A new type of fuel element, which combined the virtues of homogeneous and heterogeneous fuel elements, was conceived for a gas-cooled reactor. Tests in the laboratory and in a reactor removed the major uncertainties connected with this fuel element. The element consisted of small-diameter metal-clad fuel bodies inserted in graphite plates which served as the moderator. A satisfactory design was developed after experimentation with several graphite—metal combinations, thermal expansion studies, and graphite strength tests. Two elements were tested in-pile for a total of 2500 hr at 1500°F surface temperature. The elements retained their structural integrity after the irradiation. Although small amounts of fission products were released from the elements, this leakage was not large enough to cause an operating problem during the tests.