| The Delayed Neutron (DN) systems of some operating CANadian Deuterium Uranium (CANDU) reactors have experienced degradation in the signal Discrimination Ratio (DR), making it difficult to localize fuel channels that contain defective fuel. Two possible causes are transport times which are longer than the design value, and signal interference from tramp uranium deposited on in-core piping surfaces. A mathematical model is developed to simulate the transport from a fuel defect location to the DN water boxes. The sources that contribute to the DN signal are also discussed. This model is used to determine the impact of an increased transport time to the DN monitors and to quantify the contribution from tramp uranium to the total background activity in the Primary Heat Transport System (PHTS). |
