Justification to Disregard U-234 Mass in Certain Uranium Systems
The ANSI/ANS-8.1-1998 Standard regarding nuclear criticality safety in operations with fissionable material outside reactors states certain guidelines for dealing with uranium isotopes when setting mass limits. In ANSI/ANS-8.1-1998 235 U limits apply to mixtures “with 234 U, 236 U, or 238 U, provided that 234 U is considered to be 235U” in computing mass. In this evaluation calculations coupled with benchmark data show a superior way of handling the other common uranium isotopes when setting limits for 235 U. Currently 234 U mass must be considered as 235 U mass in order to account for the fact 234 U has a k ∞ = 1.70 meaning criticality can be reached with a finite amount of 234 U mass. 236 U and 238 U have k ∞ ’s equal to 0.71 and 0.31 meaning they remain subcritical in infinite amounts and thus no mass limits are required. In fresh uranium (un-irradiated or reprocessed) enriched to 94% 235 U by weight, bounding ratios of 234 U, 236U, and 238U exist due to the enrichment process. Utilizing these ratios it is shown that 234 U accounts for less than 0.20 of the total mass fraction of 234 U, 236 U, and 238 U and the remaining 0.80 mass fraction is in a ratio of at least 1:1 for 238 U: 236 U. Creating models conservatively based on these two material limits, the resulting k ∞ = 0.91367 ± 0.00208 suggests a mass limit for 234 U is no longer required.