Assessment of nuclear criticality potential of the S-Area Defense Waste Processing Facility (DWPF) is required to ensure the safe processing of radioactive waste for final disposal. At the Savannah River Site (SRS), high-level radioactive wastes are stored as caustic slurries. During storage, the wastes separate into a supernate layer and a sludge layer. The radionuclides from the sludge and supernate will be immobilized into borosilicate glass for storage and eventual disposal. The DWPF will initially immobilize sludge only, with simulated non-radioactive Precipitate Hydrolysis Aqueous (PHA) product. This paper demonstrates that criticality poses only a negligible risk in the DWPF process because of the characteristics of the waste and the DWPF process. The waste contains low concentration of fissile material and many elements which act as neutron poisons. Also, the DWPF process chemistry does not affect separation and accumulation of fissile materials. Experiments showed that DWPF can process all the high-level radioactive wastes currently stored at SRS with negligible criticality risk under normal and abnormal/process upset operation.