Year
2003
Abstract
Various Non-Destructive Analysis (NDA) techniques based on High Resolution Gamma Spectroscopy are available to assay Spent Nuclear Fuels (SNF). These include waste assay systems such as Tomographic Gamma Scanners (TGS). SNF contain not only fissile isotopes such as 235U but also fission products such as 137Cs. Gamma assay of 235U in the presence of high 137Cs activities is difficult because of the following reasons; (i) increased continuum in the spectral region of interest, and (ii) the backscatter from the 662 keV photons from 137Cs interferes with the 186 keV peak from 235U. The backscatter effect limits the accuracy with which the 186 keV peak area can be determined. In this work, the backscatter effect is explored using measurements as well as Monte Carlo calculations using the MCNP code. Measurements were performed using a stainless steel can filled with aluminum rivets and by locating a 0.93 g 235U source, either by itself or along with an intense 137Cs source at the center of the can. A 120% coaxial High Purity Germanium (HPGe) detector was used to measure the gamma ray spectra. The measurements were performed for Cs/U ratios that were different by a factor of 20, and were used to benchmark MCNP calculations. The emerging 662:186 keV intensity ratio was on the order of 56:1. For the MCNP calculations, a validated model of the HPGe detector was used. The experimental geometry was simulated faithfully, and MCNP runs were performed by placing a source at the center of the can, emitting either 661.66 keV or 185.715 keV photons. To test the change in the ratio of the 185.715 keV peak area to the backscatter region, a series of MCNP runs were conducted by varying the matrix density. The degradation of the signal-to-noise ratio for uranium was studied.