Environmental Engineering Reference
In-Depth Information
deformation or ballooning of the cladding. Depending on the temperature,
the cladding ductility and the rod internal pressure, the cladding will either
stay intact or may burst. Ballooning of the fuel rods may result in a block-
age of the coolant sub-channel that, in turn, may impact the fuel coolability.
If large fuel clad burst strains occur at the same axial elevation, co-planar
deformation in the fuel assembly can result and the coolability may be sig-
nifi cantly degraded. The extent of the ballooning is also dependent on the
fuel clad hydrogen content (picked up during the water-zirconium alloy
corrosion reaction during reactor operation prior to LOCA). Hydrogen
decreases the
phase transformation temperature, which means that
increasing the hydrogen content in the fuel cladding will lower its ductility
and result in more fuel rod bursts during a LOCA.
α
/
α
+
β
Oxidation of the cladding
The increasing temperatures and presence of steam will cause the intact
cladding to oxidize on the outer diameter (OD) and the burst cladding
to oxidize on both the OD and ID (two-sided oxidation) (Strasser
et al
.,
2010b). The oxidation process at the high LOCA temperatures will increase
the oxygen and hydrogen content in the cladding, reducing its ductility
and resistance to rupture. Two sided oxidation can have signifi cant effects
on the post-quench ductility (PQD) of the cladding as a result of high
but localized hydrogen pickup in addition to the oxidation (Strasser
et al
.,
2010b). The cladding continues to oxidize until the ECCS becomes effec-
tive and a peak-cladding temperature (PCT) is reached. The maximum
PCT is regulated to be a maximum of 2200°F by the USNRC and 1200°C
internationally. The length of time the system may remain at the PCT is
determined by the reactor system and regulated by the equivalent clad-
ding reacted (ECR) limit, defi ned as the total thickness of cladding that
would be converted to stoichiometric ZrO
2
from all of the oxygen con-
tained in the fuel cladding as ZrO
2
and oxygen in solid solution in the
remaining metal phase.
Embrittlement of the cladding
ECCS activation will stop the temperature rise and start to cool the core
by injection from the bottom of the core in a PWR and from the top of the
core in a BWR (Strasser
et al
., 2010b). The 'cooling' process as shown in Fig.
5.9 is relatively slow until the emergency coolant contacts the fuel that has
been at the PCT. At that point, in the range of 400-800°C and identifi ed as
'quenching' in Fig. 5.9, the water from the ECCS will reduce the cladding
temperature at a rapid rate (1-5°C/s) by re-wetting the cladding heat trans-
fer surface. The process will collapse the vapour fi lm on the cladding OD
Search WWH ::
Custom Search