Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 190

The axial split formation is schematically shown in Fig. 5.8 (Strasser et al .,
2008). Initially, the control rod is inserted during the time when the primary
defect occurs (1 in Fig. 5.8). The same scenario as for transversal breaks in
BWRs occurs, but the secondary hydrides are distributed to several fuel
clad locations which means that each hydride becomes too small to encom-
pass the whole fuel clad circumference (2 in Fig. 5.8). The tensile stresses in
the cladding which are necessary for crack propagation result from a power
increase in the failed rod, for example, when a control rod adjacent to the
failed rod is pulled out of the core. This will increase the temperature in
the fuel stack resulting in a thermal increase of the pellet diameter. If these
stresses become large enough the sharp defect may propagate if the result-
ing K I exceeds the critical value for crack propagation (3 in Fig. 5.8). It is
proposed that the mechanism for crack propagation forming an axial split
is a delayed hydrogen cracking (DHC) type failure process (see e.g. Efsing
& Pettersson, 1998; Edsinger, 2000; Lysell et al ., 2000 for more details). The
lower bounds of the crack velocities are in the range 4 10 − 7 ms − 1

based on assumed constant growth rates in the time between fi rst detection
of the defect and removal of the fuel (Strasser et al ., 2008 ).

× 10 − 8 -5 ×

Steam

1 2

￿ ￿ ￿ ￿ ￿ ￿

Axial split formation

3

5.8 Schematic showing the events resulting in axial split formation. The
numbers in the fi gure relate to the sequence of the different events that
may lead to an axial crack as described in the text (Strasser et al ., 2008).