Steve Connor: The problem with the Fukushima reactors is their age

The Fukushima reactors were installed in the 1970s, but their design goes back to the 1960s, when the nuclear industry was still learning about the myriad of things that can go wrong when nuclear fission is used to generate electrical power. Since then, the industry has strived to make important safety improvements to lessen the risks of radioactive leaks, fires and meltdowns.

All six reactors at Fukushima are boiling water reactors (BWRs), meaning that the flow of water used to cool the reactors is the same water used to turn the steam turbines of the electricity generators. In more modern pressurised water reactors (PWRs), the heat from the primary cooling circuit is transferred by a steam generator or heat exchanger to a secondary circuit of water used to turn the steam turbines.

This is an inherently safer design because it means that the primary cooling circuit can be built to much smaller specifications than in the older BWR design. More importantly, it means that there is no "phase transition" from water to steam in the primary cooling circuit – it was this phase transition to steam that speeded up rather than slowed the nuclear fission reactions at Chernobyl that led to a steam explosion and fire.

One if the key safety concerns of any nuclear power station is keeping the nuclear fuel rods from overheating. These rods contain enriched uranium, meaning they have higher-than-normal levels of the uranium-235 isotope, which releases heat when bombarded with the neutrons released in a nuclear chain reaction.

To control the chain reactions at the heart of a nuclear reactor, a key safety feature is the ability to insert neutron-absorbing rods made of boron between the fuel rods. But even the insertion of these control rods does not keep the rods from overheating if there is not enough water circulating in the cooling circuit, which is why it is so critical to maintain emergency electrical power to a nuclear plant to keep pumping the water around the cooling circuit.

Colin Brown, director of engineering at the Institution of Mechanical Engineers, said that the safety controls of the latest "generation III+" nuclear reactors have become more passive, meaning that in an emergency, the cooling of the fuel rods occurs spontaneously without the need for human intervention.

"Older nuclear reactor designs rely on active systems, a mix of human intervention and computer-control to automatically shut-down the reactor if a major fault or emergency occurs," Mr Brown said.