A metallic fuel-element modeling code (BEMOD) has been developed to describe the irradiation behavior of EBR-II driver fuels. BEMOD has been applied to both the present Mark LA and the advanced Mark II driver fuels. Good agreement on cladding diameter changes as a function of burnup is obtained between calculations and measurements on irradiated fuel elements. At a reactor power of 50 MW(th), the code calculations indicate that the Mark IA element is capable of about 3.5 at.% before a cladding ΔD/D of 2% is expected, while the Mark II design should be capable of about twice that burnup before a similar cladding ΔD/D is attained. The increase in reactor power to 62.5 MW(th) appears to have no appreciable effect on the above values.