Over the past few decades, the international community has been cooperating to reduce nuclear proliferation risks. One U.S.–led global initiative is to reduce or eliminate the use of highly enriched uranium (HEU) for civilian purposes. Historically, highly enriched uranium was a commonly used fuel for research reactors, which continue to be used for testing environments to qualify materials for the nuclear power industry; to produce medical or industrial isotopes; and to produce neutron beams for research. In order to maintain the value of these research facilities, significant efforts have focused on converting them to use low-enriched uranium (LEU), which poses much less risk of being diverted for use in weapons.
Between 2004 and 2014, all 20 of the research reactors in the U.S. capable of being converted to use existing LEU fuel were converted, leaving five high-performance research reactors still using HEU. Outside of the U.S., at least 100 facilities have either been converted from HEU to LEU or have been shut down. About five high-performance research reactors that use HEU remain in Western Europe.
Research into new LEU fuels for these high-performance reactors is currently being conducted. Not only must these new fuels have a higher uranium density to offset the lower enrichment, but they also need to be tested and qualified for service.
Canadian Nuclear Laboratories has tested candidate LEU fuels in its NRU reactor. One promising new fuel consists of uranium–molybdenum (“U-Moly”) particles inside aluminum plates. However, development of this U-Moly fuel was set back when the fuel failed during testing.
“Knowing why the fuel failed is now guiding CNL’s ongoing development of a new fuel design to eliminate or reduce the reactions between the U‑Moly and the aluminum.”
To determine why it failed, CNL accessed the Canadian Neutron Beam Centre to examine the failed fuel, which was now highly radioactive. CNL and the CNBC developed a special hot cell to contain the radiation safely during the neutron beam measurements. Neutron beams were used to determine the crystallographic phases within the fuel because neutrons provide a high penetration depth (as compared to x‑rays, for example, which would be absorbed at the surface of the fuel).
The measurements helped CNL to identify how U-Moly behaves in a highly radioactive reactor core, including the chemical reactions that occur between the U-Moly particles and the aluminum. The results provided strong evidence that these chemical reactions—which occurred at a rate that was faster than expected—ultimately led to the fuel failure. Knowing why the fuel failed is now guiding CNL’s ongoing development of a new fuel design to eliminate or reduce the reactions between the U-Moly and the aluminum.
CNL has also accessed the CNBC to examine uranium silicide fuels, which have been used successfully in the NRU reactor for decades and are therefore of great interest as a model for the conversion of other reactors. When a rare fuel failure occurred in the NRU reactor, the CNBC assisted in the root cause analysis by examining the chemical composition in the highly radioactive fuel sample. The results clearly indicated that the sample’s chemical composition was within specifications and therefore not the cause of the failure, guiding CNL’s search for the root cause elsewhere.
This research story was republished with the permission of the Canadian Institute for Neutron Scattering.