Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 191

Axial cracks in PWRs/VVERs - Long axial cracks do not form in PWRs
as readily as in BWRs (Strasser et al ., 2008). The reason for the difference is
that in PWRs, the power regulation is done slowly and without pronounced
increases in local power by decreasing the boron coolant concentration,
while power regulation in BWRs is done by a combination of control rod
movements and variations in coolant fl ow, with the control blade move-
ments leading to rapid increases in local power. However, axial cracks may
form in PWRs/VVERs by essentially the same mechanism as formation of
long axial cracks in BWRs due to (Strasser et al ., 2008 ):

￿ A class II transient and/or

￿ Due to control rod movements in load-following plants.

5.3 Materials performance during accidents

Having considered normal operating conditions, we now move on to cover
accident scenarios.

5.3.1 Materials performance during loss of coolant
accidents (LOCA)

The LOCA event starts with a decrease and then the loss of coolant fl ow
due to a break in the coolant pipe; at the same time the reactor is depres-
surized, scrammed and shut down (Strasser et al ., 2010b ). The fuel starts
heating up due to its decay heat until the emergency core cooling systems
(ECCSs) are activated and fuel cooling commences. Hypothetical LOCA
events are analyzed for each reactor to ensure that the safety criteria, as
defi ned by the regulators for the reactor system and the fuel, are met.
The design basis accidents (DBAs) which are analyzed fall into two gen-
eral categories. The large break, or large break loss of coolant accident
(LBLOCA), assumes a double ended break of a primary coolant cold leg
of a PWR or a break in the recirculation pump intake line of a BWR,
either of which could cause the loss of all the coolant from the core. The
small break, or small break LOCA (SBLOCA), assumes a break in one of
the smaller primary circuit lines that will cause less coolant loss than the
LBLOCA.
The effect of a LOCA cycle on the fuel is shown schematically in Fig. 5.9,
plotting the fuel and cladding temperatures as a function of time in the acci-
dent (Strasser et al ., 2010b). The loss of coolant fl ow and reactor pressure
at the initiation of the accident will decrease heat transfer and allow the
fuel and cladding to heat up until the reactor scrams. The fuel will then cool

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