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.
Theoretical investigation of the MTC noise estimate in 1-D homogeneous systems
In this paper, the accuracy of the noise-based determination of the moderator temperature coefficient (MTC) is investigated theoretically and quantitatively. It is known from earlier work that the noise method systematically underestimates the MTC. In this paper, it is found that the main reason for the underestimation lies with the radial incoherence of the temperature fluctuations. The deviation of the reactor response from point-kinetics is another possible reason, but it was found to play a quite insignificant role. The theory of neutron noise, induced by spatially random perturbations is elaborated and by its help the inaccuracy (bias) of the noise based MTC estimation was quantitatively investigated. It was found that a relatively short correlation length of the temperature fluctuations, which is in agreement with experimental evidence, can explain the observed underestimation of the MTC by the noise method.
Correlation of the Signals from Detectors in the Presence of a Stochastic Neutron Field
The source of correlation for the signals from detectors in the presence of a stochastic neutron field is analyzed for a variety of circumstances. A general methodology, based on a master equation approach, is compared with the Langevin/Schottky method with the result that there is consistency if the detection process is included in detail. For cases where the detector removes the detected neutron, the only sources of correlation are elementary processes that produce more than one neutron; consequently, the Schottky prescription for the noise equivalent source must be corrected accordingly. An additional term because of the finite electronic resolution is also found and added to the noise equivalent source. Because of the relevance of the subject to the theory of the /sup 252/Cf source noise method to measure reactivities, the general results are applied to interpret a recently performed experiment with this type of source.