Holdup Characterization of the UO2F2 Hockey Stick

A method had to be developed that could fully characterize themass, shape, location, and composition of a large deposit ofenriched (3.5 wt percent 235U) uranyl fluoride (UO2F2) in ahockey-stick-shaped section of pipe in the K-29 building of theformer Oak Ridge Gaseous Diffusion Plant so that a strategy forsafe removal of the pipe could be developed. This large deposithad been formed by leakage of humid air into the UF6 process gaslines over a period of years. The resulting UO2F2 is hygroscopic,readily absorbing moisture from the air to form hydrates of theform UO2F2•nH2O. The ratio of hydrogen to uranium can varyfrom 0 to 16, and its presence can have significant nuclear criticalitysafety impacts for large deposits. To properly determine theappropriate course of action for removing the pipe, the followingproperties had to be determined by a nonintrusive technique: (1)the distribution of the fissile material within the pipe, (2) thetotal mass of the deposit, and (3) the amount of hydration present.The Nuclear Materials Identification System (NMIS) (previouslydeveloped for identification of uranium weaponscomponents in storage containers) was used to successfully characterizethis deposit. The distribution, mass, and hydrogen to uranium(H/U) ratio obtained from NMIS agreed with the visualobservations after the section of pipe containing the deposit wasdisassembled. Earlier attempts using conventional gamma-rayspectrometry had predicted more than twice the mass (1,300 kg)and a symmetric distribution of material in the pipe. This paperdiscusses the details of how NMIS was used to image this depositand briefly describes some of the improvements that have beenincorporated into the NMIS imaging capability since the time ofthis measurement that make it more useful for nuclear materialcontrol and accountability, arms control and nonproliferation,and counterterrorism applications.