Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 203

Creep-rupture is the most likely cladding failure mode during dry storage
and there is general consensus on this mechanism among the industry and
the regulators (Adamson et al ., 2010). The parameters that determine the
potential for creep rupture are the cladding stress level, the cladding tem-
perature and the rate of decay heat decrease, all parameters that are burnup
dependent.
The cladding temperature , currently one of the primary USNRC licens-
ing criteria, is determined by the decay heat generated in the fuel, the heat
transfer capability of the cask and the surface temperature of the cask in its
storage environment (Adamson et al ., 2010). The decay heat is generated
primarily by absorption of the alpha decay either directly or indirectly from
the plutonium (Pu) isotopes. Increasing burnup will increase the level of Pu
isotopes formed by transformation of the 238 U, and increase the cladding
temperature in dry storage conditions. In comparison, MOX fuel will have
signifi cantly higher temperatures under the same conditions.
Several other potential failure mechanisms were considered, but elim-
inated as highly unlikely (Rashid, 2006). They are summarized below
(Adamson et al ., 2010 ).
Stress corrosion cracking ( SCC ) is not a credible failure mechanism in dry
storage because:

￿ There is insuffi cient elemental iodine present to cause SCC.

￿ At the stress and strain rates in dry storage, initiation of intergranular

cracking is nearly impossible; the 180-200 MPa stresses needed for SCC
are well above those for high burnup fuel rods.
Hydrides, including radial hydrides, will not affect iodine induced SCC. ￿

￿ The occurrence of all the conditions that cause DHC is highly unlikely,

but cannot be ruled out. The initial conclusions are based on the follow-
ing evaluation:
Analyses indicated that at a hoop stress of 250 MPa (well above dry ￿

￿ ￿ ￿ ￿ ￿ ￿

storage conditions) in a cladding wall thickness reduced by 100 μ m

oxide with an 83 m crack size, the stress intensity factor is below

that needed to initiate the DHC process,
Hydride re-orientation that might assist crack propagation is

μ

￿

intended to be minimized or prevented by current regulations and
industry practices, but cannot be ruled out.
In addition, propagation of a crack assisted by radial hydrides may ￿

not occur for many of the hydride morphologies.

Also under accident conditions during storage or subsequent transporta-
tion, the fuel must remain subcritical and should be recoverable by nor-
mal methods (Adamson et al ., 2010). The hypothetical accident conditions