Fuel plates under development for high-performance U. S. reactors con­sist of a UMo alloy foil (“U-10Mo Foil” in Figure 2-1) surrounded by a zirconium fission recoil barrier (“2X Zirconium Interlayer” in Figure 2-1) in an aluminum cladding (“Al 6061 Cladding” in Figure 2-1). The barrier is intended to prevent interactions at the interface of the fuel meat and cladding. A key issue for this fuel is the stability of this interface. Although the interface is mechanically stable, swelling of the fuel meat during irradia­tion could lead to the development of porosity at the interface and eventual delamination of the foil from the cladding. Such swelling and delamination could prove to be a life-limiting factor for this fuel system.

Qualification testing of this fuel for three high-performance research reactors (MITR, MURR, and NBSR) is currently under way. A partial fuel assembly[32] is currently being irradiated in ATR at the Idaho National Laboratory (Figure 2-2), and irradiation of ATR fuel elements is planned

to begin in 2012. Lead test assembly irradiations are planned once these irradiations are completed.

Testing of this fuel system for use in the highest-performance U. S. reactors (i. e., ATR, HFIR) is planned to begin in late 2011. Bounding- condition irradiation tests (greater than 500 W/cm2 and greater than 60 percent burnup) on a full-size fuel plate will be carried out at the ATR in late 2011. Fuel qualification testing will be initiated after these irradiation tests are completed.

Aside from the challenges discussed above, there are several other po­tential challenges that MITR will face during the transition to an LEU-fueled core. First, MITR is likely to be the first reactor to convert using UMo monolithic LEU fuel. MITR staff is presently working to better understand how best to introduce this first-of-a-kind fuel into the reactor. The current plan is to gradually introduce LEU fuel into the HEU core. To evaluate this plan, a mixed-core analysis will be carried out prior to conversion. Two challenges are foreseen in the mixed-core transition: Power peaking is gener­ally higher in new LEU elements, and steady-state HEU and LEU margins to ONB decrease with an increasing number of LEU fuel elements in the mixed core. MITR staff project that partially burned HEU elements may need to be kept in reserve as the reactor is transitioned to the full LEU core.

Second, the heating from gamma ray absorption outside the fuel is significantly less for LEU than for HEU, resulting in a lower heat load on the deuterium reflector and shield system. However, some in-core materials experiments rely on gamma heat for temperature control and may need to be redesigned.

Finally, it is also possible that mechanical stresses from the heavier loading of the denser LEU fuel will necessitate some redesigning of the fa­cility. However, current analyses indicate that heavier loading is not likely to pose a problem.