A method for using a 252CF shuffler calibrated at one enrichment to measure uranium--plutonium mixtures at different enrichments

Increasing attention has recently been focused on measuring Waste Isolation Pilot Plant (WIPP)-destined transuranic (TRU)-waste stored in 55-gallon drums at Los Alamos National Laboratory's plutonium facility. Particularly troublesome are those drums containing both plutonium and uranium embedded in noncombustible matrices. About the only practicable way to treat this situation nondestructively is to employ an active-passive neutron technique such as that used by a 252Cf shuffler instrument. The primary drawback to this method is that the count rate signals from the uranium and plutonium are coupled together. The problem is further exacerbated if the uranium and plutonium in the drums are at enrichments differing from those of the available calibration standards, a circumstance frequently encountered in plant environments. The difficulty arises because the 238U isotope possesses a fission cross section which must be properly accounted for when enrichments are varied. A method will be presented for separating the effective total measured signal from those of the plutonium and the uranium. A theoretical technique will be developed to uncouple the totalU component from the 235Ueff. Several functional forms for the 238U contribution will be graphically displayed, compared and evaluated before choosing the final hyperbolic form. Uncertainty estimates will be indicated.