Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 197

temperature gradient is introduced by local oxide spallations (Strasser et al .,
2010b). Thus the probability of signifi cant hydride blisters forming depends
on the average hydrogen content of the cladding, the thickness of the oxide
pieces fl aked off (and the difference in oxide layer thickness adjacent to the
spalled region and at the position of oxide spallation) and on the heat fl ux.
Hydrides precipitate in the form of thin platelets on planes that depend
on cladding microstructure, heat fl ux and stress (Strasser et al ., 2010b ). The
orientation and continuity of these platelets with respect to residual or
applied tensile stresses strongly infl uences the embrittlement. The orienta-
tion of hydrides in clad tubes is affected by the thermo-mechanical treat-
ment of the tubes under manufacturing, and by the stress state prevailing
under precipitation. In the presence of a radial heat fl ux, hydride platelets
are generally oriented with their surface normals preferentially aligned to
the clad tube radial direction, and the width of the platelets along the tube
axial direction is signifi cantly larger than in the circumferential direction.
These hydrides, usually termed 'circumferential' hydrides, have only a mod-
erate embrittling effect, since there is no tensile stress in the clad tube radial
direction, that is in the direction perpendicular to the hydride platelets
(Northwood & Kosasih, 1983).
However, there are also hydride platelets oriented with their surface
normals more or less aligned to the clad circumferential direction (Strasser
et al ., 2010b). These hydrides, which are usually termed 'radial' hydrides, are
much more deleterious, since they are perpendicular to the dominating ten-
sile stress in clad tubes of high-burnup fuel rods. The fraction of these det-
rimental radial hydrides is larger in recrystallization annealed (RXA) clad
materials than in SRA cladding (Northwood & Kosasih, 1983). The former
heat treatment results in a larger fraction of grain boundaries in the radial
direction, and since hydrides tend to precipitate along grain boundaries, this
could to some extent explain the differences in hydride orientation between
RXA and SRA materials. However, there are also other causes to these dif-
ferences, such as difference in hydride size.
It is noteworthy that VVER fuel cladding will not fail due to PCMI
because of the very low hydrogen clad contents in VVER fuel claddings
( Strasser et al ., 2010b). Instead VVER fuel rods with rod internal overpres-
sures may fail due to creep burst.
If the cladding fails, fragmented fuel may disperse into the coolant
(Lespiaux et al ., 1997). This expulsion of hot fuel material into water has
the potential to cause rapid steam generation and pressure pulses, which
could damage nearby fuel assemblies and possibly also the reactor pres-
sure vessel and internal components. Hence, 'the potential consequences
of fuel dispersal are of primary concern with respect to core and plant
safety ' (Strasser et al ., 2010b). The fuel dispersal tendency for high burnup
fuel is very much dependant on the pulse width. For energy deposition with

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