The developers claim that no one would be able to remove the fissile material from the reactor because its core would be inside a tamper-proof cask protected by a thicket of alarms.

Known as the small, sealed, transportable, autonomous reactor (SSTAR), the machine will generate power without needing refuelling or maintenance, says Craig Smith of the DoE's Lawrence Livermore National Laboratory in California.

Conventional reactors pose a threat of proliferation because they have to be periodically recharged with fuel which later has to be removed. Both steps offer operators the chance to divert fissile material to weapons programmes- as is thought to have happened in North Korea and Iran.

Another reason to provide a small reactor is that conventional nuclear stations generate around a gigawatt of electricity, and that's overkill for plants in developing countries without an extensive electricity grid to distribute it.

In an SSTAR the nuclear fuel, liquid lead coolant and a steam generator will be sealed inside the housing, along with steam pipes ready to be hooked up to an external generator turbine.

A version producing 100 megawatts would be 15 metres tall, 3 metres in diameter and weigh 500 tonnes. A 10-megawatt version is likely to weigh less than 200 tonnes.

The US will deliver the sealed unit by ship and truck and install it. When the fuel runs out it will collect the old reactor for recycling or disposal. The DoE hopes to have a prototype by 2015. For this to happen, it will have to overcome many technical challenges.

In conventional reactors, the nuclear chain reaction depletes the fissile isotopes in the fuel rods, which is why they have to be replaced every few years.

To sustain power generation for 30 years, the sealed reactor will have to be engineered to act as a breeder, using some of the neutrons to convert non-fissile isotopes such as uranium-238 into fissile plutonium-239.

To further extend the reactor's life, the cylindrical core will be engineered to sustain fission only when surrounded by a metal cylinder that reflects neutrons back into the fuel.

This mirror will start at one end of the core, and over the course of the reactor's lifetime move slowly along to the opposite end, consuming the fuel as it goes. Engineering long-term reliability into such a system will be a major task.

Automated controls will monitor the sealed reactor, Smith says, adjusting its electrical output and shutting it down if faults or tampering are detected.

Alerts will be sent over secure satellite radio channels to the DoE or to an international agency overseeing the reactors. The project faces strong political obstacles.

Michael Levi of the Brookings Institution, a think tank in Washington DC, questions whether developing countries will be prepared to leave the keys to their electricity supply in the hands of the US. He also doubts that SSTAR will be as proliferation-proof as the DoE hopes.

While the design makes it hard for countries hosting the reactors to cheat without getting caught, "what happens if they don't care about what we think?" he asks. It would then be possible to break into the reactor and reprocess the plutonium-rich fuel to make weapons, he says.

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