Environmental Engineering Reference
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narrow pulse widths, heat conduction from the rim region is low and leads
to higher local temperatures in the rim region due to the radial power peak-
ing. Energy deposition with wider pulses allows for heat conduction from
the pellet to the cladding, thus minimizing the temperature peaking in the
pellet rim and maximising the fuel clad ductility. Lowering the pellet rim
temperature decreases the potential for fuel particle dispersal.
5.4
Materials performance during interim
dry storage
Most countries that generate nuclear power are in the process of develop-
ing criteria, designs and sites for the permanent disposal of spent nuclear
fuel, but they have yet to become licensed realities (Adamson
et al
., 2010 ).
The most signifi cant fuel related criteria for dry storage are compared in
Table 5.1. Meanwhile the pools at the nuclear plant sites are fi lling up with
spent fuel and the utilities are transferring the spent fuel from the pools to
dry cask storage sites that are mostly located at the plant sites. Exceptions
are the central, large intermediate pool facilities that serve all the plants in
Sweden (CLAB facility) and all the plants in Finland (KPA-STORE). The
lack of a licensed permanent fuel repository in any country has placed total
reliance on intermediate storage. As a result the dry storage technology has
become a major activity and business component of today's back-end fuel
strategies.
The key differences between dry storage and in-reactor performance of
fuel are (Adamson
et al
., 2010 ):
Long storage time - 40 years or more.
Inert gas, helium (He) storage atmosphere instead of pressurized or
boiling water (decreased heat transfer, but no corrosion).
Decay heat that can raise the cladding temperature to 400°C or higher,
then decreases over time.
Atmospheric storage pressure that, combined with high fuel rod temper-
ature and internal gas pressure, results in a high clad
Δ
P and clad stresses
that decrease with time as well.
No external radiation (no additional radiation damage).
Dry cask storage containers dissipate the fuel decay heat by natural con-
vection of the cask He atmosphere and conduction through the cask
container walls; there are no moving parts or forced cooling in this sys-
tem. As a result, the cladding can reach temperatures of several hundred
degrees Celsius. The pressure differential across the cladding can be sig-
nifi cant since the fuel rod internal gas pressure is made up of (Adamson
et al
., 2010 ):
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