A high-resolution gamma tomograph for void fraction distribution measurements in fuel element bundles

We would like to present our recently developed high-resolution gamma ray measurement system for analyzing dry-out effect and determine void fraction distributions in sub-channels of electrically heated fuel element bundles at the thermal hydraulic test loop KATHY in Karlstein (AREVA NP GmbH, Germany). The instrumentation setup enables a non-invasive measurement of cross-sectional void fraction profiles through the pressure vessel for fuel element bundles under typical nuclear reaction conditions. The gamma ray tomography system consists of a Cs137 isotopic source with an activity of about 165 GBq and a detector arc containing 320 single elements. The source radiation is restricted to a flat fan beam with a tungsten collimator. The average spatial resolution of the system is 3 mm in plane and 8 mm axial. With a special gantry vertical positioning and continuous rotation of the measurement setup is realised which is necessary for a complete tomography in different planes. Typically, transversal scans require an approximate recording time of 25 minutes. Gamma ray tomography is a relative measurement method. To determine void fraction calibration measurements are recorded at zero and one hundred percent void fraction respectively. It is a challenge to develop a tomography measurement system that is non-sensitive to temperature changes, high humidity and electrical fields to scale the void measurement to the calibration data. Cross-sectional images are reconstructed by standard filtered back projection algorithms.