Environmental Engineering Reference
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that these criteria have to meet, as defi ned by the USNRC, are specifi ed
in 10CFR71.73 (NRC, Rules and Regulations, Title 10 Code of Federal
Regulation, Chapter 71). Of all the accident scenarios the most limiting
scenario is a free drop of the cask for a distance of 9 m (30 ft) onto a fl at,
unyielding horizontal surface, striking the surface in a position that would
cause the maximum fuel damage.
Radial hydrides in zirconium alloy cladding are undesirable because
they reduce the critical stress intensity required to propagate a radial
crack through the wall of the cladding during handling or transportation
(Adamson
et al
., 2010). The objectives of the dry storage regulations are to
limit the conditions that could result in hydride re-orientation.
A certain fraction of the hydrogen (H) picked up during the oxidation
reaction is soluble in the zirconium matrix and the remainder forms zirco-
nium hydrides (Adamson
et al
., 2010). The solubility of the H is a function
of temperature, alloy composition and microstructure. Solubility is also a
function of irradiation history, heating or cooling rates during service. The
orientation of the hydrides formed during normal reactor operation are
generally circumferential near the cooler cladding OD and remain so dur-
ing wet storage of the spent fuel.
The hydrides can reorient in the radial direction if they are precipi-
tated from solid solution by cooling the alloy from a higher temperature
under a tensile or hoop stress (Adamson
et al
., 2010 ). The hydrides will
align themselves in the direction perpendicular to the tensile stress. This
can occur during reactor operation although it is generally unlikely. It
could occur during dry storage if the internally pressurized cladding is at a
high temperature, holds suffi cient hydrogen in solution and is then cooled
while under the hoop stress. The hydrides in solution will precipitate in the
radial orientation (provided the hoop stresses are large enough), while the
hydrides that did not dissolve will remain in their original circumferen-
tial orientation. This is most likely to occur during rapid cool-down from
high temperatures after cask drying or evacuation procedures rather than
during storage when the rate of temperature and pressure reduction that
control the stress levels are extremely slow.
In summary, the factors that affect hydride re-orientation in irradiated
cladding are (Adamson
et al
., 2010 ):
Hoop stress.
Maximum temperature.
Cool-down rate and fi nal temperature.
Solubility of H in the specifi c alloy at its specifi c burnup that will deter-
mine the amount of H in solution at the maximum temperature and the
amount of circumferential hydrides.
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