Q&A: How worried should we be about nuclear catastrophe?

How serious is this incident?

On a scale from zero to seven, the incident at Fukushima Daiichi has been rated as level four, meaning "accident with local consequences". The International Nuclear and Radiological Event Scale classified the Three Mile Island disaster in the US in 1979, which resulted from a partial meltdown of a reactor core, as level five, or "accident with wider consequences" – the same category as the Windscale fire of 1957. The Chernobyl crisis in 1986, when a reactor blew apart with a catastrophic release of radioactivity, was classed as seven, a "major accident".

The seriousness of this incident, however, depends on whether the Japanese authorities are able to prevent the reactor containment vessels housing the nuclear fuel from cracking open. They are trying to keep the nuclear fuel rods from being exposed to air and overheating, which is why they are flooding the containment vessels with seawater mixed with boron.

What caused the explosions?

Two nuclear reactors suffered hydrogen explosions that ripped apart the outer containment buildings housing the reinforced-steel reactor vessels where the nuclear fuel is kept. The first explosion occurred in reactor unit 1 on Saturday and the second occurred in unit 3 yesterday. Both happened as a result of the build-up of hydrogen gas, which is highly explosive. This was a result of the decision to vent gases and superheated steam from within the pressurised reactor vessels, which were becoming dangerously overheated. The hydrogen was produced as a result of superheated water coming into contact with the zirconium casing housing the nuclear fuel rods. The explosions, which are not nuclear explosions, have not cracked the steel containment vessels housing the nuclear fuel, Japanese authorities say.

How did the crisis develop?

The earthquake caused the turbines and reactors to close down, as they are designed to do in such a seismic emergency. This involved the insertion of control rods in between the fuel rods to stop the nuclear reaction. However, heat continued to be generated, which had to be controlled by the cooling system, which relies on the pumping of water through the reactor vessel.

The shutdown, however, was also accompanied by a loss of mains power, which caused emergency diesel generators to kick in. It was crucial to maintain power to the cooling system. But within an hour, these also broke down, probably due to the tsunami, which measured at least 7 metres high, compared to the maximum 6.5 metres that the facility was designed to withstand.

Mobile battery-operated power units were brought in to cope with the loss of the electricity generators, which helped to stabilise conditions in units 2 and 3. However, the loss of water coolant from unit 1 led to higher temperatures and pressures, forcing the authorities to vent radioactive gases and water vapour into the containment area, which led to the first explosion. They then used fire engines to flood the reactor vessel with seawater and boron to keep the fuel rods from melting and preventing a nuclear chain reaction. Yesterday, a similar chain of events led to a second explosion, this time in unit 3, and a corresponding flooding of the reactor vessel with seawater and boron. The control room of unit 3, however, was still operating, according to the International Atomic Energy Authority.

Is there any danger of a Chernobyl-style disaster?

At this stage, nothing can be ruled out. The biggest danger is that the nuclear fuel rods are exposed to air. If this happens, they could overheat and cause a nuclear meltdown, although some experts believe this is unlikely.

The design of Chernobyl's nuclear reactors was very different from the boiling-water reactors at the Japanese facility. The former Soviet power plant had antiquated safety features compared with those in place throughout Japan. However, it is clear that the Japanese are still trying to stabilise the situation and, until this has been achieved, there remains a possibility, however slight, that the situation could turn into either a nuclear meltdown, or a catastrophic release of radioactive material. Yesterday, Japanese nuclear workers were still desperately battling to stabilise reactor unit 2.

What are the long-term implications of the crisis?

So far there have been relatively small amounts of radioactivity released into the environment. Yesterday, the IAEA said that radioactivity levels around a nearby nuclear power plant at Onagawa, where the plant's detectors were picking up radioactive plumes coming from Fukushima, had returned to normal levels. Some 230,000 iodine tablets, which are taken to minimise the risk of thyroid cancer from ingesting radioactive iodine, have been distributed to emergency centres around Fukushima but have not yet been distributed to residents. Nearly 200,000 people have also been evacuated.