Further Investigation of Neutron Noise Under the Condition of Fuel Failure Based on Frequency Domain Finite Element Method

Abstract Usually, some fuel elements are allowed to be damaged under normal operating conditions of the reactor. At present, the design index of fuel element damage rate of pressurized water reactor at home and abroad is 0.25%. In view of fuel element damage accidents, there are two measurement methods for operating units: fuel accident monitoring and primary loop dose monitoring. Fuel accident monitoring depends on the measurement of delayed neutron activity flowing out of the core to determine whether there is fuel damage, and primary loop dose monitoring depends on measuring the activity of fission products and the concentration of inert gas flowing into the primary coolant to determine whether there is fuel damage. It takes a long time for the two measurement methods to discharge the measured fluid from the core outlet to the measuring device, and the limited damage event is also perplexed by the lower limit of dosimeter measurement after being widely diluted by the total flow of the first loop. For the plate fuel core, the axial flow of the delayed neutron precursor after the fuel element is damaged will break the inherent balance of the normal core and cause the transient change of neutron noise. Based on this idea, on the basis of the work of the author’s team reported by ICONE30, this work assumes three different degrees of plate fuel assembly damage to the assembly position of the core, corresponding to three axial flow scenarios of delayed neutron precursor. Based on the frequency domain finite element method, the neutron dynamics calculations are carried out for the above scenarios, and the fast group and thermal group neutron noise responses of neutron detectors at four different positions in the core are obtained.