A Path for Nuclear Power
A novel but tested technology, the pebble-bed reactor, can make fission energy safe.
Nuclear Power’s Twisted History
Pebble bed reactors were first designed in 1944 in the United States, but the first to be built was developed by German physicist Rudolf Schulten. It began operation near Aachen, Germany in 1966 and ran successfully for 21 years, providing heat for a small steam power plant. Tests run during its life demonstrated safe operation in the event of a total shutdown of the helium coolant.
Although the South African pebble bed reactor project has ended, General Atomics in San Diego recently applied for government funding to commercialize a similar, 240-megawatt, gas turbine–powered nuclear plant. The Chinese currently have two pebble reactors, but they are being used to generate steam for a conventional turbine, which local officials claim is less challenging. (This approach makes no sense to me, because it has lower thermal efficiency and retains the danger of water ingress into the reactor.) There are gas-cooled reactors in operation in the United Kingdom; the first was built at Calder Hall in 1956. High-temperature, gas-cooled reactors have also been attempted, notably at Fort St. Vrain in Colorado and at a 300-megawatt pebble bed unit in Germany. Both units were shut down in the 1980s after a few years of operation.
Despite a handful of promising experiments, the overall rarity of pebble-bed nuclear reactors and similar gas-cooled reactors elicits an inevitable question: If this engineering approach is so much safer, why are the vast majority of the world’s nuclear power plant reactors water cooled?
The answer lies buried in the history of atomic energy. After Enrico Fermi created the world’s first sustained nuclear chain reaction, in the University of Chicago’s squash courts on December 2, 1942, physicists considered using helium as a coolant for future nuclear power plants. The idea was rejected as too complicated, but it soon returned in the work of Farrington Daniels, a chemistry professor from the University of Wisconsin.
For some years, Daniels had been working on a process of fixing nitrogen from air using a novel furnace that heated the air with stone pebbles. Building upon this research, he proposed that a chain-reacting nuclear pile could be constructed along similar lines. The pile would consist of uranium oxide and carbide pebbles whose heat of fission would be removed by the flow of a cooling gas. Daniels filed a patent on his idea on October 11, 1945. In the patent (2,809,931, U.S.) he calls the pile a “pebble bed reactor,” claiming that the cooling gases be used to generate steam (to power a steam turbine), or “the heated gases can be used directly in gas turbines.” The next year, researchers at Oak Ridge National Laboratory started designing a helium-cooled reactor based on Daniels’ concept.
The project didn’t get far. The Atomic Energy Commission was also formed in 1946, and one of its first acts was to cancel the Oak Ridge project. According to Alvin Weinberg, then the director of the physics division at Oak Ridge, the Daniels design team was reassigned and became the core of the group that designed the first naval nuclear power plant. Funded by the Navy under the direction of Hyman Rickover, the reactor for the submarine Nautilus was cooled by pressurized water.
Water-cooled reactors made sense within the cramped confines of a submarine hull. Once companies such as General Electric and Westinghouse formed engineering teams and manufacturing facilities to produce water-cooled nuclear power plants for subs, the die was cast and they applied the technology to commercial power plants as well. Today, water-cooled reactors are used in most of the world’s operating nuclear plants.