ABSTRACT

In Situ gamma spectroscopy is a valuable tool for supporting decontamination and decommissioning (D&D) activities at nuclear facilities. These measurements save money and time in the characterization, decontamination and release of buildings and grounds, when compared to the conventional process of extracting samples and sending them to a remote laboratory for analysis. The Canberra In Situ Object Counting System (ISOCS) is a portable gamma spectroscopy system that has been deployed at several sites performing D&D work, with successful results. This paper describes these applications and identifies specific advantages over more conventional methods.

The 903 Pad Project at the Rocky Flats Environmental Technology Site (RFETS) involves the assessment of the extent of surface and subsurface soil contamination resulting from past accidental releases of Pu and U. In situ gamma spectroscopy was selected as the preferred method for evaluating the areal surface contamination because of the method’s ability to directly measure large surface areas in short count times and to obtain immediate results. Target nuclides for in situ measurements were ²⁴¹Am, ²³⁵U and ²³⁸U, with Pu derived from ²⁴¹Am. Because of the low energy photon emissions from ²⁴¹Am and U isotopes, careful modeling of the soil contamination, which addressed vertical distributions, soil chemical composition and moisture content, was critical. Over 1000 measurements were completed, with results providing distributions as expected and showing excellent agreement with soil sampling.

An ISOCS system was deployed in a mobile on-site laboratory to provide analysis of samples collected from the area designated as Trench One at the RFETS. Rapid feedback was required to help guide the trench excavation and to decide the proper disposition of the excavated materials. Since the nature of the samples collected from the trench could not be accurately predicted, it was not practical to purchase and/or prepare actual counting standards with the same matrices. This problem was resolved by ISOCS’ ability to mathematically model the samples, and to adjust those models as needed, to derive valid efficiency calibrations on a sample-by-sample basis. This system was particularly useful in modeling the sample source/matrix configuration of unknown masses of depleted U possibly contaminated with low levels of ²⁴¹Am, since disposition of the depleted U depended upon the level of Am present.

The Barnwell Nuclear Fuel Plant in South Carolina was constructed to reprocess spent fuel but was never put into commercial operation. However, initial system testing was performed using solutions containing low levels of U, Pu and 241Am. Decommissioning of these facilities requires characterization of systems, which has been performed using ISOCS as one of the key tools. Modeling and in situ counting of complex components such as tanks, traps, glove boxes, filter banks and piping eliminates the need to open and/or dismantle systems for characterization purposes. Estimates of levels of internal contamination obtained with ISOCS measurements have been consistent with other methods where comparisons are available.