Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 386

between fuel and clad becomes established, leading to a condition called
pellet clad interaction (PCI). In extreme cases, these factors lead to cracking
of the clad and to release of the fi ssion gases into the coolant. Such situa-
tions may demand reduction of reactor power or the shutdown of the reac-
tor if safe discharge limits are crossed. The strain concentration produced in
the cladding by the sharp corners of radially cracked pellet during a power
increase is increased by (i) the increasing coolant pressure, (ii) the pellet/
cladding friction coeffi cient, (iii) the pellet radius and (iv) the circumferen-
tial temperature gradient. This severity can be marginally reduced by using
a stronger clad material and by reducing the number of radial pellet cracks.
For burnups greater than 70 MWd/kgU, a high burnup structure (HBS) with
bubble size much larger than the grain size forms and traps the fi ssion gases.
A rapid rise in temperature can lead to shattering of the HBS and can put
severe stress on the cladding. 69

1.4.2 Cladding

The standard cladding material in a LWR is a dilute zirconium-base alloy
containing some other elements such as tin, niobium, iron, nickel, chro-
mium and oxygen (Zircaloy-2 for BWR and Zircaloy-4 for PWR). Being
a hexagonally close packed (HCP) crystal structure and hence inherently
anisotropic, zirconium acquires further anisotropic properties after fabri-
cation due to induced texture (Fig. 1.25b) with the <c> axis of the HCP
crystal oriented at ~30° from the radial direction of the tube. Minor modi-
fi cations in the chemistry of the alloys are made to reduce the water side
corrosion in the clad tubes. Formations of inter-metallic precipitates (which
increase the corrosion rate) are avoided by giving the clad material a beta
quench (fast cooling from the beta phase). The fuel and the pressure bound-
ary (clad tube) experience time-related ageing and degradation; the former
may affect linear power rating while the latter can lead to catastrophic clad
failure. The burning of the fuel leads to release of fi ssion gases and to fuel
swelling. The fuel makes contact with clad which has picked up hydrogen
from the coolant and leads to degradation of the clad tube. 70 In order to
improve the structural rigidity, spacer grids made of Inconel/Zircaloy are
placed at defi nite intervals along the length of the assembly which holds the
fuel elements in the assembly with spring forces. When these forces relax
(due to creep) a gap is created between the grid and rod, and the rod can
vibrate. This fretting may result in a breach in the clad integrity. 71

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Hydride related problems in clad

Hydride related problems are viewed as an issue to be considered for
clad failure. The main sources of hydrogen for the clad are: the corrosion