Sumasu Yamada

Systematic approach for stochastic model in reactor noise analysis

Abstract A systematic approach to construct a time-series model for reactor noise analysis is discussed starting from stochastic Markovian equations for a system without measurement noise. Properties of the zeros of the model are also discussed using the system-matrix representation, and conditions are presented to calculate the number of zeros of the model.

Application of an adaptive filter for criticality surveillance systems

Abstract Applicability of the adaptive filter for an on-line criticality surveillance system has been discussed through numerical simulations and analysis of real data from a subcritical reactor, based on the fact that the break frequency in the higher frequency region of the PSD of the neutron signal fluctuation has a one-to-one correspondence to subcriticality within the framework of a point reactor kinetics. The recursive extended least squares (RELS) algorithm, a modified RELS algorithm, and the recursive maximum likelihood (RML) algorithm have been examined and it was found that: 1. 1. The model parameters estimated by the adaptive filters do not converge to the theoretical values, however, the PSD of the model agrees well with that of the theoretical one in a higher frequency region. 2. 2. The modified RELS method is best for ARMA model identification of the neutron signal fluctuation from the viewpoint of parameter convergence when the sampling frequency, an anti-aliasing filter and a high-pass filter are appropriately selected. 3. 3. The order of ARMA model is best at (2,2) for subcriticality resolution. 4. 4. The break frequency of the first-order high-pass filter should be set within the range 1/10-1/100 of the break frequency of the PSD of the neutron signal in a higher frequency region. These results indicate that: this approach cannot provide the absolute value of subcriticality without criticality measurement in contrast to Mihalczos method, however, it can be applicable to an inexpensive on-line monitoring system of reactivity change with a relatively small amount of numerical calculations.

Counting losses due to saturation effects of scintillation counters at high count rates

Abstract The counting statistics of a scintillation counter, with a preamplifier saturated by an overloading input, are investigated. First, the formulae for the variance and the mean number of counts, accumulated within a given gating time, are derived by considering counting-loss effects originating from the saturation and a finite resolving time of the electronic circuit. Numerical examples based on the formulae indicate that the saturation makes a positive contribution to the variance-to-mean ratio and that the contribution increases with count rate. Next the ratios are measured under high count rates when the preamplifier saturation can be observed. By fitting the present formula to the measured data, the counting-loss parameters can be evaluated. Corrections based on the parameters are made for various count rates measured in a nuclear reactor. As a result of the corrections, the linearity between count rate and reactor power can be restored.

Observation noise and zero loci of the time series model

The properties of zeros of time series models are examined in a linear stochastic system with white Gaussian observation noise. Each zero has a locus in the complex plane as the variance of observation noise changes from zero to infinity. An application of zero loci is presented for understanding the properties of the autoregressive model. >

Feynman-α correlation analysis by prompt-photon detection

Abstract Two-detector Feynman-α measurements were carried out using the UTR-KINKI reactor, a light-water-moderated and graphite-reflected reactor, by detecting high-energy, prompt gamma rays. For comparison, the conventional measurements by detecting neutrons were also performed. These measurements were carried out in the subcriticality range from 0 to

Properties of Autoregressive Model in Reactor Noise Analysis, (I): Convergence of Parameters and Power Spectral Density

1.8. The gate-time dependence of the variance-and covariance-to-mean ratios measured by gamma-ray detection were nearly identical with those obtained using standard neutron-detection techniques. Consequently, the prompt-neutron decay constants inferred from the gamma-ray correlation data agreed with those from the neutron data. Furthermore, the correlated-to-uncorrelated amplitude ratios obtained by gamma-ray detection significantly depended on the low-energy discriminator level of the single-channel analyzer. The disriminator level was determined as optimum for obtaining a maximum value of the amplitude ratio. The maximum amplitude ratio was much larger than that obtained by neutron detection. The subcriticality dependence of the decay constant obtained by gamma-ray detection was consistent with that obtained by neutron detection and followed the linear relation based on the one-point kinetic model in the vicinity of delayed critical. These experimental results suggest that the gamma-ray correlation technique can be applied to measure reactor kinetic parameters more efficiently.