A nuclear reactor designed to generate power in the basement of an apartment block is being developed in Japan. In the past few months government-backed researchers have been testing a fail-safe mechanism for the reactor, which will close down automatically if it overheats. The Rapid-L reactor was conceived as a powerhouse for colonies on the Moon. But at six metres high and only two metres wide this 200-kilowatt reactor could relatively easily fit into the basement of an office building or apartment block, where it would have to be housed in a solid containment building.