Pebble-Bed Modular Reactors.

If nuclear magazines had centerfolds, every month they'd show a picture of a PBMR.
[Source: MIT]

Whatever anyone could want a nuclear power plant to do, these sweethearts deliver.

They can't go out of control and overheat. You can shut off the cooling at full power and they just warm up a little and stop. They're made in modules; you get whatever size you want by assembling modules. You want small? Buy one module. You want big? Buy a bunch. They don't require heavy forgings so they can be mass-produced. They're cheap. They never have to shut down for refueling. They're gas-cooled so water chemistry is never an issue. They can drive hydrogen generators while generating electricity. They are so safe they can be built close in; the leftover heat can be used to heat homes and businesses instead of heating up the outdoors.

How is all this possible, you're wondering. Here's the deal:

The big bugaboo with conventional reactors is that the fuel elements stay in the reactor for a long time, a couple of years or so. During that time they build up fission products that give off heat even when you shut the reactor down. So the challenge is to ensure that cooling is always available to the core, no matter what. You may recall that during the Three-Mile-Island accident the operators deliberately turned off the cooling pumps and, sure enough, the core overheated and melted.

The concept here is to continually refuel. The fuel elements only spend a couple of weeks in the reactor before they are put into storage and the few fission products they hold are allowed to decay away; then they are cycled back through the reactor. Actually, a number of concepts for doing this have been proposed; it happens that the PBMR is the concept going into commercial operation.

How it works is that spherical fuel elements, called pebbles, are fed into the top of the reactor while others are withdrawn at the bottom. The pebbles are mixed graphite and uranium, coated with an abrasion-resistant ceramic. They're like billiard balls.

The design feature of greatest interest is that the reactor has a strongly negative void coefficient, which is a physicist's way of saying the reactivity rate goes down when the temperature goes up. So they don't need control rods or shutdown rods, although some versions have them. You control the power of the reactor by controlling the flow of gas coolant through the pebble bed. If you want less power you cut back the flow of gas; as the temperature rises the reactivity rate drops. If you want no power you shut off the flow; the temperature rises to the shutoff point and the reaction stops.

Could all reactors be this kind? Possibly. The catch is that the world probably will need some advanced-cycle reactors and an advanced-cycle PBMR hasn't been invented yet. So it could be that the future will include a mix of PBMRs and advanced-cycle reactors.

In the meantime, customers in China and South Africa are trying them out.[source]