Some designs are claimed to be able to circulate the primary coolant via convection without requiring pumps, at least in emergency shutdown conditions.

The scheme that Dr Bradfield proposed in 1938 requires large pipes, tunnels, pumps and dams.

Unlike storage in pools next to reactors, this does not require pumps and filters, which can break down.

The locks themselves are fairly elaborate, but the Japanese captain knew they required hardly any machinery or pumps.

Mr. Chapin speculates that the natural flow of water makes it easier to use those two reservoirs; the other three would require expensive pumps.

Water, seeking its former level, rushed down the pipe and surged back up the other side without requiring pumps.

Any further new wells requiring pumps will be added to the programme.

Even as a liquid, hydrogen has low density, requiring large tanks and pumps, and the extreme cold requires tank insulation.

However, these systems required numerous pumps to return the fluid from the spread-out radiators and proved to be extremely difficult to keep running properly.

These were required because the route of the ditch gradually descended along natural ridges so it could be built without requiring pumps to move the water.