Tsuyoshi Misawa

Measurement of Prompt Neutron Decay Constant and Large Subcriticality by the Feynman-α Method

The Feynman-{alpha} experiments have been carried out using light-water-moderated and -reflected cores loaded with highly enriched uranium fuel at the Kyoto University critical assembly. An experimental technique using a multichannel scaler was developed to improve the accuracy of measurement and to shorten measuring time. Then, the {beta}{sub eff}/l values of single and coupled cores with different neutron spectra were measured to demonstrate the capability of the present technique for measuring the prompt neutron decay constant {alpha}. Moreover, the Feynman-{alpha} method was applied to measuring large subcriticalities. Through these experiments, it is found that the present technique greatly improves the accuracy of {alpha} measurement, and the one-point reactor approximation is applicable to a tightly coupled core. It is also found that the subcriticality down to approximately {minus} 35

Formulation of data synthesis technique for Feynman-α method

can be measured by this method if the position of the neutron detector is chosen carefully, and the present Feynman-{alpha} method can be applied to a subcriticality monitoring system.

Study on reactivity worth of beryllium by (n,2n) and (γ,n) reactions

Abstract We intend to develop a subcritically monitor based on the Feynman-α method, which detects a change in the prompt neutron decay constant caused by a change in the reactivity of a system containing nuclear fuel materials. For the practical use of this method, quick data processing is necessary to facilitate a real time measurement and it needs to be applicable to a wide range of count rates in the neutron detector. The Feynman-α method, based on pulse counting, has a serious problem relating to the counting statistics, namely the count loss effect due to the dead time of counter itself and the following counting circuit. To eliminate the count loss effect, especially in a high count rate region, a quick correction technique should be established to ensure accurate measurement. As a candidate for this, we have developed the data-synthesis technique which processes synthesized data generated by adding multiple time-series data acquired by independent neutron detectors. The present paper provides the derivation of a formula for this data-synthesis technique and the result of a numerical examination on its applicability.

Application of Nodal Method to Lambda Mode Higher Harmonics Code

Experiments on the reactivity worth of beryllium metal were performed using the Kyoto University Critical Assembly, and they were analyzed to examine the validity of the computational method to treat (n,2n) reactions in calculations. The experimental results demonstrated that beryllium metal has positive reactivity worth compared with graphite. In the analysis, (n,2n) reactions were treated as modifying scattering cross sections in a transport calculation, whereas both scattering and absorption cross sections should be modified in a diffusion calculation. The results of calculations for the reactivity worth of beryllium agreed with experimental data within a few percent in the calculated-to-experimental ratio. Calculated results indicated that (n,2n) reactions of beryllium contribute by {approximately}85% to the positive reactivity worth compared with graphite in these experiments at a thermal reactor. Moreover, through the improved neutron and gamma-ray coupled calculation, the effect of ({gamma},n) reactions of beryllium on reactivity was estimated. It was found that ({gamma},n) reactions of beryllium can be negligible so far as this reactivity worth is concerned.