Investigation of the validity of the point-kinetic approximation and of the break-frequency method in 2-D subcritical systems
The accuracy and applicability of the break-frequency method for estimating the reactivity in a subcritical 2-D, 2-group reflected system with a distributed source is investigated for two different system sizes. The source is distributed according to the static thermal flux. This method could be useful in power reactor cores during loading conditions for determining the subcriticality. The validity of the point-kinetic approximation in the same systems is also tested. A neutron noise simulator is used to calculate the actual neutron noise induced by fluctuations of the source which is compared with the analytically calculated point-kinetic term. The break-frequency method is used to estimate the reactivity from the neutron noise calculated by the simulator. The conclusion of this work is that the point-kinetic approximation in general is not valid in 2-D, 2-group reflected systems and hence the break-frequency method is not valid either.
Development of a 2-D 2-group neutron noise simulator
In this paper, the development of a so-called neutron noise simulator is reported. This simulator calculates both the direct and the adjoint reactor transfer function between a stationary noise source and its induced neutron noise for any 2-dimensional heterogeneous critical system. The main advantage of this neutron noise simulator is that any realistic core can be modelled, since the simulator is designed to rely on a set of material constants corresponding to the actual reactor operating conditions. The calculations are performed in the 2-group diffusion approximation and in the frequency domain. The spatial discretisation is carried out with respect to the finite difference scheme. The noise source, expressed as an "absorber of variable strength" type, is defined directly from the fluctuations of the macroscopic cross-sections and can be spatially distributed over the core or concentrated in a few discrete nodes. If the noise source is a point-source, the simulator actually estimates the 2-dimensional 2-group discretised Green's function of the system. From the calculated Green's function, the neutron noise induced by a "vibrating absorber" type of noise source can also be determined. Different benchmark cases show that this neutron noise simulator works satisfactorily.
Subcriticality determination of nuclear fuel assembly by Mihalczo method.
To establish a technique of on-site subcriticality determination suitable for the criticality safety management of nuclear fuel assembly, the applicability of the method proposed by Mihalczo was examined with the Tank-type Critical Assembly (TCA) of the Japan Atomic Energy Research Institute. In the Mihalczo method, cross power spectral densities and auto power spectral densities are evaluated from the output currents of an ionization chamber containing 252Cf neutron source and two neutron detectors. The principle of this method is that the spectral ratio formed by the power spectral densities mentioned can be related to the subcriticality by the help of a stochastic theory. Throughout our data processing, an improved formula taking account of the neutron extinction at a detection process was used. Up to the subcriticality of 15 dollars, the Mihalczo method agreed with the water-level worth method, which has been a standard method of reactivity determination at the TCA facility. The systems treated in the present report hold symmetry concerning the nuclear fuel configuration and the 252Cf chamber position. It was clarified that, contrary to Mihalczo's assertion, the factor converting the spectral ratio to a subcriticality depends on subcriticality itself.