NDA issues with RFETS vitrified waste forms

摘要

A study was conducted at Los Alamos National Laboratory (LANL) for the purpose of determining the feasibility of using a segmented gamma scanner (SGS) to accurately perform non-destructive analysis (NDA) on certain Rocky Flats Environmental Technology Site (RFETS) vitrified waste samples. This study was performed on a full-scale vitrified ash sample prepared at LANL according to a procedure similar to that anticipated to be used at RFETS. This sample was composed of a borosilicate-based glass frit, blended with ash to produce a Pu content of (approximately)1 wt %. The glass frit was taken to a degree of melting necessary to achieve a full encapsulation of the ash material. The NDA study performed on this sample showed that SGSs with either (1/2)- or 2-inch collimation can achieve an accuracy better than 6 % relative to calorimetry and (gamma)-ray isotopics. This accuracy is achievable, after application of appropriate bias corrections, for transmissions of about (1/2) % through the waste form and counting times of less than 30 minutes. These results are valid for ash material and graphite fines with the same degree of plutonium particle size, homogeneity, sample density, and sample geometry as the waste form used to obtain the results in this study. A drum-sized thermal neutron counter (TNC) was also included in the study to provide an alternative in the event the SGS failed to meet the required level of accuracy. The preliminary indications are that this method will also achieve the required accuracy with counting times of (approximately)30 minutes and appropriate application of bias corrections. The bias corrections can be avoided in all cases if the instruments are calibrated on standards matching the items.