Tested method to minimize plutonium assay discrepancies between laboratories

Plutonium assay differences are frequently observed between laboratories exchanging plutonium dioxide powders. These differences are commonly the result of chemical changes and/or nonhomogeneities in sampled materials. The irregularities are often caused by moisture absorption during sampling, packaging, shipment, and storage of the materials. A method is proposed which eliminates the effects of chemical change in samples, particularly moisture absorption, and minimizes sampling error. A nondestructive thermal watts/gram test on every preweighed sampled and total dissolution of these samples for chemical assay are the primary features which make this method effective. Because this method minimizes the error related to exchange material, it is possible to design an interlaboratory exchange program which demonstrates the assay capabiliies of the participants. In an experiment performed to demonstrate the effectiveness of this method, three PuO/sub 2/ batches of varying isotopic composition were synthesized at Mound to be used in the exchange tess. Powder sample aliquots from each batch were weighed directly into their vials under controlled atmospheric conditions. Calorimetric heat measurements were made on each vial to test homogeneity and verify sample weight. Six vials of each batch were chemically assayed at Mound and six at NBL (New Brunswick Laboratory). Both laboratories chose controlled-potential coulometry as the chemical assay technique because of its demonstrated precision and accuracy. Total dissolution of preweighed exchange samples eliminated the need for laborious and usually futile heating to return the material to its original condition. The mean chemical assay values obtained by Mound and NBL agree to within 0.01% for each of the compositions tested. Testing of both chemical assay and calorimetric data revealed no sampling error throughout the experiment