Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 385

100

80

60

40

5

20

1

0

0

15 28
Stack fault energy (mJ/m 2 )

36 61

1.32 Effect of stacking fault energy and dose on the strain to failure. 66

specifi c structures and materials that experience degradation during reactor
operation.

1.4.1 Fuel

The fuel U-235, used in PWRs in its oxide form UO 2 , is enriched up to 5%
(maximum) and is used as dried pellets of about 10 mm in diameter and
10 mm in height with theoretical density (TD) ~95%. The stoichiometry
of UO 2 is kept such that the ratio of O:U is never allowed to go beyond
2:1. The surface temperature of the fuel can reach ~1400°C with the centre
temperature still higher. The oxide pellets are enclosed in a Zircaloy clad
tube and the tube is capped on both sides to make a fuel rod. The fuel-clad
gap is fi lled with high thermal conductivity helium gas and the conductiv-
ity degrades slowly with the fi ssion gases diluting helium. Many such fuel
rods (17 17) are bundled to form a fuel assembly. Many such fuel assem-

blies (~200) are immersed in a pool of light water, fl owing at a pressure of
~16 MPa, which is the heat transfer fl uid in the primary loop of a PWR.
During start up, the pellet-clad gap gets reduced due to thermal expansion
of the fuel but soon increases as the fuel densifi es under irradiation. At high
burnups, the gap slowly reduces and eventually an intimate contact is estab-
lished with the fuel swelling and the clad collapsing under creep due to the
coolant pressure ( clad creep-down). All cladding tubes have some ovality
which increases with creep-down and the direct impact of creep-down is the
increased gap (between rods) and increased water volume. This increased
water volume increases the moderation effect and gives rise to power peaks
in the neighbouring fuel pellets. 68 The swelling of the fuel applies a severe
hoop stress on the clad which slowly gets embrittled by irradiation and by
absorption of hydrogen. The thin oxide layer on the clad breaks and bonding

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