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Reactor No 4 at the Chernobyl nuclear power plant, covered by a cement sarcophagus following the 1986 meltdown. S Ryan
Nuclear Meltdown

What happens in a nuclear meltdown?

Our guide to what happened at Chernobyl in 1986 and what’s going on in Fukushima, Japan, today.

JAPANESE AUTHORITIES ARE STRUGGLING to prevent further explosions at nuclear facilities after damage caused during Friday’s earthquake and tsunami.

However, officials in Japan have denied that their situation could escalate into a Chernobyl-like scale of nuclear devastation.

On 26 April, 1986, reactor number four at the Chernobyl nuclear power plant suffered a meltdown.

But what does that mean?

Electricity is produced in nuclear power stations through steam. The nuclear reactors heat up, raising the temperature of water around the reactor core to produce steam which fuels an electricity turbine. In turn, water pumped around the core helps to cool it down and prevent the reactor from overheating.

Plant workers also insert control rods to absorb excess neutrons by-produced in the nuclear reaction.

In the Chernobyl incident, plant operators were carrying out a safety test which triggered a power surge. That surge prevented the control rods from working properly, allowing the reactor to continue heating up until it exploded.

The core then melted down into the ground. The roof on the reactor was blown open in the explosion before meltdown, allowing toxic radioactive substances to leak out into the air.

In March 1979, the Three Mile Island facility in Pennsylvania suffered a partial meltdown after reactor coolant escaped and the core overheated. The situation was brought under control before a full meltdown occurred.

The Fukushima power station

At the Fukushima nuclear facility in Japan, coolant pumps and their back-up diesel generator were knocked out by the earthquake and tsunami on Friday, meaning the core’s heat was not being reduced.

Officials then began injecting seawater into the affected parts of the plant as an emergency effort to cool the reactors. Using seawater to cool a reactor means it can never be used to generate electricity again.

This morning the IAEA confirmed there had been a hydrogen explosion at one of the reactors at the Fukushima plant – the second at the plant in three days. Hydrogen gas can form when the hot metal-covered fuel rods chemically react with the water. Officials say that the explosions damaged the outer shell of the buildings and not the containment units inside.

Traces of radiation have been detected locally after plant operators released some of the hydrogen in an effort to ease pressure on the containment unit. That hydrogen contained small quantities of radioactive elements.

Plant operator TEPCO says it continues to monitor radiation levels in the “site’s immediate surroundings”.

Today, Japanese nuclear authorities said they believe three reactors at the Fukushima plant are now melting. The IAEA has created a Facebook page which will be regularly updated with information on the situation in Japan.

According to the IAEA’s International Nuclear Event Scale (INES), Chernobyl ranked as a seven out of seven, or a “major accident”.

On Saturday, Japanese authorities said that their nuclear situation was ranked as a four on the INES, which is considered an “accident with local consequences”.

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