Void fraction distribution measurement in fuel element bundles with a new high-resolution gamma tomograph

The knowledge of the void fraction distribution in nuclear fuel element bundles under boiling or pressurised water reactor conditions is of importance for the investigation of critical heat flux phenomena and the evaluation of sub-channel codes. However, performing measurements at pressures of more than 6 MPa and temperatures above 250 C is a challenge. Tomography with gamma radiation is a promising tool for such applications. At AREVA NP thermal hydraulic tests of fuel assemblies are conducted at the Karlstein Thermal HYdraulic (KATHY) test facility. This test loop allows experimental access to several important thermal hydraulic quantities, like CHF, pressure losses and thermal hydraulic stability. Since 1999 also void distributions inside the fuel assemblies can be measured. The first void measuring device has been a gamma densitometer which provided integral information for an alley of sub channels. In 2005 the gamma densitometer was replaced by a new developed gamma ray computed tomography (CT) device allowing the measurement of 2D-void distributions. (orig.)