A. M. Degtyarev

Assessment of the Effect of Microconstants Uncertainties on the Computational Error in the Neutron-Properties of a Brest Reactor Cell

A methodology for assessing the errors arising in calculations of keff and the 239Pu breeding ratio of a BREST cell as a result of microconstants uncertainties and the results of such an assessment are presented. The analysis is performed for the uncertainties in the 239Pu and 238U constants, taken from the BNAB-MIKRO library and most important for BREST characteristics, and the incertainties in ENDF/B-VI.2 library values of σin for lead. The correlations of the group uncertainties for individual constants and between constants are taken into account. A specially developed mathematical model of a BREST cell is used to determine the coefficients of sensitivity with respect to a perturbation of the microconstants for the neutron-physical properties. The model is based on computational results obtained using the WIMS-D4 code. The constants component of the computational error determined for a 1σ confidence interval is 0.018 for keff and 0.051 for the 239Pu breeding ratio.

Power Regulation Feature of a Reactor with Negative Feedback

The theoretical possibility of anomalous manifestation of negative feedback on coolant temperature in the form of a change in the reactor power with sign opposite to that of the introduced reactivity is analyzed. The analysis is performed using a simplified one-dimensional model of a reactor with one feedback on the coolant temperature. It is shown, on the basis of the approximations adopted, that the properties of a reactor with respect to a given effect are determined by a certain criterional complex of physical characteristics, for which numerical estimates corresponding to the appearance of anomalous self-regulation were obtained. Application of the criterion to real loads for VVÉR showed that there are prerequisites for appearance of such anomalies. It is shown that the manifestation of negative feedback examined here is consistent with the neutron-physical stability of a reactor.

The Stability of the Neutron Field of the Conversion Load of Commercial Uranium-Graphite Reactors with Respect to High-Frequency Oscillations

The stability of the neutron field in a conversion load of commercial uranium-graphite reactors on the basis of highly enriched uranium in connection with the intrinsic positive reactivity effect with respect to fuel temperature is studied. This effect is small compared with the negative reactivity effect with respect to the coolant temperature, but nonetheless this feature creates the prerequisites for the appearance of neutron-physical instability during reactor operation and requires a corresponding study.The analysis was based on the computational investigation of the behavior of simplified one-point and one-dimensional models of the core in transient regimes with a mutually consistent description of the dynamics of the neutron- and thermophysical properties of the load. The basic assumptions concerning the characteristics of the reactor which were used in the computational model are presented. The results obtained, on the basis of the approximations adopted, preclude the possibility of the appearance of spontaneous high-frequency oscillations, due to positive effect of reactivity with respect to fuel temperature, in the conversion load.