Consideration of Nuclear Criticality When Directly Disposing Highly Enriched Spent Nuclear Fuel in Unsaturated Tuff-I: Nuclear Criticality Constraints

• Published in: Nuclear technology Vol. 144; no. 2; pp. 201 - 221
• Main Authors: Rechard, Rob P., Sanchez, Lawrence C., Trellue, Holly R.
• Format: Journal Article
• Language: English
• Published: La Grange Park, IL Taylor & Francis 01.11.2003; American Nuclear Society
• Subjects: Applied sciences; CONCRETES; CRITICAL MASS; CRITICALITY; Energy; ENRICHMENT; Exact sciences and technology; FISSILE MATERIALS; Fuels; GOETHITE; HIGHLY ENRICHED URANIUM; HOMOGENEOUS MIXTURES; IRON; MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; MOISTURE; Nuclear fuels; PLUTONIUM; PLUTONIUM 239; Preparation and processing of nuclear fuels; SANDSTONES; SATURATION; SODDYITE; SOLIDS; SPENT FUELS; TUFF; URANIUM 235; URANIUM DIOXIDE; WATER; YUCCA MOUNTAIN; Yucca Mountain; United States; North America; America; Nevada; critical limits; radioactive waste disposal; Tuff; Enriched uranium; Concrete; Nuclear fuel; Iron; Uranium 235; Plutonium 239; Spent fuel storage; Irradiated nuclear fuel; Criticality
• ISSN: 0029-5450; 1943-7471
• DOI: 10.13182/NT03-4

This paper presents the mass, concentration, and volume required for a critical event to occur in homogeneous mixtures of fissile material and various other geologic materials. The fissile material considered is primarily highly enriched uranium spent fuel; however, 239 Pu is considered in some cases. The non-fissile materials examined are those found in the proposed repository area at Yucca Mountain, Nevada: volcanic tuff, iron rust, concrete, and naturally occurring water. For 235 U, the minimum critical solid concentration for tuff was 5 kg/m 3 (similar to sandstone), and in goethite, 45 kg/m 3 . The critical mass of uranium was sensitive to a number of factors, such as moisture content and fissile enrichment, but had a minimum, assuming almost 100% saturation and >20% enrichment, of 18 kg in tuff as Soddyite (or 9.5 kg as UO 2 ) and 7 kg in goethite. For 239 Pu, the minimum critical solid concentration for tuff was 3 kg/m 3 (similar to sandstone); in goethite, 20 kg/m 3 . The critical mass of plutonium was also sensitive to a number of factors, but had a minimum, assuming 100% saturation and 80-90% enrichment, of 5 kg in tuff and 6 kg in goethite.