IRIS is a 335-MW(electric) pressurized water reactor (PWR) with integral primary system. Its design and optimized maintenance are compatible with long cycles, up to 4 yr. Advanced fuel management techniques are employed to support this objective; among others, novel use of burnable absorbers is considered. Erbium is one absorber that is currently utilized in PWRs. It has many desirable properties that ideally suit the extended cycle and low soluble boron concentration targeted by IRIS. However, erbium also leads to a cycle length penalty due to incomplete depletion and residual absorber isotopes, plus uranium displacement following erbium incorporation in the fuel matrix. This paper proposes to consider modifying the isotopic composition of erbium, evaluates its impact, and demonstrates that it is possible to obtain a considerable reduction of the penalty while retaining the positive features of natural erbium fuel. The economic benefit of the reduced penalty is appealing for the industrial production of isotopically modified erbium fuel.