In the ‘metal waste treatment’ step, the dissolution residues obtained in the anode processing step are melted together with zirconium and NFP (recov­ered from the bottom of the electrorefiner) to synthesize a metal ingot, called a metal waste form (MWF). At the same time, the entrained salt is distilled and separated from the MWF to be recycled in the electrorefiner. Zirconium or stainless steel is added to the metal waste before melting to produce MWF ingots that have a consistent composition, phase assemblage and microstructures. The target composition of the MWF is stainless steel with 15 wt.% zirconium, because the Fe-Zr phase diagram indicates that Fe-15Zr has a relatively low solidus temperature of 1325 °C (Arias, 1993). MWF is a multiphase alloy comprising two Fe rich solid-solution phases and several FeZr2-type intermetallic phases. NFP are distributed in either one or both of the main solid solution and FeZr2-type intermetallic phases, while actinide elements are preferentially incorporated into the intermetal­lic phase. According to extensive examination and testing undertaken by ANL and INL to gain acceptance for the Yucca Mountain geological reposi­tory, MWFs are extremely robust with regard to retention of NFP in repos­itory-like conditions (Ebert, 2005).