Pebble Beds

I recently began to think again about the Pebble Bed Reactor. In the early days of Nuclear Green, I was encouraged by what I thought was the potential of the Pebble Bed Reactor. The Pebble Bed Reactor is cooled by a gas; most likely helium and the pebbles resemble tennis balls in size. They are made from graphite and other carbon based materials. In the Pebble based reactor, the helium coolant blows the pebbles up into the reactor core until they reach a critical geometry and begin to support a chain reaction. This heats the helium which exits the reactor core drawing off the surplus heat. The helium passes through a cooling system that transfers the heat to a secondary gas which then drives a turbine or in some cases the heat is transferred to water which turns to steam and the steam drives the turbine.

When I first began to think about post carbon energy, the Pebble Bed Reactor ranked high on my list of coal replacement candidates. Later on I was to discover there were problems. The Molten Salt Reactor always ranked high on my list and continues to do so, but I believe one of the primary functions of fourth generation nuclear technology is to lower energy costs. I held that the Molten Salt Reactor had real potential for doing so and I continue to hold this view. The Pebble Bed Reactor does not hold as much potential for lowering energy costs.

The core of the Pebble Bed Reactor must be quite large and strong enough to withstand the pressure of heated helium passing through the reactor core. The core of the Pebble Bed Rector may not be as heavy as the core of the commercial Light Water Reactor, but it is quite large. Larger in fact than the core of conventional reactors. There are other parts of the Pebble Bed Reactor that will be quite large. All of these large parts add up to a considerable cost disadvantage when it is compared to the Molten Salt Reactor.

Seven or eight years ago reactor scientists at Oak Ridge National Laboratory and at the University of California at Berkley came up with a solution to the problems of Pebble Bed Reactors. That solution was to replace the helium core coolant with a Molten Salt Reactor coolant. The core of a molten salt cooled Pebble Bed Reactor would be quite small compared to a helium cooled Pebble Bed Reactor yet the molten salt cooled Pebble Bed Reactor would have many of the advantages of a helium cooled Pebble Bed Reactor.

The molten salt cooled Pebble Bed Reactor could take advantage of research conducted in West Germany and South Africa on Pebble Bed Reactors as well as research done on molten salt coolant and molten salt cooling systems conducted at ORNL. The resulting reactor would be quite inexpensive and could be build in factories and transported by truck, railroads, and river barges. Most of the safety advantages of Molten Salt Reactors would apply to a molten salt cooled Pebble Bed Reactor. Pebble bed safety technologies could be used when molten salt technologies were not appropriate.

Small Pebble Bed Molten Salt Reactors could be build in factories and buried underground. Larger Pebble Bed Molten Salt Reactors could be constructed in factories through modular design and then assembled on site. Many of the safety regulations applied to light water cooled reactors are redundant for Pebble Bed Molten Salt Reactors. A nuclear regulatory agency ought to recognize the superior safety characteristics of molten salt nuclear technology and regulate accordingly.