Quantitative Analysis of the Gamma Spectra Measured with SFAT

Spent fuel attribute tester (SFAT) is a method for the gross defect verification of the spent fuel. The verification is based on the gamma spectrum analysis of the spectra acquired with NaI or CdTe detector. If the spectra measured above an assembly contains statistically significant amount of fission products like 137Cs or 144Pr, the assembly can be categorised as spent fuel. The detection of the fission products may be difficult, because the geometry of the measurement is usually rather poor and the detector resolution may affect the analysis. Therefore, the most advanced and statistically correct analysis methods has to be used to ensure the verification correctness. Finnish authorities are using a special version of SAMPO 90 [1-3] in the acquisition and analysis of the SFAT spectra. The most powerful part of the SAMPO 90 has been its gamma peak area determination using either linear or non-linear data fitting. In order to automate the whole measuring and analysis job, number of commercial MCA's are directly controllable using SAMPO 90. There is not any SFAT specific version of SAMPO 90, but it was very trivial task to build one using the macros and programmable report generator of the SAMPO 90. Measurements were performed using 100 second real integration time. The 50 Mhz Wilkinson type ADC gave tolerable live time, dead time was between 10 and 30 %. Also shorter integration times were tested. It was found that when using NaI detector even one second integration time may be enough for spent fuel verification purposes. In addition to the pointwise measurement, so called online measurement was tested during the session. In the online measurement the detector is moving in the pool and acquiring data simultaneusly. The verification is based on visual detection of the 137Cs or 144Pr in a special online graph that is build into SAMPO 90. Because of the SAMPO 90 user applications, all these verification methods are easy to use and fast, for example even more than 300 assemblies can be verified in an hour using online measurement.