Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 388

solubility limit is exceeded for the hydrides to form and grow, before the
crack extends further. The crack-tip can undergo a corrosion reaction and
the hydrogen released can either be absorbed by the matrix close to the
crack-tip or the hydrogen can diffuse through the matrix to the crack-tip.
The former is called corrosion hydrogen cracking and the latter is known as
DHC. Knowing the crack velocity enables prediction of the failure time of
the tube. Since there is no way to measure the crack velocity in the reactor,
it is assumed that the crack starts at the centre of the tube and proceeds in
both directions with a velocity in the range of 2.5 10 − 7 m/s which

is determined from out-of-pile unirradiated laboratory specimens. 75 It is
possible that the velocity may be much higher in reactor as the stress state
is more severe. The stress arises partly from the increased pellet volume
because of increased temperature (due to reduced thermal conductivity as
the pellet cracks up with burnup) and partly from the increased volume of
the oxide layer of the Zr-liner on the interior of the tube. The increase in
stress due to this volume expansion is faster than the creep relaxation by
the clad.

× 10 − 7 -6.6 ×

Fuel assembly bow

Various reports of incomplete control rod cluster assembly insertion trig-
gered investigations to identify causes of the sticking problem. The root
cause was understood to be excessive deformation (bowing) of fuel assem-
blies. When the bowing exceeds the limit, it increases friction between the
control rod and the guide thimble and can result in the breaking of the con-
trol rod cluster assembly. 72 The mechanism of bow, though not clearly under-
stood yet, is believed to be caused by creep affecting the overall assembly
and guide thimble. If the fuel assembly experiences a fl ux gradient, the tubes
at the lower fl ux side will grow less than those in high fl ux side causing
the fuel rod to bow. Provision is made in the design, to accommodate an
increase in length of the fuel rod (due to creep and growth) on either end
and any restriction in the free movement leads to bowing of the rod. Rods
under cold-worked stress-relieved (CWSR) conditions show more elonga-
tion than those under fully recrystallized conditions. 72

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Creep of fuel cladding

Fuel failure occurs when the cladding barrier is degraded and breached.
The fuel rod failure rate in LWRs has been signifi cantly reduced since
1987. 75 This is due, besides design improvements, to the introduction of
many improved variants of Zr-base alloys over the years. With the recent
developments in LWR fuels, Zr-1%Nb-Sn-Fe alloys with higher resistance
to irradiation-induced growth, creep and corrosion, are being used for guide