OBJECTIVES High Purity Germanium detectors allows for measurements with high energy resolution. However, a more robust system of detecting gamma radiation using commercial off the shelf scintillator detectors such as LaBr3, NaI or BGO might be needed for used fuel verification for a long-term reliable operation for e.g. verification in connection with a deep geological repository. We aim to: • Evaluate if different scintillator detectors can be used to

determine parameters such as Burnup, Cooling Time and Decay Heat of used nuclear fuel assemblies.

• Evaluate in what range of fuel parameters different scintillator detectors can be used.

These studies have been performed by collecting gamma-ray energy spectra from used nuclear PWR fuel with four different detector types at the Swedish central interim storage of used nuclear nuel (CLAB).

• Pool wall collimated passive gamma measurement station for axial scans of nuclear fuel assemblies.

• Distance between fuel assembly and collimator end is ~2.5 m. • An elevator in the pool allows vertical movement and rotation of the fuel assembly. • Detector and data acquisition (DAQ) system located in a room behind the pool wall that

allowed for easy exchange of various detectors (see [5,6] for geometrical setup details). • Two DAQ systems based on digital signal analysis were used:

• For the HPGe detector: the LYNX system and the Genie 2000 software from Canberra Inc. were used.

• For scintillator detectors: the DSPEC 50 system and the MAESTRO software from ORTEC Inc. were used.

In both cases, trapezoidal shaping that allows for good energy resolution over a wide range of counting rates was used.

• Using the built-in capabilities of the DAQ software, the 137Cs peak for all measured spectra was quantified.

EQUIPMENT AND METHOD

RESULTS We present here results from analyses focused primarily on determination of decay heat using the 137Cs peak. The figures show the measured gamma-ray energy spectra. The table show values on the 137Cs peak in all measured spectra for the HPGe and LaBr3 detectors. As can be noted, the ratio between measured intensities approximately constant, indicating that the LaBr3 could be used instead of HPGe for all the fuel assemblies measured in this study.

CONCLUSIONS

• LaBr3 is a viable option for passive gamma assay of used fuel, with its energy resolution of about 3% being good enough to resolve the most dominant gamma peaks in the energy spectrum even at short cooling times (in the order of 5 years).

• For long cooling times even NaI or BGO detector can be used to evaluate the intensity of the dominant 137Cs peak at 662 keV. According to the data measured in this study, they are usable for cooling times beyond 18 years. Further studies are needed to determine at which cooling times (earlier than 18 years) that NaI and BGO detectors become a usable option.

ACKNOWLEDGEMENTS This work was performed in the framework of the collaboration agreement between the EU and the US-DOE-NNSA in the field of Nuclear Material Safeguards and Security Research and Development. The personnel at the interim storage facility for used fuel (CLAB) in Oskarshamn, Sweden, are acknowledged for their support.

Peter Jansson*, Sophie Grape*, Stephen J. Tobin**, Henrik Liljenfeldt*** * Uppsala University, Sweden ** Los Alamos National Laboratory, USA *** Swedish Nuclear Fuel and Waste Management Company (SKB AB), Sweden

Comparing HPGe and Scintillator Detectors for Gamma Spectroscopy Assay of Used Nuclear Fuel

Assembly Id Type BU [GWd/tU] CT [years] IE [%] PWR 5 17×17 47 5.2 3.94

PWR 19 15×15 35 28.3 3.20 PWR 24 17×17 23 18.1 2.10

CT: 5 y

CT: 18 y

CT: 28 y

PWR 5 PWR 19 PWR 24

137Cs: Intensity [cps]

FWHM [%]

Intensity [cps]

FWHM [%]

Intensity [cps]

FWHM [%]

HPGe: 2755 ± 4 0,37 1112 ± 1 0,28 817 ± 2 0,27

LaBr3: 838 ± 11 3,5 332 ± 6 3,7 292 ± 5 3,6

Ratio: 3,29 ± 0,04 3,35 ± 0,06 2,8 ± 0,05

CLAB