Core issues: worst-case nuclear reactor scenarios

The China Syndrome may be the stuff of Hollywood films - where the unstoppable core plunges through the Earth from one side of the globe to the other - but what does a full reactor meltdown mean?

And what is its impact on the environment, food stocks and humans?

Nuclear experts point out that Japan's boiling water reactors are nothing like the reactors at Chernobyl.

Chernobyl, in Ukraine, was the site of the world's worse civilian nuclear power plant accident in 1986. Graphite - combustible at higher temperatures - was used to cool the fuel rods and there was no container structure around the reactor. When the rods failed to control the nuclear fission chain reaction, explosions occurred, releasing radioactive plumes that blew across Europe.

The General Electric-designed reactors at Fukushima Daiichi plant use water rather than graphite, so a similar explosion is not possible.

Experts also stress that a nuclear explosion is impossible, even if there were no container structures or if they all failed, as the fuel in the rods are not sufficiently enriched.

“The suggestions of a possible nuclear explosion are ill-founded and not based on scientific fact,” Professor Richard Wakeford of Manchester University's Dalton Nuclear Institute told the Financial Times.

Concerns instead centre around the possible melting of the fuel rods in the reactors.

The role of the fuel rods

At the core of the Fukushima Daiichi reactors, zirconium metal rods hold uranium fuel pellets.

When the reactor is operational, the neutrons from the uranium fuel break-up the atoms, causing a chain reaction that generates heat and produces radioactive elements like strontium and caesium as well as more neutrons.

The heated rods are kept cool by water pumped between them. The heated water then turns to steam, which drives a turbine to generate electricity.

The core is within a reactor vessel made of thick steel. Around it is a concrete containment structure. The top layer is a building structure that does not play a role in containing the core. It was these structures that were damaged when the explosions occurred at Daiichi's No.1 and No.3 reactors. Officials have not commented on the damage caused by the explosion at No.2 this morning.

Emergency shutdown - but where's the water?

When Friday's earthquake and tsunami occurred, all of Japan's 55 nuclear plants shut down automatically as they were designed to do.

But the loss of power and tsunami damage to back-up generators crippled reactor cooling systems.

This left a hot fuel mixture of radioactive materials including uranium, plutonium, strontium and caesium to "decay" or cool down over time.

Workers pumped the reactors with seawater to cool down the rods. But the temperatures were so high that water was boiling faster than the water being pumped in, and the fuel rods became exposed to air.

"If [the rods] are exposed for a sufficiently long time, there is a point at which the rising temperatures can start to basically melt the rods and they lose their vertical configuration in the core," said the spokesman for the US nuclear energy institute, Steve Cerekes.

The exposed rods and soaring temperatures are already believed to have damaged the zirconium metal, releasing radioactive gases and hydrogen.

There have been hydrogen explosions at all three reactors, although officials have not been clear about what caused them.

Worst-case scenario

The fear is that a full meltdown will occur. The rods melt to a point where they breach both containment structures and escape into the environment, contaminating the soil and releasing radioactive particles into the air.

"The worst case scenario is ... the fuel rods fuse together, the temperatures get so hot that they melt together in a radioactive molten mass that bursts through the containment mechanisms," nuclear expert Joe Cirincione of the anti-nuclear group Ploughshares Fund told Agence France-Presse.

These radioactive particles are then picked up by winds, and depending on where they are blowing at any given time, a nuclear cloud could, for example, move across the Pacific to the west coast of the United States, causing possible health problems and contaminating food stocks.

Australia would not be contaminated by such radioactivity as it is not part of the northern hemisphere jet stream.

The World Meteorological Organisation said today that current winds would blow radioactivity over the ocean, away from Japan and other Asian countries. But it warned that weather conditions could change.

Four countries in Asia - Hong Kong, the Philippines, Singapore and South Korea - are already testing Japanese food imports for radiation.

Radiation levels near the Daiichi nuclear power plant reached as high as 400 millisieverts (mSv) an hour, Japanese officials said.

In comparison, people are exposed to about 2mSv of natural radiation a year. A chest X-ray exposes you to about 0.02 mSv of radiation, while about 5 per cent of people exposured to a cumulative level of 1000 mSv would suffer from a fatal cancer years later, Reuters reported.

Partial meltdown in 1979

A partial meltdown of the core of a nuclear reactor took place at the Three Mile Island plant in Pennsylvania in the United States in 1979, the last time a major nuclear emergency occurred.

The site was permanently closed, but only "very small" amounts of radioactivity were released, the US Nuclear Regulatory Commission said.

That incident was rated five on a scale of seven (Chernobyl was rated seven). Japanese authorities have so far rated the Fukushima problems at four, although the head of France's nuclear safety agency said this morning it was more on the scale of level five or six.

Will a full meltdown take place?

Nuclear scientists said a full meltdown is unlikely to occur.

"It's entirely credible that the worst-case now does not involve break-out of a pressure vessel," nuclear and technology expert Malcolm Grimston at the British think-tank Chatham House told Reuters.

Professor John Gittus of the University of Swansea told the Times the likelihood of a radioactive disaster as on the scale of Chernobyl was "one in a hundred".

"But if it does happen, the most likely consequence would be a few dozen people dying of radiation poisoning and a large area of land contaminated."

They point to the natural decaying process, which means that the heat from the fuel would reduce over time.

"The major period that you have to be concerned about once a reactor has gone off line like this is the first 72 hours," said Mr Cerekes.

"After that, [the temperatures] begin to really drop off. It doesn't mean you are out of the woods at that point, but it means that you have a more manageable situation."

'Feed and bleed'

If the seawater levels pumped into the cores stay high and the partial meltdowns that reportedly took place at the reactors do not breach the containment structures, the worst may be over.

A resumption of normal electrical power is also critical to this process.

But engineers may still have to balance the venting of the steam from the boiling water with the feeding in of more water in a procedure called "feed and bleed".

If the steam is not vented, the pressure that builds up in the core could cause an explosion that in turn could threaten the integrity of the containment structures.

Damage to the suppression pool of No.2, which was reported this morning, could also reduce the ability of engineers to manage the pressure within the core.

How much do we know?

Compounding the situation is the apparent lack of frankness from Japanese officials about the nuclear plants, as highlighted by local and international media over the past few days.

A 2008 US embassy cable from Tokyo, obtained by WikiLeaks and published by The Guardian today, canvassed the fears expressed by a Japanese politician about power plant cover-ups.

Taro Kono reportedly told US diplomats the Japanese ministry in charge of nuclear energy had been "covering up nuclear accidents and obscuring the true costs and problems associated with the nuclear industry".

"The ministries were trapped in their policies, as officials inherited policies from people more senior to them, which they could then not challenge."

The Tokyo Electric Power Company (TEPCO), which runs the Fukushima Daiichi plant, is also not free from controversy.

In 2002, five executives from the firm, including its chairman, resigned after reports of safety cover-ups at their plants surfaced.

Further reading:More about the Chernobyl accidentMore about the Three Mile Island accidentAbout radiation and safety