Going from HEU to LEU: Conversion of the Oregon State TRIGA® Reactor

The U.S. Domestic Reactor Conversion Program continues its efforts to support the Global Threat Reduction Initiative by converting US reactors from Highly Enriched Uranium (HEU) fuel to Low Enriched Uranium (LEU) fuel by 2010. To date, five reactors have been successfully converted. The recent conversion of the Oregon State TRIGA® Reactor (OSTR) was completed in December 2008, concluding a nearly three -year effort. In April 2006, work began on a safety analysis report in support of conversion from 70% enriched, 8.5 wt% HEU TRIGA® fuel to 19.75% enriched, 30 wt% LEU TRIGA® fuel. The nominal erbium content of the LEU fuel was specified as 1.1 wt%. This erbium concentration was selected to produce a core configuration similar to the original HEU core configuration and a core geometry better suited to OSTR irradiation facilities. The new LEU fuel was received in June 2008 and was placed in dry storage. The original HEU core was unloaded in September 2008. The new LEU fuel was loaded in October 2008. Normal operation resumed in December 2008 after two months of start-up testing. Flux measurements in the various irradiation facilities indicate that the epithermal and fast components of the neutron spectrum are essentially unchanged while the thermal component of in-core irradiation facilities has been reduced. This change in the spectrum is not expected to significantly affect performance since 1) many of the in-core irradiations primarily utilize the fast portion of the spectrum and 2) irradiation facilities distant from the core (i.e., beam ports) have been minimally affected due to optimized neutron reflection and coupling. Cooperation between the government agencies, fuel manufacturer, and the licensee proved fundamental to the success of the project.