Engineering Scale Drying of Aluminum-Clad Spent Nuclear Fuel: Experiment Report

Abstract

Engineering-scale tests were performed to evaluate the relative effectiveness of forced gas and vacuum-drying processes on aluminum-clad spent nuclear fuel (ASNF) in preparation for extended dry storage. Drying models were developed to improve confidence in the likely range of conditions at the start of dry storage and to understand the factors most significant to each process. Testing was performed at the Holtec International Forced Helium Dehydration training facility in Camden, New Jersey, by students from the University of South Carolina in collaboration with Idaho National Laboratory.

Technical basis for extended dry storage of aluminum-clad spent nuclear fuel

“…22  Removal of residual or physisorbed water from ASNF dry storage configurations is achievable with commonly used processes such as vacuum drying or FHD. 39 There are indications that under specific drying conditions and processes, part of the chemisorbed water could be removed from the (oxy)hydroxide layers as well. 22 Compared to vacuum drying, FHD demonstrated a better performance in partly removing chemisorbed water and in achieving uniform ASNF cladding temperatures.…”

“…22 Compared to vacuum drying, FHD demonstrated a better performance in partly removing chemisorbed water and in achieving uniform ASNF cladding temperatures. 39  Only a weak relationship between drying duration and mass loss can be observed during the thermogravimetric analysis of pre-corroded aluminum samples. Further, FHD and vacuum drying experiments using various thermal conditions demonstrated the removal of most bulk water within 3 hours and 4 hours, respectively, of the drying process initiation.…”

“…However, an increase in drying temperature can reduce the time needed to complete an ASNF FHD process. 39  Even without removing any of the chemically bound water in a DOE Standard Canister loaded with ASNF, the canister pressurization is expected to remain well within the structural capacity of the canister during extended storage (i.e., <34 atm). 29,34  For all test temperature conditions, water mass loss from (oxy)hydroxide substrates on ASNF and pre-corroded aluminum samples demonstrated a positive correlation with an increasing drying temperature, although most mass change occurred between 200 and 300°C.…”

“…19 Significant (although incomplete) dehydration of the tri-hydrated bayerite film could be practically achieved for ASNF by drying at temperatures slightly above 200°C. 39  Phase changes in the (oxy)hydroxide layer were observed during drying at ~220°C. There are suspicions that these phase changes could be accompanied by the significant removal of chemisorbed water.…”

“…There are suspicions that these phase changes could be accompanied by the significant removal of chemisorbed water. 39  Changes in the aluminum-clad meat microstructures and material properties could take place at temperatures below 200°C. These changes may be undesirable due to their potential of challenging the integrity of the fuel structure, but preventing these effects by limiting the drying temperature may be of competing interest in the selection of optimal thermal conditions for drying.…”