HEU Fuels in Use when the RERTR Program Began in 1978
Three main fuels were in use with highly-enriched uranium (HEU) when the RERTR Program began in 1978. These were UAlx-Al dispersion fuel with uranium densities up to 1.7 g/cm3, U3O8-Al dispersion fuel with uranium densities up to 1.3 g/cm3, and UZrHx alloy fuel with 0.5 g/cm3 (8.5 wt% U). UAlx-Al (or aluminide) fuel had been developed at the Idaho National Engineering Laboratory in the 1960s for use in the 250 MW ATR reactor. U3O8-Al (or oxide) fuel had been developed at the Oak Ridge National Laboratory in the 1960s for use in the 100 MW HFIR reactor. UZrHx (or TRIGA) fuel had been developed by General Atomics and used in TRIGA reactors with power levels up to 14 MW.
Qualified LEU Fuels
The objectives of the fuel development effort in the RERTR Program were to extend the uranium densities for these existing fuels to their practical limits using low enriched uranium (LEU < 20% U-235) and to develop new LEU fuel types with higher uranium densities.
For the existing fuels, LEU uranium densities were qualified up to 2.3 g U/cm3 for UAlx-Al fuel, up to 3.2 g U/cm3 for U3O8-Al fuel, and up to 3.7 g U/cm3 for UZrHx fuel. Each fuel was tested extensively up to these densities and, in some cases, beyond them. All of the data needed to qualify these fuels with LEU were collected and published.
For the new fuels, U3Si2-Al dispersion fuel was qualified with uranium densities up to 4.8 g/cm3. The U.S. Nuclear Regulatory commission (NRC) issued a formal approval  for use of this fuel with uranium densities up to 4.8 g/cm3 in domestic research and test reactors. A whole-core demonstration using this fuel was successfully completed in the Oak Ridge Research Reactor (ORR) in 1987 using a mixed-core approach. Many other tests using this fuel have been successfully completed in research reactors around the world. Plates with up to 6.0 g/cm3 have been fabricated by CERCA in France using a proprietary process, but irradiation tests have not yet been completed.
Information on Selected Thermal Properties and Uranium Density Relations for Alloy, Aluminide, Oxide, and Silicide Fuels was published in IAEA-TECDOC-643 (April 1992) Volume 4: Fuels, Appendix I-1.1, and is made available here in Adobe Systems’ Portable Document Format (PDF) for the convenience of the reader.
Other LEU Fuels
Other fuels such as U3Si-Al fuel were also tested. Since U3Si is more dense than U3Si2, U3Si-Al fuel meat will contain a higher uranium density than U3Si2-Al fuel meat for the same volume fraction of the dispersed phase.
Miniplates with U3Si-Al fuel and up to 6.1 g U/cm3 were fabricated in the mid 1980s by Argonne National Laboratory and by the Comision National Energia Atomica in Argentina. These miniplates were irradiated in the ORR reactor to a burnup of 84-96% of the initial U-235 content. Postirradiation examination of these miniplates gave good results, but showed that burnup limits would need to be imposed for higher uranium densities. Four full-sized plates fabricated by CERCA in France with up to 6.0 g U/cm3 were successfully irradiated in the SILOE reactor to a burnup of 53-54% of the initial U-235. A full-sized fuel assembly with U3Si-Al fuel and 6.0 g U/cm3 was also fabricated by CERCA and irradiated in SILOE to 55% U-235 burnup. However, conclusive evidence indicating that that U3Si becomes amorphous under irradiation convinced the RERTR Program that this material as then developed could not be used safely in fuel plates beyond the limits established by the irradiation tests in SILOE.
Advanced Fuel Development
Development of the fuels described above was terminated by DOE in 1989. The effort to develop new advanced LEU fuels was restarted in 1995 after a pause of about six years. Results of these investigations can be found under "Advanced Fuels" on this web page and in the Proceedings of the 1995, 1996, and 1997 RERTR International Meetings. Many of the papers from these proceedings can also be found on this web page.
 U.S. Nuclear Regulatory Commission: "Safety Evaluation Report Related to the Evaluation of Low-Enriched Uranium Silicide-Aluminum Dispersion Fuel for Use in Non-Power Reactors," U.S. Nuclear Regulatory Commission Report NUREG-1313 (July 1988).