Performance and inspection of zirconium alloy fuel bundle components in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 195

5.3.2 Materials performance during reactivity-initiated
accidents (RIA)

The design basis RIA in a PWR is the control refection rod accident (REA)
and in a BWR the control rod drop accident (RDA) (Strasser et al ., 2010b ).
The REA is based on the assumption of a mechanical failure of the control
rod drive mechanism located on the reactor vessel top, followed by the ejec-
tion of the mechanism and the control rod by the internal reactor pressure.
The resulting signifi cant power surge is limited partly by Doppler feedback
and fi nally terminated by the reactor trip. The BWR RDA is assumed to
occur if a control rod is detached from its drive mechanism in the core bot-
tom, stays stuck while inserted in the core and then, if loosened, drops out
of the core by gravity, without involvement of a change in reactor pressure
as in the REA. As a result the BWR power pulses are slower and the pulse
widths wider than for a PWR. The pulse widths for PWRs are in the range
of 10-30 ms and for BWRs in the range of 20-60 ms.
The reactivity transient during a RIA results in a rapid increase in fuel
rod power leading to a nearly adiabatic heating of the fuel pellets (Strasser
et al ., 2010b). In a fresh fuel rod, the fi ssile material consists predominantly
of U-235, which is usually uniformly distributed in the fuel pellets. Hence,
both power and fi ssion products are generated with a relatively small varia-
tion along the fuel pellet radius. However, with increasing burnup, there is a
non-uniform build-up of fi ssile plutonium isotopes through neutron capture
by U-238 and formation of Pu-239 and heavier fi ssile isotopes of plutonium.
Since the neutron capture takes place mainly at the pellet surface, the dis-
tributions of fi ssile material, fi ssion rate and fi ssion products will develop
marked peaks at the pellet surface as fuel burnup increases. The highest
temperatures are occurring at the fuel pellet periphery.
The RIA-simulation experiments conducted in the 1960s and 1970s using
zero or low burnup test rods showed that cladding failure occurred primar-
ily by either (Strasser et al ., 2010b ):

￿ ￿ ￿ ￿ ￿ ￿

￿ Post-DNB brittle fracture of the clad material occurring during the

re-wetting phase of the overheated heavily oxidized (and thereby
embrittled) clad due to the abrupt quenching resulting in large thermal
clad stresses. This failure mode is imminent if the cladding is severely
oxidized due to the RIA fuel clad temperature excursion.
Cladding contact with molten fuel. ￿

Contrary to low burnup rods, the failure mechanism for BWR/PWR high
burnup rods not subjected to DNB is PCMI and potentially creep burst (for
rods with a rod internal overpressure and subjected to DNB) (Strasser et al .,