Pebble Bed Modular Reactor (PMBR)

The press has recently carried a number of stories about the potential resurgence of nuclear power as an option to deal with both generation shortages and global climate/emissions concerns. Most recently, of course, the Vice-President has raised it.
[e.g., Boston Globe, 11 Feb; Business Week, 23 Apr; WSJ, 2 May — I have copies]

One of the more remarkable “new” technologies mentioned is the Pebble Bed Modular Reactor (PMBR), actually an old idea. In the heyday of reactor development, helium gas cooled designs were pursued by the U.S. (using a fuel block concept) and by the Germans, who used a “pebble” fuel configuration. The US program fell apart in the mid-90’s, though General Atomics kept pushing it as a means to burn up Soviet plutonium stockpiles. The Japanese and Chinese also continue to have programs, each with operating developmental reactors.

While the Germans dropped their program, their pebble idea later took hold with research in China and Indonesia, and finally in South Africa, where the story picks up speed. Eskom, the huge utility there, faced serious pollution problems with bad coal, and they needed smaller power plants that could be located near the coast, closer to population centers. The country also wanted to create high tech industry and jobs.

Eskom set up a new venture called PBMR (Pty) Ltd, and attracted development funds from the their government, British Nuclear Fuel (BNFL), and Exelon.
BNFL 22.5%
Eskom 30%
Exelon 12.5%
Industrial Development Corp of S. Africa (IDC) 25%
(the remaining 10% is reserved for black empowerment investment)

What is PBMR? From the company’s website:

“The PBMR is a helium-cooled, graphite-moderated high temperature reactor (HTR).

The PBMR consists of a vertical steel pressure vessel, 6m (19,7 ft) in diameter and about 20m (65 ft) high. It is lined with a 100cm (39 inch) thick layer of graphite bricks, which serves as a reflector and a passive heat transfer medium. The graphite brick lining is drilled with vertical holes to house the control rods.

The PBMR uses silicon carbide and pyrolitic carbon coated particles of enriched uranium oxide encased in graphite to form a fuel sphere or pebble about the size of a billiard ball. Helium is used as the coolant and energy transfer medium to a closed cycle gas turbine and generator system. When fully loaded, the core would contain 330 000 fuel spheres and 110 000 pure graphite spheres. The latter serve as an additional nuclear moderator.”

A major appeal is the inherent passive safety of the design. From the website:

“How safe is the PBMR? The PBMR is based on a simple design, with passive safety features that require no human intervention and that cannot be bypassed or rendered ineffective in any way. In all existing power reactors, safety objectives are achieved by means of custom-engineered, active safety systems. In contrast, the Pebble Modular Reactor (PBMR) is inherently safe as a result of the design, the materials used, the fuel and the physics involved. This means that, should a worst case scenario occur, no human intervention is required in the short or medium term.”

Another is “modularity”, at a scale of ~100 MW. Also, without the huge burden of auxiliary systems and containment, it should be relatively cheap to build.


The website is very comprehensive, so no need to try to paraphrase it here.


Questions remain, of course. The fuel elements must be well made, and the problem of spent fuel disposal is still hugely unresolved, especially in the US. On the regulatory front, the NRC is being urged to move rapidly to develop a new unique set of licensing criteria that would be appropriate for this inherently safe design, as the old framework simply doesn’t apply. One has to wonder, though, if not-invented-here will hinder progress in the US.

Here’s an Feb 2001 NRC “Fact Sheet” about the many different “next-generation reactors” on their plate:

The DOE Office of Nuclear Energy has it’s Generation IV Initiative, which seems to be taking the view that certification (much less deployment) of some yet-unidentified new small modular reactor technology won’t happen til 2030. A two-year “Roadmap” effort was announced last November. Argonne and Idaho are the lead labs in the program.


Exelon has started the process of education in Washington and around the country. See their congressional testimony available on the Nuclear Energy Institute’s website:

Meanwhile, in March PBMR let a contract for the design of the fuel fabrication plant:

Contact: Ward Sproat, Exelon Nuclear

0 replies

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply