Regional Basic Professional Training Course in Korea

this stage.
Control rod degradation
❙ 923 ❙
16. In‐Plant Accident Management
Under the effect of temperature, the control rod components (Ag, In, Cd) vaporize; cause
the clad to burst (at about 1300‐1500°C) and these elements to be released into the reactor
coolant system.
Zirconium fusion and eutectic formation
At about 1800°C, the non‐oxidized part of the clads will melt and start to run, like candle
wax. If this liquid zirconium comes into contact with the uranium oxide, it will cause
eutectic reactions, which will degrade the pellets at a temperature lower than the UO2
fusion temperature.
Fusion of zirconium oxides and uranium
At a higher temperature (about 2700 – 2800°C), the oxides melt. The fusion of the
different elements will cause the breakdown of core geometry in the form of local or total
collapses.
High pressure situations
If core degradation occurs at high pressure, its clad, by deforming, will be crushed on the
fuel pellets. On contact, the previously mentioned reaction, between zirconium and
uranium oxide, will occur. This will lead (from 1200 – 1500°C) to the formation of liquid
eutectics (U‐Zr‐O). The degradation mechanisms (oxidation, melting) are similar to the
ones described for the low‐pressure situations.
Flow of molten material into the lower plenum
The melting process will follow the route of candling, blockage at the lower edge of the
core, melt pool formation in the core, its break through either on the side or at the bottom
to pour into the lower head.