Cesium and iodine speciation in irradiated UO2 fuel

The presence of CsI in nuclear fuel has long been debated. Its formation significantly decreases volatility, thereby reducing the rate at which iodine and cesium are released from the reactor core during a nuclear accident. A series of samples were investigated by Knudsen Effusion Mass Spectrometry (KEMS) in order to determine whether CsI is present in irradiated nuclear fuel. The examined samples were pure CsI, CsI exposed to gamma radiation, CsI-doped UO 2 simulated fuel and irradiated LWR fuel samples. The CsI and CsI-doped samples were examined to establish boundary conditions for the detection of CsI by KEMS. These samples indicated that the presence of CsI in fuel is characterized by three mass spectrometric signals Cs + , I + and CsI + , with a peak ratio of CsI + and I + of 1:0.7. The examinations of irradiated fuels showed none of these characteristics and hence no evidence that CsI is present in irradiated LWR nuclear fuel, at least after a storage period of years.