PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00045/1452
Formation Decomposition of Hydrated Phases on Nuclear Fuels
Brady Hanson, Bruce McNamara, John Abrefah, Steve Marshaman
Recent studies at this Laboratory suggest that the presence of small quantities of hydrated uranium oxide can be
deleterious to dry storage of spent nuclear fuels. An understanding of the mechanisms and kinetics of formation of these
hydrated phases and a technical basis for a drying technique to remove the physisorbed and chemisorbed water from spent
fuel is necessary to mitigate these undesirable consequences.
Project Description
The purpose of this project is to determine whether
hydrated phases of spent fuel are expected to form under
the typical conditions a failed fuel rod would experience
in a spent fuel storage pool and to establish the technical
basis for a drying technique to remove these phases. The
presence of water, even in an absorbed phase, may be
detrimental to long-term dry storage of spent fuels.
Samples of unirradiated UO2 and spent fuel were hydrated
at various temperatures by placing them in deionized
water. The hydration products were observed and
identified using x-ray diffractometry. These hydrated
phases were then subjected to thermogravimetric analysis
under a variety of temperature and atmospheric conditions
to observe the decomposition of the hydrates. It appears
clear that hydrated phases will form during pool storage
and the industry standard vacuum drying techniques are
inadequate to remove the absorbed water from the fuel.
Introduction
A fraction of fuel rods are known to fail (via pin-holes in
the cladding or larger defects) during in-core operation
and storage in spent fuel pools. These failed rods may
become waterlogged and allow hydrated phases of the
fuel to form. The presence of small quantities of hydrated
fuel have been shown to increase the dry-air oxidation
rate by up to three orders of magnitude over fuels where
no hydration has occurred. This rapid oxidation is
important to consider in the event of off-normal or
accident scenarios involving the ingress of air during drystorage,
transportation, or repository conditions. In
addition, free and absorbed water remaining in a cask may
be subjected to radiolytic decomposition and form species
that could degrade the fuel, cladding, or system
components.
Current industry practices for drying commercial fuel
assemblies call for subjecting the assembly to a vacuum
of approximately 3 torr for 30 minutes and then repeating.
Based on studies of N-Reactor fuel at PNNL, such a
vacuum technique without applied heat and a properly
controlled atmosphere was inadequate for removal of
physisorbed and chemisorbed water from failed rods. The
aims of this project were first to determine whether
hydrated species are expected to form under the
conditions experienced in a spent fuel pool and second, to
determine the conditions necessary to remove these
phases. Our goal was to develop a simple and economical
means by which free and absorbed water could be
removed from failed fuel rods and minimize any future
degradation associated with the presence of water.
Approach
Approximately 1-gram samples of unirradiated UO2
fragments and powders were placed in glass vials. The
fuel was covered with about 8 mL of water and the vial
was then capped. Batches of samples were heated at
25°C, 60°C, 75°C, and 95°C. Each week, the samples
were shaken and stirred with a glass stirring rod to
provide maximum surface area contact of the fuel with
the water. Subsamples (~10 mg) from selected vials were
then removed for semi-quantitative analysis using x-ray
diffraction. The samples were prepared by combining a
known quantity of fuel with a similar, known quantity of
Al2O3, which served as an internal standard. Duplicate
samples were prepared for each of the unirradiated
specimens. A similar procedure was followed using spent
fuel powders. Samples of the original material had been
similarly analyzed to ensure that there were no hydrated
species prior to placement in the water.
Subsamples (~250 mg) of those specimens where x-ray
diffraction had positively identified hydrated phases were
then heated under a controlled atmosphere using a
thermogravimetric analysis system. The rate of weight
change as a function of time and/or temperature was
measured. The decomposition of the hydrated phases was
observed at heat ramp rates ranging from 0.2°C min -1 to
Nuclear Science and Engineering 365