US DOT SPECIFICATION 6M RADIOACTIVE FISSILE MATERIAL PACKAGE NUCLEAR CRITICALITY SAFETY RE-EVALUATION

A recent nuclear criticality safety analysis of the Department of Transportation (DOT)Specification 6M fissile material pac!cage is reported for a broad variety of fissile materialloadings including 233U, 235U and 239Pu as metal and the dioxide forms with varying degreesof homogeneous water moderation. The reported results satisfy appropriate criticality safetycriteria and specifications of Titles 49 and I 0 of the Code of Federal Regulations and theInternational Atomic Energy Agency Safety Series No. 6 on Regulations for the Safe Transportof Radioactive Material. The results of the nuclear criticality safety analysis form the bases forextending previous Fissile Class I (transport index= 0.0) package mass limits for specific packagesizes. Each of the fissile isotopes e33U, 235U and 239Pu) were evaluated with four differentdegrees of water moderation at variable material densities. Also, Fissile Class II (0. 1 <transportindex< I 0.0) package mass limits were determined for the same materials which are applicableto 30, 55 and 110 gallon (US) 6Ms. The re-evaluation (for specific sizes of US DOT Specification6M packaging) demonstrates substantial safe increases of permitted fissile material loadings overgeneric mass limits previously applied to all sizes of 6M packages (I 0 to II 0 gallon). PreviousFissile Class I (transport index of 0.0) load limits for all 6M package sizes were 1.6 kg 235U,0.5 kg 233U and 0.9 kg 239Pu for the single moderation limit of hydrogen to fissile material atomratio of 3 (considering all sources of hydrogen within the 2R vessel). The re-evaluation justifiesincreasing the load limits for a 30 gallon 6M by factors of 4.3 for 235U, 8.4 for~ and 6.2 for 239Pu. These particular increases in package utility are the direct result of a specialized anddetailed analysis which avoids excesses in safety conservatism.