Holdup measurements of plutonium in glove box exhausts

A new measurement technique has been developed to quantify plutonium in process glove box exhausts. The technique implemented at Rocky Flats Plant utiltizes a shielded, collimated 0.5in. x 0.5in. bismuth germanate (BGO) gamma-ray detector. Pairs of measurements are made at one foot intervals along the duct. One measurement is made with the detector viewing the bottom of the duct with the detector crystal approximately 2 inches from the duct surface. The second measurement is made on the top of the exhaust pipe with the detector crystal 2 inches from the top of the duct. When the detector is placed in the bottom assay position, the area of the holdup material is assumed to extend beyond the detector field of view. The concentration of plutonium in g/cm2 is obtained from this bottom measurement. The deposit width is determined from a model developed to relate the deposit width to the ratio of the count rates measured at the two positions, above and below the duct. Once a deposit width has been calculated, it is multiplied by the concentration determined from the bottom measurement to yield a mass- per-unit-length at the duct location. Total plutonium mass is then determined by multiplying the duct length by the average of the mass- per-unit length assays performed along the duct. The applicability of the technique is presented in a comparison of field measurement data to analysis results on material removed from the ducts. 3 refs., 3 figs., 1 tab