PNNL-13501 - Pacific Northwest National Laboratory

to 300°C and 400°C. Unfortunately, the amount of tramp
oxygen in the system was enough to allow oxidation of
the fuel once the temperature approached 275°C, even
with a 3% hydrogen gas mixture flowing through the
system.
Figure 4. Thermogravimetric analysis decomposition of
metaschoepite at 300°C and 400°C
X-ray diffraction of the samples after thermogravimetric
analysis testing showed that the metaschoepite had been
removed. However, dehydrated schoepite remained. It
was not clear if the dehydrated schoepite was a
decomposition product of the metaschoepite or if the
decomposition product was UO3, which then readily
rehydrates even at ambient conditions. A truly inert
environment must be maintained, even for preparation of
the x-ray diffraction sample to address this issue.
Thermogravimetric analysis testing on the dehydrated
schoepite demonstrated that even with temperatures of
about 300°C for over 10 hours, this phase was not
completely removed. Testing under more controlled
conditions and under different atmospheres is necessary
to optimize the method for removal of the hydrated
phases.
Summary and Conclusions
Testing on unirradiated UO2 and spent fuel powders
clearly demonstrated that hydrated phases formed rapidly
under conditions expected under spent fuel pool storage.
While the large surface area of the samples tested was a
factor, the time in storage for spent fuel supported the
conclusion that hydrates will form. The week-to-week
fluctuations in the relative quantity of hydrated phases are
not explained, but they raise the question as to whether
hydrated phases are formed by a dissolution/
reprecipitation reaction as assumed or if they are formed
through direct reaction with the fuel. The stability of
these hydrated phases, especially metaschoepite, in an
aqueous environment was unresolved. The observation of
floating phases raised questions regarding the expected
transport of radionuclides from areas contaminated with
uranium.
Standard drying techniques that do not apply heat will not
remove the absorbed water. Additional atmospheres must
be tested to determine the conditions for removing
hydrated phases from failed spent fuel rods.
References
CRWMS M&O. 2000. Clad Degradation-Dry
Unzipping. ANL-EBS-MD-000013 REV00. Las Vegas,
Nevada.
Oliver BM, GS Klinger, J Abrefah, SC Marschman,
PJ MacFarlan, and GA Ritter. 1998. Drying Results of
K-Basin Fuel Element 1164M (Run 6). PNNL-11896,
Pacific Northwest National Laboratory, Richland,
Washington.
Nuclear Science and Engineering 367