blister, Blisters on Spent Nuclear Fuel, circumferential hydrides, Cladding deformation, cracks, delayed hydride cracking, DHC, dry cask storage, dry storage, failure, HR, hydride reorientation, Radial hydride precipitation, Radial hydrides, SFC, SNF, spent fuel cladding, spent fuel rod cracks, Spent Nuclear Fuel, Spent Nuclear Fuel failure, zirconium hydrides, Zry-4
Comment Deadline tonight 11.59 PM Eastern for “Dry Storage and Transportation of High Burnup Spent Nuclear Fuel“, draft NUREG-2224, “Comments received after this date will be considered if it is practical to do so, but the NRC is able to ensure consideration only for comments received on or before this date…” Comment on document here: https://www.regulations.gov/document?D=NRC-2018-0066-0001 https://www.regulations.gov/docket?D=NRC-2018-0066 (Comment is easy and can be anonymous. NRC allowed less than 2 months to comment, so please ask that they extend the deadline as well. Remember to put the docket ID: NRC-2018-0066-0001)
See more here: https://miningawareness.wordpress.com/2018/09/23/dry-storage-and-transportation-of-high-burnup-spent-nuclear-fuel-comment-deadline-monday-night-11-59-pm-eastern-there-was-less-than-2-months-for-comments-request-extension/
The article below discusses some of the topics under consideration, though it doesn’t specify High Burnup Fuel.
Excerpted from full article below by Luppo et al. (2018): “Dry storage is currently being considered as a method of managing commercial nuclear fuels before delayed reprocessing or final disposal. Creep has come to be regarded as a dominant mechanism of cladding deformation. However, hydride reorientation (HR) and/or delayed hydride cracking (DHC), and blisters and rims formation are mechanisms of failure feasible to occur in spent fuel cladding (SFC) during dry storage. The HR phenomenon usually involves the dissolution of circumferential hydrides and the formation of zirconium hydrides oriented perpendicular to the hoop, hereby referred as radial hydrides .
Hoop stresses are mainly produced by internal pressure of gasses released during fuel cladding operation. In fuel claddings of Atucha 1, the maximum (conservative) end of life pressure at 360°C is 100 bar. At 200°C, the maximum nominal temperature during dry storage, this pressure drops to 75 bar and it would produce a hoop stress of 81 MPa…. Hydrogen enters in Zry clads during fuel element life in service; when hydrogen exceeds the terminal solid solubility, circumferential hydrides are precipitated. In the presence of thermal gradients, produced for instance by oxide spallation, localized concentration of hydrides – known as blisters – can be formed. Due to the fact that zirconium hydrides have a lower density that zirconium alloys, stresses are produced around blisters. These stresses may overcome the necessary threshold to precipitate radial hydrides. Moreover, cracks inside blisters may act as initiators of delayed hydride cracking (DHC).
In the dry storage facilities developed for Atucha 1, Zry – 4 clads of spent fuel elements (SFE) must withstand environmental conditions which include maximum temperature of 200°C and internal SFE pressure of ~ 75 bar (value at end of cycle). During dry storage, if blisters are present, stresses around them are superposed to circumferential stresses generated by the internal pressure due to fission gasses helping precipitation and/or growth of radial hydrides and DHC process. In order to assess this possibility, in the present work sections of Zry-4 cladding were hydrided. Blisters have been grown by thermal gradient under different conditions of temperature and shapes of thermal contact in order to obtain different blister dimensions and fraction of radial hydrides. After blister formation, some specimens were subjected to Hydride Reorientation Test (HRT) by applying an internal pressure of 75 bar at 200°C. Finally, tubes were sectioned and metallographically analyzed looking for radial hydrides.
In some samples radial hydrides were observed, which effectively grew by the internal pressure of Zry-cladding during HRT“.
Emphasis added. Original here: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1517-70762018000200509
Related information of interest:
“Defects oriented in the longitudinal direction … have a tendency to fail from hoop stress… while defects oriented in the circumferential direction… have a tendency to fail from axial stresses…” https://www.netl.doe.gov/research/oil-and-gas/project-summaries/completed-td/de-fc26-02nt41633
“Like other hydride-forming metals, zirconium is susceptible to embrittlement by hydrogen when hydrides are formed. The embrittlement takes two forms: short term loss of toughness and a stable, time-dependent crack growth mechanism called delayed hydride cracking (DHC). During DHC, hydrides nucleate and grow slowly in the high stress region of a stress-raiser such as a crack tip. When they reach a critical condition, probably related to size, they fracture, the crack extends and the process is repeated./ DHC has been responsible for several failures in components….” See: “DELAYED HYDRIDE CRACKING OF ZIRCONIUM ALLOY FUEL CLADDING“, INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 2010
In the US: “10 CFR 72.122(h)(1) states in part – “The spent fuel cladding must be protected during storage against degradation that leads to gross ruptures in the fuel or the fuel must be otherwise confined such that the degradation of the fuel during storage will not pose operational safety problems with respect to its removal from storage.” https://www.nrc.gov/reading-rm/doc-collections/isg/isg-22.pdf
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