Research On Reactor Noise In A Molten Salt Reactor

In the operation of a reactor,various kinds of normal or abnormal processes can lead to the fluctuations of all the parameters around their mean(static)values.The fluctuation is also known as the reactor noise,by detecting and analyzing its space-and time(frequency)-dependent behavior,some dynamic properties can be obtained.Moreover,the reactor noise can also provide useful information for the the operation monitoring and fault diagnostic purposes.Differing from the cases of traditional reactors with solid fuels,the fuel used in the molten salt reactor(MSR)is dissolved in the coolant and can thus circulate throughout the primary loop together with the coolant.Therefore,the effects of spatial neutronic coupling,loss of delayed neutron and temperature feedback become much stronger and thus the reactor noise in MSR becomes more prominent and complicated than that in traditional reactors following the same perturbation.Moreover,the non-uniform distribution of fuel salt caused by the on-line refuellig process indicates the more frequent generation of the reactor noise.Although the MSR concept was proposed in 1940s,the studies of MSR noise were just started in recent years,especially after MSR was chosen as one of the six candidates for the Gen-IV reactors.All the early studies were performed in simplified models with a one-dimensional bare homogeneous core,the solutions are thus expected to deviate far away from the realistic ones.To obtain more accurate dynamic properties and noise characters of MSR,more realistic system and theoretical models are established in this thesis to investigate the space-and frequency-dependent behavior of reactor noise.The main works and results are as following:(1)Assuming the core as a three-dimensional(3D)bare homogeneous cylindrical blanket and establishing the theoretical model based on one-group diffusion theory,the dynamic properties and frequency-dependent behavior of neutron noise in MSR are investigated with the reactor transfer function technique,the effects of fuel velocity,frequency and system size(dimension)are also analyzed.From the viewpoint of the reactivity loss or delayed neutron loss,the maximal deviation between MSR and traditional reactors occurs at the velocity of infinity or a certain finite one.In the case of low frequencies,the maximal magnitude of the amplitude of reactor transfer function is found at the velocity where the effective delayed neutron fraction reaches its minimum,indicating the maximal influence of the perturbation on the MSR system;for high frequencies,the magnitude of the amplitude of reactor transfer function as well as the influence of the perturbation on MSR system increases monotonically with the increasing velocity.(2)Establishing the neutronic model based on two-group diffusion theory and the thermal-dynamic model based on conservation law,building the core as a cylindrical blanket surrounded by radial graphite reflector,the effects of both the temperature feedback and fuel property on the noise in the Thorium-based MSR are studied,the contributions of fluctuations of different cross sections on the neutron noise and their interactions are also investigated.It is found that the temperature feedback effect is important only at low frequencies,the pure neutronic model is sufficient in the case of high frequency.Because of the larger decay constant and the smaller delayed neutron fraction,both the neutron noise and precursor noise are not that sensitive to the velocity change.Moreover,due to the interference among the effects of fluctuations of different cross sections,the fast and thermal neutron noise are in opposite phases and their spatial distributions become more complicated.(3)The adiabatic approximation is applied to reconstruct the neutron noise and interpret the interference between the point kinetic and space-dependent components,the solutions are compared with those obtained in the point kinetic approximation and the exact ones to investigate the applicability of adiabatic approximation on MSR noise problem.The results show that the adiabatic approximation is accurate enough for low velocities.When the velocity increases,the adiabatic approximation can still provide good estimation of the neutron noise at the plateau region,the characteristic peak induced by the fuel recirculation can also be reconstructed.Thus the adiabatic approximation can be regarded as an effective tool for interpreting the results of neutron noise in MSR.