Atomic Energy

Situations with local boiling of the coolant become more likely when VVER-type power units are transferred to a heightened power level. In a VVER core, neutron flux detectors are not present in all FA. An important problem for evaluating the possibility of detecting local boiling in FA which are not equipped with detectors is the study of the spatial distribution of the change brought about in the neutron flux noise by local boiling of the coolant. The results of an investigation of the spatial distribution of the neutron flux noise initiated by fluctuations of the coolant parameters at the ingress into the core with non-equilibrium local surface boiling are reported. The sensitivity of the neutron flux noise in FA with no boiling to local boiling in neighboring FA is analyzed. The results could be helpful in analyzing the data recorded by the in-reactor noise diagnostics system in commercial operation in operating power-generating units.

A comparative analysis is presented of the influence of the rheological characteristics of concrete on the stress strain state of NPP containment shells. It is based on creep and shrinkage tests performed on concrete, field observations, and calculations of the strains and stresses engendered in the reinforcement and concrete by the shrinkage and creep of concrete. The results showed discrepancies in the obtained data. For this reason, it is recommended that field data and detailed working models, in which the cross-section parameter and the pitch of the reinforcement rods correspond to the nominal values, be used in evaluating the stress-strain state of NPP containment under operational and accidental loads.

A mathematical model of the technological process of cadmium distillation at the stage of pyrochemical reprocessing of spent mixed uranium-plutonium nitride fuel for the purpose of studying the serviceability of and developing monitoring and process control systems is presented. The basic thermodynamic and kinetic characteristics and the principles of mathematical modeling of vacuum distillation of liquid mixtures, specifically, cadmium, are described. The requirements of the model and the assumptions are formulated. The information structure and mathematical description of the model were developed. Computational experiments were performed on the model and showed its serviceability and possibility of use for developing monitoring and process control systems.

The results of experimental studies of the effect of the magnetic field of the well casing on the operation of the gas-filled neutron tube used in the logging equipment are reported. The operation of the ion source was evaluated in terms of the parameters of the pulsed currents of charged particles recorded at the target of the neutron tube. Analysis shows that a longitudinal magnetic field, as compared with a transverse magnetic field, has a greater influence on the performance of the tube. Of the screening methods studied the one with a robust magnetic-steel housing for the equipment turned out to be most effective.

The recovery of non-uniform surface contamination density from measurements of instrumental spectra performed by the airborne survey method is examined. An algorithm based on Tikhonov’s method of regularization is proposed for solving the inverse problem. The results of testing the chosen algorithm on model problems of recovering the ¹³⁷Cs contamination of a soil surface in the form of spots with different activity and size are presented. The influence of the regularization parameter on the recovery error is analyzed, and the optimal values of the regularization parameter are obtained.

The results of computational modeling of experiments with heating of a water-filled and dewatered system are reported. The aim of the experiments was to test the computational methods used to analyze accidents in RBMK spent fuel storage, which is an example of a system with low heat fluxes. The calculations were performed using the RELAP5 code and a model specially developed for analyzing such experiments. It is shown that the RELAP5 code with the Reflood option gives acceptable results, but for lack of a description of surface evaporation in the code water level reduction in the calculations starts with a delay (after water boils) and occurs faster than in the experiment. When surface evaporation is taken into account by means of Balter’s empirical relation, the time dependence of the water level coincides with the experimental dependence. The variation of temperature at the control points of the heater is also close to the experiment. The heating of a dewatered system is described well by both models. Heat transfer by radiation from the heater to the vessel walls and into the ambient environment plays the main role.