Scientists develop nuclear reactor for use in office or apartment blocks

The so-called Rapid-L reactor was conceived as a powerhouse for colonies on the moon, but researchers at Japan’s Central Research Institute of Electrical Power Industry (CRIEPI), are designing a 200-kilowatt reactor which, they hope, could also be used in developing countries where remote regions cannot be conveniently connected to the main grid.

Small reactors have already been built to drive submarines and ships, but this is the first time that they have been designed for residential or corporate use. “In the future it will be quite difficult to construct further large nuclear power plants because of site restrictions,” says Mitsuru Kambe, head of the research team at CRIEPI. “To relieve peak loads in the near future, I believe small, modular reactors located in urban areas such as Tokyo Bay will be effective.”

The government-backed researchers have been testing a fail-safe mechanism for the reactor which will close down automatically if it overheats. The Rapid-L, which would have to be housed in a solid containment building, unlike normal nuclear reactors, has no control rods which have to be inserted and withdrawn to regulate the reaction. Instead, it uses reservoirs of molten lithium-6 – an isotope that is effective at absorbing neutrons. The reservoirs are connected to a vertical tube that runs through the reactor core. During normal operation the tube contains an inert gas. But as the temperature of the reactor rises, the liquid lithium expands, compressing the inert gas and entering the core to absorb neutrons and slow down the reaction.

The lithium acts as a liquid control rod. And unlike solid control rods, which have to be inserted mechanically, the liquid expands naturally when the core gets warm. The Rapid-L uses the same principle to start up and close down the reaction. The reactor would be cooled by molten sodium and run at about 530°C. Kambe’s main concern now is to test the long-term durability of the “fail-safe system”.

The research “is part of the effort being made in the US and in Japan to develop reactors which do not need hardware to keep them safe,” says John Gittus of the University of Plymouth in the UK. However, Kambe, admits that the success of such a reactor depends on the acceptance of the public, the electricity utilities and the government.

Malcolm Grimston, a nuclear expert at the Royal Institute of International Affairs, is sceptical that the Japanese people could be persuaded of the reactor’s safety. “There’s nothing wrong with the concept,” he says. “But if the Japanese public won’t now accept big reactors for safety reasons, then you have to wonder what the response would be building lots of small reactors in the middle of cities.”