Year
1996
Abstract
The amount of spent nuclear fuel is increasing. Spent fiel attribute tester (SFAT) has been designed especially for cases when traditional verification with CVD (Cerenkov Viewing Device) is impossible. This happens if cooling time of the fiel assembly is long enough or burnup is low. Also the facility circumstances, i.e. non-transparent or rippling water, can prevent the use of CVD. SFAT is needed also in cases when higher assurance of the presence of irradiated fiel assemblies is needed. This has happened several times during the intonational safeguards inspections in recent years and the need for SFAT is increasing. The principle of SFAT is to measure the gamma radiation emitted by definite fission products of the spent fiel. No fhel movements are needed. The most important fission products for SFAT measurements is 137CS.It has a long half life (30.2 years) and a linear relationship with the burnup. With proper analysis of measured gamma spectra, lots of information is available horn the measured object. Good analysis requires that the number of gamma counts in the 137CSpeaks can be determined precisely. This may be complicated because the resolution of the detectors normally used, NaI(Tl) or CdTe, is lower than that of HPGE detectors. If the cooling time is short, multiples from short-lived fission products like 134CS, are also detected in the region of interest. In addition to this, the strong and changing background including scattered radiation also from irradiated metal structures and from water causes difllculties to the analysis. The only way to properly calculate tic area of an individual gamma peak is to use curve fitting. In the spectra analysed so far, up to eight different gamma peaks, depending on the case, have been fitted in order to calculate the area of ‘37CSphoto peak (662 keV) precisely. After the exact peak analysis, rough estimations of the burnup and cooling time can be made. The procedures mentioned above can be automated allowing fast verification of the declared parameters while the SFAT is still in the measurement position.