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These dry cask brands belong to French State owned AREVA.

As illustrated above, “Spent fuel pools are needed to unload failed canisters and cask. However, the NRC allows pools to be destroyed after all fuel is loaded into dry storage, claiming nothing will go wrong. This December 1, 2010 Peach Bottom TN-68 cask event report is an example of why the pools are needed in case of cask or canister failure. http://pbadupws.nrc.gov/docs/ML1100/ML110060275.pdf
[NOTE: The Areva thick steel TN-68 cask worked as designed. It has a lid monitoring early warning system, so casks can be unloaded and repaired (e.g., seal replaced) before a radiation leak. If this had been a thin steel canister that leaked, there is no early warning system and the canister cannot be repaired.]

Rather interesting that the French used to know about galvanic corrosion in the 1880s but AREVA apparently hadn’t heard of the concept! “A spectacular example of galvanic corrosion occurred in the Statue of Liberty when regular maintenance checks in the 1980s revealed that corrosion had taken place between the outer copper skin and the wrought iron support structure. Although the problem had been anticipated when the structure was built by Gustave Eiffel to Frédéric Bartholdi’s design in the 1880s, the insulation layer of shellac between the two metals had failed over time and resulted in rusting of the iron supports“. https://en.wikipedia.org/wiki/Galvanic_corrosion

Acid rain, apparently long beloved by Judge Scalia, may have contributed to the corrosion of the Statue of Liberty, according to the Christian Science Monitor: http://www.csmonitor.com/1981/0701/070135.html
By extension, it may impact the spent fuel storage. Shortly before joining the Supreme Court, Judge Scalia dismissed an acid rain lawsuit: http://www.nytimes.com/1986/09/19/us/acid-rain-lawsuit-rejected-by-court.html

Some excerpts from United States Nuclear Waste Technical Review Board “Evaluation of the Technical Basis for Extended Dry Storage and Transportation of Used Nuclear Fuel” December 2010
Galvanic corrosion. Dissimilar metals are sometimes in contact with each other in some dry-storage systems (see subsection When dissimilar metals are connected electrically in the presence of an electrolyte (liquid water or medium to high humidity), a galvanic cell is established and electrochemical reactions can occur. All casks contain varying amounts of residual water. For example, if bolts are anodic to large component of a cask system, the bolts may corrode quickly and lose their ability to function as fasteners in the system. For instance, if a relatively active metal like zinc, in the presence of large areas of ferrous surfaces that are cathodic to zinc, there may be a potential problem. It is important to systematically consider the various metals in use in a given dry cask storage system to anticipate galvanic corrosion. Coatings, electroplating, weld overlays, and metal seals are all a source of dissimilar metals.” (p. 107)

Aspiration of water on metal surfaces. Finally, dry-storage systems employed for CSNF are of the passive vented type, meaning outside air circulates next to the external surface of the metal canister to cool it. For an intact canister, as the canister cools, it may condense water and other particles on its surface from the external atmosphere as a function of external temperature and dew point. If cracks or holes develop in the canister, and as the fuel continues to cool, water may ingress to reside on the external surface of the cladding. Aspirated “water ingress during ‘dry’ storage may significantly increase the overall chemisorbed water content of the SNF over the storage period, especially if the SNF is badly damaged.”81 So there may be a point in time when a significant source of chemisorbed water appears on the surface of the fuel rods. Free water “can be released by direct decomposition of the chemically bonded species, vaporization of the physisorbed and the free water, and radiolytic decomposition. These forms of water and decomposition products could cause corrosion, pressurization and possibly embrittlement issues for the storage of spent fuel.”81 It is expected, however, that if holes or cracks develop in the canister, it will be able to be measured in terms of loss of helium and increased radioactive releases from the cask system and remedial actions such as re-packaging could be taken.” (p. 78)

Bare-fuel casks use seals between the cask and the lids, and helium pressure between the primary (inner) and secondary (outer) lids is monitored. Failure of the secondary lid seal has occurred on several casks leading to fuel retrieval, inspection, and resealing.20 Canisters in canister-based systems are sealed by partial-penetration welds between the canister wall and the primary and secondary canister lids.21 One of the welds is helium-leak tested after welding; helium pressure between the lids is not monitored after the canister-based system has been moved to its storage location,… 22 More details about the dry-storage system components can be found in subsection 4.1.3.” (p.33)

Summary: degradation behavior of used fuel-pellets. The design of dry-storage systems rely on two independent barriers to prevent the oxidation of used fuel-pellets: helium in the fuel rod and helium in the canister. Helium is an important part of the safety system of dry cask-storage systems and its presence needs to be verified and ensured for preventing activation of extended dry storage degradation mechanisms. Currently, neither pressurized helium in fuel rods nor in welded canisters is monitored. Research into monitoring the presence of helium within welded casks, preferably nondestructively, would be helpful. Helium detection may not be of the highest priority if, after some years, dry-storage fuel temperatures are low enough to prevent significant oxidation to U3O8, as current research appears to indicate (but the potential for low-temperature oxidation of fuel needs to be better understood). If the helium cover gas escapes the cask system, or for the case of a cask breach during accidents, it would be helpful to further characterize the oxidation behavior of various types of fuel in air, with temperature. If even a small proportion of CSNF fuel rods are discovered to contain oxidized fuel, research is needed to better understand how the powdered fuel may be dispersed and spread upon opening a canister under water or in air. Fission-product gases are hazardous (radioactive) and may also contribute to overpressure of fuel rods. Validating the modeling that has been done and extending it to high-burnup fuel would be useful. Helium gas microcracking appears to damage fuel pellets and is especially active in MOX fuels, but not much data are available for understanding this phenomenon and whether it triggers more release of fission gas or other problems.” (p. 92)
Summary – degradation behavior of used fuel cladding: The key cladding degradation mechanisms noted above are known. What has not been sufficiently studied is the effect of these degradation mechanisms under the conditions of very-long-term storage. Although analytical models can predict consequences of individual mechanisms fairly well, coupled models may be needed to better predict the effects of multiple mechanisms. To validate these models, it is important to inspect representative low- and especially high-burnup used fuel after 30 to 40 years of dry storage.. Creep rupture is a known degradation mechanism that should be researched for better understanding of how much creep occurs in actual stored cladding over time, especially for high-burnup fuels, and to validate creep modeling. Creep that occurs at lower temperatures may be the result of a different mechanism from that of high-temperature creep. Stress corrosion cracking is another mechanism that is believed to be active but needs to be better understood. Hydrogen-related degradation mechanisms and effects are a recent renewed area of research with the potential of having significant effects on cladding degradation over time. The hydriding phenomenon is important from the perspective of embrittlement of the cladding which would affect its integrity during transportation to either a reprocessing plant or a repository. One of the key findings is the importance of the helium cover gas in the canisters in limiting and potentially avoiding certain key degradation mechanisms. The extent of cladding degradation over long periods is an important aspect of subsequent used-fuel handling systems and transportation concerns.” (p. 104)
Summary: degradation behavior of dry-storage system components. As is evidenced by deteriorating concrete bridges, atmospheric corrosion and degradation of concrete structures do occur. The additional influence of heat and radiation damage can compound environmental damage. Fortunately, the loads on the concrete overpacks are limited. In addition, the external surface of the concrete overpack and foundation pad, as well as a number of other dry-storage system components, cannot easily be inspected. Other components that are housed inside of welded containers also cannot be easily inspected. Aging of these component materials should be studied to better understand the possible extent and under what conditions they age. Monitoring the condition of concrete overpacks to identify damage before it becomes significant will be important. Once damage is found, a follow-up maintenance program will correct the damage and help minimize future degradation problems. There are already reported observations of visible metal and concrete degradation of dry-storage systems in areas near the sea. Having an inspection program at all sites for monitoring degradation on all metal and concrete exterior surfaces to better understand how degradation evolves in various climates is important. The dust and chemical species that comprise an atmospheric-corrosion environment can affect several different mechanisms.” (p.113) Excerpted from: US NWTRB “Evaluation of the Technical Basis for Extended Dry Storage and Transportation of Used Nuclear Fuel” December 2010 This report was prepared for the U.S. Nuclear Waste Technical Review Board by Dr. Douglas B. Rigby, staff member, in support of the Board’s analysis of issues associated with extended dry storage and transportation, an effort lead by Dr. Andrew C. Kadak, Board member. http://www.nwtrb.gov/reports/eds_rpt.pdf

Discusses some frightening dry cask spent fuel “incidences”, including explosions: https://www.nirs.org/radwaste/atreactorstorage/drycaskfactsheet07152004.pdf

Of Related Interest:
Oregon Office of Energy, “Staff Evaluation of Holtec Design for Portland General Electric’s Independent Spent Nuclear Fuel Storage Installation (ISFSI), September 20, 2002
Pages 35-37 are of special interest.