What is 'nuclear melting'?

The melting of nuclear fuel rods when the temperature in the reactor's core rises to too high a level is called "nuclear melting."

When the water level is too low, the nuclear fuel sticks stick out of the water
and became too hot.Photo: NHK.

However, this is not the official term of the International Atomic Energy Agency.

Nuclear fuel rods are partially melting in two or three reactors of Japan's Fukushima Dai-ichi plant. But many people still don't understand what 'nuclear melting' is.

' Nuclear melting is the melting of uranium dioxide (UO2). The melting temperature of UO2 is 2.865 degrees Celsius , 'Livescience quoted Martin Bertadono, a nuclear expert at Purdue University in the US.

Nuclear melting often occurs when the cooling system cannot deliver water to the core of the reactor, which contains fuel rods.

Livescience said that under normal conditions, the fuel rods UO2 are placed in the water so that the temperature in the bar does not reach the melting point. Water is pumped continuously through the core of the reactor. But if the water is not pumped at a sufficiently high rate, it will heat very quickly due to the large amount of heat. When the temperature reaches the boiling point, water will evaporate.

' Due to boiling water, the water level in the reactor will decrease. When the water level drops below the top of the fuel rods, the bars will heat up. If the fuel bar protrudes from the water for about an hour, the uranium begins to melt. In case the water evaporates, all the fuel rods in the reactor will melt , 'Bertadono explained.

By injecting seawater into the reactor when the cooling system is not working, the technical experts of the Fukushima Dai-ichi plant want to prevent the melting of all fuel rods. However, the fuel rods have partially melted.

If the fuel rods protrude from the water long enough, they will melt.Photo: NHK.

What happens when a partially molten fuel bar?

'When nuclear fuel melts, the products of nuclear decay will escape from the core and penetrate into the compression tank. They then escaped from the compression tank if water leaked out of the tank , 'said Taiwo Temipote, a nuclear researcher at the Argonne National Laboratory of the United States.

Of course, after escaping from the compression tank, the radioactive material still had to pass another ' gate ' to spread out. It was a metal cover that surrounded the reactor. This cover is designed to prevent the spread of radioactive material. If this shell is damaged or destroyed by an internal or external explosion, the situation will become serious.