Year
1999
Abstract
Plutonium scrap from another Department of Energy site is to be converted at Savannah River Site (SRS) to a form for permanent storage. For accountability and criticality safety, the material must be measured at SRS, and handling restrictions require assay in 9975 shipping drums. A Multiplicity Neutron Counter is available to perform the measurements, but requires about 12 hours per assay, too long to support the measurement schedule. The assay time has been reduced to 2 hours by use of the Known-M method, the first known routine application of Known-M. The approach involves expression of the multiplication in terms of the effective 39Pu mass and a quadratic polynomial. Because only a few measured values of multiplication were available, values from Monte Carlo neutron transport calculations (using code MCNP) were used. Because the scrap cans have variable fill heights and fill height affects multiplication, an algorithm to correct the effective 239Pu mass values for that effect was developed. Testing of the Known-M calibration with limited data suggests a 2-sigma uncertainty of about 5%. Drums can contain one or two individual scrap cans, and an algorithm for measuring the combined plutonium content in two cans was developed. The Known-M assay calculations will be performed off line using a spreadsheet.