The risk of a meltdown at a damaged Japanese nuclear plant has raised questions about how quickly older nuclear reactors should be phased out.
The stricken reactor at the Fukushima Daiichi plant, one of the world's largest nuclear power stations, was almost at the end of its planned 40-year commercial life when the earthquake struck on Friday.
The plant has been in operation since March 1971. Its six reactors were designed to generate a combined 4.7 gigawatts of power - about the same as the capacity to be installed in the first phase of the UAE's nuclear programme.
The nuclear units at the Fukushima Daiichi plant, 240 kilometres north of Tokyo, are "light water" reactors, the most common type used for nuclear power generation.
All such designs use ordinary water, either fresh or sea water, to cool the hot nuclear fuel rods, inside which the nuclei of radioactive uranium atoms disintegrate to release of vast amounts of energy. As the water is heated by the fuel rods, it vaporises, forming steam that drives turbines to generate electricity.
Reactors such as those at Fukushima, built to the original "boiling water" design of General Electric (GE) in the late 1960s, produce steam directly from water flowing past the fuel rods. The speed of the nuclear reaction is controlled by the twin mechanisms of lowering or raising the rods and speeding or slowing the flow of cooling water.
Under normal circumstances, a continuous flow of water would be pumped past the fuel rods, but with a total power failure, that flow would cease, allowing the entire reactor unit to overheat even if the rods were fully lowered.
"Reactors are not like your car that you can turn off and walk away. They're going to continue generating a great amount of heat until the core is disassembled," said Ron Chesser, the director for the Centre of Environmental Radiation Studies at Texas Tech University.
Without cooling water, there is a real chance of a meltdown of the reactor core that could result in a large release of radiation. Usually in case of a failure of the main cooling system, an "emergency core cooling system" would extinguish the nuclear chain reaction by dousing the rods with water treated with boron. This element has a high affinity for the neutrons ejected from split uranium atoms, which are responsible for most of the steam-producing heat transfer.
But stores of boronated water at the Fukushima Daiichi reactors may have been swept away by the tsunami, which also damaged the electric generators that pump water through the plant.
Most of Japan's nuclear reactors are of a newer design called the "advanced boiling water reactor", developed in the 1980s and 1990s. It has an excellent safety record, and the design was standardised after GE merged with the nuclear unit of Japan's Hitachi in the early 1990s. Because the designs of the earlier boiling water reactors were not standardised, less is known about the weaknesses of specific plants, which makes the current Fukushima crisis more difficult to assess.
Hitachi has also developed a design for smaller nuclear reactors of the boiling water type, featuring a "passive" safety system that automatically shuts down the reactor in an emergency. The rods are cooled by the water already on hand without the need for any pumping. The only human intervention required is to top up the water supply every few days. That design, however, would be difficult to implement in large plants because sufficient cooling could not be achieved without flowing water.
Another reactor class used in power plants is the "pressurised water reactor" - the type that a South Korean consortium will soon start installing at Braka, a remote location on the coast of western Abu Dhabi. In this design, the water surrounding the fuel rods is maintained at a pressure of about 158 atmospheres, or twice the 75-atmosphere pressure typically used in boiling water reactors. This prevents the water in the reactor core from vaporising. It does so only after passing to a separate vessel outside the reactor core.
There are certain safety advantages inherent in this design. The crucial one is that the water in the reactor core can continue to expand for longer as the temperature rises during an emergency. The further apart the water molecules move, the less efficiently they intercept high-energy neutrons emitted from the fuel rods, bouncing them back at a lower "thermal" energy that stimulates the nuclear chain reaction. Instead, more high-energy neutrons hit the reactor casing, which absorbs them, helping the rods to cool.
Even in a dire emergency, a meltdown in the UAE's planned nuclear plants would be significantly less likely than in Japan, which not only is prone to earthquakes but still has a number of ageing plants in service. In a worst-case scenario the UAE reactors would pose less of a threat to human health and safety than in crowded Japan, as they are to be built in remote desert regions, far from towns and cities.
As nightfall approached in Japan yesterday, the main question surrounding events at the Fukushima Daiichi plant was whether they would culminate in a mild release of radioactivity, as in the US Three Mile Island incident, or in a nightmare Chernobyl-type scenario.
It was still unclear yesterday afternoon whether a large explosion at the plant had damaged that reactor's container. A breach of the container would allow large amounts of radiation to escape into the atmosphere and contaminate groundwater.
At about 9pm in Tokyo, Japan's chief cabinet secretary Yukio Edano said the container had not been breached and radioactivity levels were falling.
"We believe the levels are within expectations," he said. "We will work hard to grasp the situation with the radioactive material, and are making utmost efforts for the safety of the nearby residents."
Nevertheless, fears mounted as reports poured in of a collapsed ceiling at a containment building billowing smoke.
Officials at the Tokyo Electric Power Company (Tepco), the plant's operator, declined to confirm whether an explosion had directly affected the facility's No 1 light water reactor. They said radioactive caesium was detected inside the plant, suggesting a radiation leak from the reactor core from fuel rods exposed to air.
Various radioactive caesium isotopes are produced by atomic fission of the material inside nuclear fuel pellets. One isotope, Caesium-137 with a half-life of more than 30 years, is responsible for most of the residual radioactivity remaining from the Chernobyl accident and from spent nuclear fuel that has been cooled for several years.
Nuclear Engineering International had reported that units 1 to 3 at Fukushima had been automatically shut down, bringing the entire plant to a standstill as the other three reactors were closed for maintenance. After the earthquake knocked out the gas supply to the electricity generators powering the nuclear plant's cooling system, back-up diesel generators were pressed into service. They ran for only an hour before failing due to damage from the tsunami.