aluminum boron carbide, dry cask storage, Friction Stir Welding, galvanic corrosion, High Level Nuclear Waste, Holtec, Holtec Dry Casks, joint failure, Kissing bonds, material failure, materials, metamic, MPC, NRC, nuclear dangers, nuclear energy, nuclear fuel baskets, nuclear waste storage, public safety, reducing safety margins, safety margins, SNF, Spent Nuclear Fuel, US NRC, weld failure, welding
Deadline imminent (11.59 pm Mon. March 9th EDT/ 4.59 am Tuesday UTC) for US NRC comment regarding storage of damaged spent nuclear fuel rods, and other safety related exemptions to be given to Holtec for their dry cask storage. Make the comment uniquely yours, because they may not count repeats. https://www.federalregister.gov/articles/2015/02/05/2015-02310/list-of-approved-spent-fuel-storage-casks-holtec-international-hi-storm-100-cask-system-certificate They may accept at 11.59 in your (US) time zone. But, better to get it in by 11.59 EDT, if possible.
Besides wanting to store spent nuclear fuel, which is more damaged (see previous two posts), Holtec wants to do some other things, which decrease safety-safety margins.
Kiss of death?
USFW: “Black widows take terminal trips…” http://www.fws.gov/refuge/Kenai/community/2013_article/10252013.html
One is using a new welding process, from 1991, which can give rise to “kissing bonds”, and other defects, for the spent fuel baskets, within the casks: Holtec “intends to employ a new qualified welding process called Friction Stir Welding (FSW), for external basket joints.” This matters because, as the “Final Safety Analysis Report for Holtec” stated “Criticality control is maintained by the geometric spacing of the fuel assemblies and fixed borated neutron absorbing materials (Boral) incorporated into the fuel basket assembly… The MPC fuel basket is positioned and supported within the MPC shell by a set of basket supports welded to the inside of the MPC shell. Between the periphery of the basket, the MPC shell, and the basket supports, heat conduction elements are installed…” 
Thus, “kissing bonds”, and other welding defects, could lead to collapse of baskets holding the spent nuclear fuel, which could lead to a deadly criticality accident. Also, it would seem that such a failure could lead to puncture of the flimsy MPC shell, which is apparently only 1/2 inch thick. 
Wikipedia provides a summary of the downside of Friction Stir Welding (FSW), which appears unfortunately relevant to spent fuel safety: “… FSW is associated with a number of unique defects. Insufficient weld temperatures, due to low rotational speeds or high traverse speeds, for example, mean that the weld material is unable to accommodate the extensive deformation during welding. This may result in long, tunnel-like defects running along the weld which may occur on the surface or subsurface. Low temperatures may also limit the forging action of the tool and so reduce the continuity of the bond between the material from each side of the weld. The light contact between the material has given rise to the name “kissing-bond”. This defect is particularly worrying since it is very difficult to detect using nondestructive methods such as X-ray or ultrasonic testing. If the pin is not long enough or the tool rises out of the plate then the interface at the bottom of the weld may not be disrupted and forged by the tool, resulting in a lack-of-penetration defect. This is essentially a notch in the material which can be a potential source of fatigue cracks.” http://en.wikipedia.org/wiki/Friction_stir_welding
From the Holtec-NRC document. Staff is the NRC; Applicant is Holtec. Original in italics, our comment in brackets:
“3.0 STRUCTURAL EVALUATION
The staff has reviewed Amendment 8, R.1 for the HI-STORM 100 which requests two changes relevant to structural performance:
(1) Changes to testing requirements for Metamic HT and (2) Changes to Minimum Guaranteed Values (MGVs) for Metamic HT
The first proposed change, as described by the applicant in their revision request, does not have any direct bearing on structural performance.”
[Note that they say no “direct” bearing. However, weld failure-fatigue cracks would indirectly bear on structural performance.]
“Further review of the change request letter and the FSAR change pages (Section 1.III.2.4 Qualification of Metamic-HT, (e) Welding of Metamic – HT) shows that the applicant intends to employ a new qualified welding process called Friction Stir Welding (FSW), for external basket joints. Allowing the use of friction stir welding of the Metamic HT basket does not change the safety basis as evaluated by the staff in HI-STORM 100, Amendment No. 8, with respect to basket structural performance. Since the basket corners utilize the same welded joint configuration specified in amendment No. 8 and prior amendments, the primary consideration is that of weld process and qualification, rather than structural performance of the weld itself.”
[Huh? Structural performance of weld is literally of critical importance.]
“Based upon a review of the application, the staff determined that the methods employed to structurally qualify the weld joint were sufficiently robust to demonstrate comparable structural performance to the welding method described in previous amendments. The staff’s conclusion is based upon the fact that the loading conditions and the fully supported boundary conditions (via shims) of the peripheral basket panels result in joint stresses significantly below their full capacity. This results in significant margin which should account for any differences in welding procedures, should they arise in the future. These conclusions only apply to the basket corner welds and shim arrangement defined by this revision.”
[The NRC just said that it is ok to reduce safety margins in nuclear waste casks! Aren’t corner welds the most important? What is wrong with them?]
[More reduction in safety margins. They are reducing the Minimum Guaranteed Values (MGV) for stress-strength for the fuel basket material, Metamic HT. They report a 20% reduction of MGV during a tensile test: “Tensile testing, also known as tension testing, is a fundamental materials science test in which a sample is subjected to a controlled tension until failure. The results from the test are commonly used to select a material for an application, for quality control, and to predict how a material will react under other types of forces.” http://en.wikipedia.org/wiki/Tensile_testing. And, they report 10% average reduction for material yield stress, ultimate strength and Young’s modulus. They do not tell us what kind of average they are using. If it is an arithmetic average (mean) the reduction could be huge.]
“The second part of the change request addresses changes to the MGVs for Metamic HT. The MGVs for Metamic HT are used in calculations to demonstrate that the structural components will satisfy engineering requirements such as stress limits or deflection limits. By providing MGVs, all calculations performed with those values will represent a bounding calculation for a given engineering requirement. The applicant referenced its engineering change order (ECO) process supported by a 10 CFR 72.48 evaluation to make changes to the MGVs, but elected to additionally submit the proposed changes in those values to the NRC for review and approval through the revision process. A review of the material properties submitted by the applicant indicates an average reduction in MGVs of approximately 10%, for material yield stress, ultimate strength, and Young’s modulus. A reduction of 20% of the MGV was reported by the applicant for the reduction in area criteria measured during a tensile test. The applicant applied these changes to structural calculations (stress strain curve development for finite element analysis) and determined that positive margin remains for basket performance criteria. The positive margins include the areas of peak stress criteria, maximum deflection criteria, and crack propagation criteria. The staff reviewed these results and, because positive margin remains for basket performance criteria even with the reduced MGVs, finds this acceptable”
[Are some of the margins now negative then, instead of positive?]
“3.1 Evaluation Findings
Based on evaluation of the supporting documentation and calculation for Amendment No. 8 to CoC No. 1014, the staff finds that the revision acceptably meets the review criteria identified in NUREG-1536, REV. 1. Specifically, the staff finds:
F3.1 The structural properties of the CoC No. 1014, Amendment No. 8, SSCs remain in compliance with 10 CFR Part 72, and the applicable design and acceptance criteria have been satisfied. The evaluation of the structural properties provides reasonable assurance that the CoC No. 1014, Amendment No. 8, Revision No. 1, will allow safe storage of spent nuclear fuel (SNF). This finding is reached on the basis of a review that considered the regulation itself, appropriate regulatory guides, applicable codes and standards, and accepted engineering practices.” http://pbadupws.nrc. gov/docs/ML1426/ML14262A476.pdf
“SAFETY EVALUATION REPORT DOCKET NO. 72-1014 HOLTEC INTERNATIONAL HI-STORM 100 CASK SYSTEM CERTIFICATE OF COMPLIANCE NO. 1014 AMENDMENT NO. 8, REVISION NO.1”
WHAT THE F(UKUSHIMA)IS “REASONABLE ASSURANCE”? REDUCING SAFETY MARGINS IN THE CONTEXT OF THE UNKNOWN RANDOM IMPACTS OF RADIATION ON MATERIALS, ESPECIALLY OVER TIME, IS NOT “REASONABLE”! FURTHERMORE, ONE YEAR AGO THE US CSB CONCLUDED THAT THE ANALYSIS USED FOR HYDROGEN DAMAGE IN THE PETROLEUM INDUSTRY WAS WRONG! CORROSION EXPERTS ARE SURPRISED AT THE HYDROGEN DAMAGE AT THE DOEL AND TIHANGE NUCLEAR REACTORS IN BELGIUM. TOO MANY THINGS ARE UNKNOWN AND UNEVALUATED. THEY MAY BE UNKNOWABLE.
We wonder if this Metamic (Aluminum boron carbide) material is appropriate or if it may set up galvanic corrosion, or other problems, especially in the context of damaged fuel rods? If it is a problem, you can be certain that we won’t learn about it from either Holtec or the US NRC.
“All MPCs have identical exterior dimensions which render them interchangeable.” [The insides vary according to fuel rod types and numbers.]
“Criticality control is maintained by the geometric spacing of the fuel assemblies and fixed borated neutron absorbing materials (Boral) incorporated into the fuel basket assembly…
The construction features of the PWR MPC-24 and the BWR MPC-68 are similar. However, the PWR MPC-24 canister in Figure 1.2.4, which is designed for high-enriched PWR fuel, differs in construction from the MPC-68 in one important aspect: the fuel storage cells are physically separated from one another by a “flux trap”, for criticality control. All MPC baskets are formed from an array of plates welded to each other, such that a honeycomb structure is created which resembles a multiflanged, closed-section beam in its structural characteristics.
The MPC fuel basket is positioned and supported within the MPC shell by a set of basket supports welded to the inside of the MPC shell. Between the periphery of the basket, the MPC shell, and the basket supports, heat conduction elements are installed. These heat conduction elements are fabricated from thin aluminum alloy 1100 in shapes which enable a snug fit in the confined spaces and ease of installation. The heat conduction elements are installed along the full length of the MPC basket to create a nonstructural thermal connection which facilitates heat transfer from the basket to shell. In their installed condition, the heat conduction elements contact the MPC shell and basket walls.” “FINAL SAFETY ANALYSIS REPORT for the HOLTEC INTERNAT[ONAL STORAGE AND TRANSFER OPERATION, REENFORCED MODULE CASK SYSTEM, (HI- STORM 100 CASK SYSTEM), DOCKET 72- 1014, VOLUME I OF II” http://pbadupws.nrc. gov/docs/ML0724/ML072420254.pdf (NRC Documents can be found with either the title or the ML number which serves as an ID.)
Note  The MPCs are 1/2 inch thick. The variations come from the over-pack. The Holtec Star is apparently for transport; Holtec Hi-Storm is not. Some over-packs are above ground; some underground. Hi-Storm overpacks are vented, meaning that the inner MPC is the only real protection for the outside world. The concrete overpacks can be subject to degradation.
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