to eliminate hot cracking and other types of microfissures (SPA 5.9 ER3OS austenitic stainless steel tiller metal and welding grade gases for shielding the weld puddle.
[Reference 2]
- 6. Canisters are oriented in the vertical position during welding such that the weld is performed in the flat welding position (the most forgiving welding orientation).
[References 2,3 and 4]
- 7. Weld roots are typically about 1/8 inch or slightly thicker Which is good practice for OTAW machine welds. [Reference 4]
- 8. Weld layers are thin (betweenil/16 inch and 1/8 inch) requiring multiple layers (and multiple weld~passes) to assist with developing weld deposit consistency via remelting; Layers become thinner as the groove is flled becaus& the widthis greater. [Reference 4]
- 9. ARE VA-TN's historical record with these welds is excellent having a significant history of welds made with this system and these welding procedures that shows 1% repairs rates. [Reference 4]
The welding procedures and welder control documentation were reviewed in detail and~specifics of that review are reported in Reference 2. The review concluded that
.*'... the procedures the CiTAWI welds in the subject spent fuel canisters can reasonably be expected to bel of good quality dnd free of injurious defects, The expectation was based on te characteristics of the C)TA W wel4 the excellent controls outlined for the welding program, and the fact that the welds and base materials are austenitic stainless steel. Also the welding consumables are compatible with the structural materials used in the design..."/Reference 2]
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James F. Becka Report No. 1301415.405.R0 October 23, 2014 Page 3 of 6 The welding application itself is performed entirely in the flat position - a welding position that eliminates any comlplications related to welding out of position or having to negotiate restricted access. The reason for this viewpoint is that out of position welds have to compete against the forces of gravity and the joint design provides adequate access for arc manipulation. The result of a welding in the flat position is that defects are less likely to be introduced than might be expected with other weld orientations or restrictions.
The spent fuel canister welding system is robust and is proven. The welding head is mounted on a non-metallic shielding material weighing over 1500 lbs and is shown in Figure 1 below.
Figure 1 Photo of the robust wedldng head that Ii positioned on the dry storage cask -s shown hI FMgure 2.
The wedldng tordh is visible in the photo just behind the rope. (Photo provided by AREVA-TN)
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° James F. Becka Report No. 130 1415.405.R0 October 23, 2014 Page 4 of 6 Figure 2 Welding system poultlmned mn the storage canister fer welding (Photo provided by AREVA-TN)
The entire welding system rotates simfilar to a "lazy-susan" and the welding torch is manipulated in and out as required for proper positioning. There are other torch adjutuments such as tilt, lead, height, etc. Leading and trailing cameras are mounted to provide video of the front and rear of the torch and weld puddle. Welding videos have been reviewed [Reference 3] in an attempt to assess whether or not weld quality could be assessed. One objective of the video review was to look for key discontinuities such as porosity and evidence for any lack of fUSiorL The conclusions from the video review were that circumstances were observed at various limes during welding that might suplrt the generation of defects such as oxide buildup, weld root burn-thru, localized contanination on the surface, weld deposit surface irregularities, and tungsten drift requiiring realignment. However, nothing could confim either the generation of defects or the lack of defects. Since each weld is a unique entity one must rely on tendencies or trends if post weld inspections are not available. There were also observations of good welding practices as well as those events stated above. The*se included root repai, periodic adjustment of tungsten positioning, tungsten electrode replacement, electrode steering as needed, etc. Most of the videos were very sinilar (all having the same types of observation at about the same frequency). Canister No. 16 also had the same types of observations but the freqtumecy appeared to be about twice any of the others. This was a judgment call by the reviewers and not quantitative. It was carefully poined out that even so, there was no evidenace to indicate that any specific discontinuities were generated - only that welding conditions were observed that sonmetimes lead to the various types of discontinuities. In addition, sice these welds use multiple weld beads to complete the weld, there is the opportunity to "heal" conditions created by welding over them.
Historca Perspective ARE VA-TN was asked to describe their historical perspective on the welding of the canisters with this system. It is recognized that all of the canisters were not welded by ARE VA-TN but File No.: 1301415.301 Revision: 0 Page B-5 of B-7 F0306-01R2
James F. Becka October 23, 2014 Report No. 1301415.405.R0 Page 5of6 might include a contractor or the utility themselves. However the same welding system likely would have been used (often rented from AREVATN). AREVA-TN noted that typical discontinuities might include local porosity (rare), occasional tungsten inclusions, usually resulting from torch tip contact with the solidifying weld puddle, lack of fusion or overlap.
Regarding the potential for any linear indications (holidays or breaks), cracking typically does not occur with austenitic stainless welds. Maximum size of indications typically would be less than 1" to 2". Irregularities at starts and stops can occur, and rollover has been seen in some cases.
ARE VA-TN also was asked for their historical experience regarding canister closure weld acceptance rates (i.e. first tine PT rate). The response indicated that a best estimate would be less than 1 UNSAT PT per 10 canisters, with an average of 10 PT e~xaminations per canister (includes root and final layer on inner top cover, vent port cover, siphon port cover and test port, with root, mid and final layer on outer top cover for certain DSC models). Therefore, the historical experience suggests a rate of about 1% UNSAT PTs for field closure welds. Further, the recent field experience as the weldinlg process matured produced no weld repairs at all - on 50+ canisters the findings were I PT indication from starts and stops was found to hold developer, but light grinding was performed to smooth the surface and eliminated the indication.
Thus, these minor indications required no weld repairs.
ARE VA-TN was also asked regarding how many stainless steel canisters have been loaded and closed by welding to date. The estimate was for approximately 750 loaded/closed NUHOMS canisters, with closure performed by ARE VA-TN, end user or other contractor. This represents an extensive sampling that indicated an indication rate of less than 1% and that rate appeared to significantly improve over the last 50 that have been welded.
There were no applicab~le mockups that had been used to examine for discontinuities or defects, so that information was unavailable. The historical evidence seems to paint a favorable picture lending a degree of comfort that the canisters in question at M'NGP are not likely to have indications of a significant size.
Finally, literature was examined to find information regarding generation of defects in stainless steel weldments. The best paper found is indicated in Reference 5. This paper written by The Welding Institute in Cambridge, UK was published in Welding and Cutting, November 2012.
The paper titled "Repair Rates in Welded Cons-rution - An Analysis of Industry Trends" provided good insight. More than 800 professionals were contacted with about 10% responding.
There were different kinds of resoss such as % of welds requiring repair or % weld lengths requiring repair being the most prevalent. The following applicable conclusions were noted.
GTAW stainless steel welds returned under 2% repair rates. The impact of different welding factors were parsed and suggested the following impacts: root repairs at 22.5%, fill layers 7.5%,
joint type 15%, access limitations 26%, and other welding factors 11%. Most of these are not present in the canister welds as pointed out previously. It appears that the ARE VA-TN canister weld repair experiences are slightly lower, but nevertheless are considered consistent with inutilexpectations for a variety of manufactured and installed components. Since all welding is in the flat position using a proven welding system, the 1% defect rate appears to be reasonable. In addition it was pointed out that experience with the past 50 canisters has been even better.
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James P. Becka Report No. l301415.405.R0 October 23, 2014 Page 6 of 6 Conclusions Based on the sum of the information reviewed, it can be said that the likelihood for the occurrence of large defects is not supported by historical evidence. While there remains the potential for long lack of fusion defects either interbead or sidewall, the thin muitilayer design and potential for subsequent bead healing by remelting would significantly limit the through-thickness dimension of any long defect. In fact, the most likely lack of fu~sion indication(s) would be intermittent in nature and not expected to have a thtrough-thickness dimension greater than one weld bead. While a quantitative estimate of a limiting flaw size cannot be produced, the qualitative likelihood that large defects would not be present is assuring.
It is suggested a bounding subsurface defect condition is conservatively represented as an intermittent lack of fusion (LOF) defect evenly distributed along the canister weld. Further, the total length for LOP is conservatively estimated at 25% of the canister cover plate weld circumference. The estimated through thickness dimension is I/8 inch, because this dimension represents a maximum weld bead thickness. One eighth inch is considered to be a conservative assumption, because it is recognized that most weld beads will be thinner especially as the weld cavity begins to fill. No credit is being taken for remelting even though remelting is normally associated with multipass welding."
Very truly yours, Richard E. Smith, PhD. PAWS Senior Associate res
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