Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 192

Fuel relocation

ECR

PCT

1200

Cooling

Clad

Oxidation

Quenching

800

Burst

Rupture

400

Ballooning

Coolant blockage

0

0 50 100 150

Time (s)

5.9 Typical LOCA in a PWR (Strasser et al ., 2010b).

down somewhat, partly due to cooling by the steam-water mixture that is
formed, but the cladding temperature will continue to rise.
During and after the LOCA it must be ensured that (Strasser et al .,
2010b ):

￿ The core remains coolable (which means that the maximum allowable

coolant blockage is limited)
No fuel dispersal occurs (which means that cladding rupture is not

￿ ￿ ￿ ￿ ￿ ￿ ￿

allowed; it is assumed that the cladding burst is so small that only fi ssion
gases are released)
Less than 10% of the fuel rods in the core fail through burst (but without ￿

fuel dispersal) (a requirement in Germany only).

Ballooning of the cladding

The loss of coolant fl ow decreases heat transfer from the fuel, increases
the fuel temperature and causes a signifi cant temperature rise of the clad-
ding (Strasser et al ., 2010b). The decrease in system pressure causes a pres-
sure drop across and a hoop stress in the cladding. The result is the creep