NUCLEAR FUEL
New Designs for Nuclear Industry

In the late 1940s an ORNL team led by Eugene Wigner designed water-cooled fuel elements to ensure that the Materials Testing Reactor would produce a high enough concentration of neutrons to determine which materials would hold up best for future reactors. The team designed the fuel elements of uranium sandwiched between aluminum plates surrounded by beryllium, to reflect neutrons back into the core. Wigner's best-known innovation was to curve the plates so they would bow in only one direction under intense heat, preventing constriction of water coolant flow, which determined neutron intensity. This design was a model for cores of U.S. research reactors and submarines.

In 1958 the British, using an early gas-cooled design, produced commercial nuclear electricity. With increased interest in a high-temperature gas-cooled reactor (HTGR), ORNL researchers focused on making fuel that could perform at high temperatures in a reactor cooled by helium and moderated by graphite.

They formed hundreds of spherical particles of uranium dioxide or uranium carbide, coated them with carbon to retain fission products, and embedded the fuel beads in graphite structures. Using the Oak Ridge Research Reactor, Don Trauger's team proved this fuel was stable when irradiated, unlike the design in which uranium carbide particles were dispersed in graphite. The ORNL team's findings caused the Germans to switch to coated- particle fuel in their HTGR. Improved versions of this design were used in two commercial U.S. HTGRs and in test reactors in Germany, Japan, and China.

ORNL researchers are now helping to develop meltdown-proof fuel beads for use in advanced HTGR concepts.

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