Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 194

and cooling will be by nucleate boiling. Thermal shock due to the sudden
change in heat transfer conditions can fracture the cladding at this stage and
the ability of the cladding to withstand the thermal stresses will depend on
the extent of oxidation and degree of cladding embrittlement that occurred
during the LOCA transient.
The oxidation embrittlement process and fi nal structure of the cladding
after completion of the LOCA cycle is as follows (Strasser et al ., 2010b ):

￿ First, the increasing water and steam temperatures during heat-up

increase the reaction rates with the cladding and increase the conver-
sion of the cladding surface into thicker ZrO 2 fi lms.
As the LOCA temperature passes the levels where ￿ α → β transforma-

tions start and fi nish, the resulting structure consists of:
The growing ZrO −

2 layer.
A zirconium alloy layer with a very high oxygen content which sta-

−

phase.
The bulk cladding which is now in the

bilizes the α

− phase.

The ECCS initiated quenching phase cools the cladding back down

β

￿

through the transformation temperature and the bulk cladding is

now re-transformed from the

β → α

β into the α phase and referred to as the

'prior or former β phase.'

Oxygen and hydrogen affect the formation of the structure as follows dur-
ing the oxidation (Strasser et al ., 2010b ):

￿ Oxygen diffuses from the ZrO

2 to the bulk cladding which is in the

β

phase at the high temperature (HT); however, the β phase has a low sol-

ubility for oxygen.
Increased hydrogen levels from the oxidation reactions prior to and dur-

￿

ing the LOCA increase the diffusion rate and solubility of oxygen in the

￿ ￿ ￿ ￿ ￿ ￿

β phase >1000 ° C.

Wherever the solubility limit of oxygen in the ￿ β phase is exceeded, the

excess oxygen stabilizes the α phase.

￿ The oxygen stabilized phase forms next to the ZrO 2 layer and grows,

as does the ZrO 2 layer, at the expense of the bulk cladding in the

α

α phase

and as a result after quenching in the 'prior β phase.'

The fi nal integrity of the cladding is based on the properties of the prior β

phase, since the ZrO 2 and oxygen stabilized zones are too brittle to sus-

tain a load (Strasser et al ., 2010b). 'Oxygen is the major source of cladding
embrittlement as noted above and hydrogen is less likely to contribute to
the embrittlement except to the extent that its presence increases the oxy-
gen solubility' (Strasser et al ., 2010b ).

α