Vol 126, No 2 (2019)

The results of experimental studies of heat transfer during boiling of sodium-potassium alloy in the presence of natural convection in model FA of a fast reactor with seven fuel-rod simulators are reported. The method of investigation is described. Data are obtained for three regimes of boiling of the liquid metal in FA: stable (nucleate), unstable (slug), and secondary stable (disperse-annular). The unstable boiling regime developed in two directions depending on the state of the surface of the simulators: in the presence of manufacturing roughness – in the direction of damping of pulsations, stabilization of boiling, and subsequent stable heat removal in the presence of a fi lm of liquid on the surface of the simulators (disperse-annular regime of boiling) or in the presence of small roughness – in the direction of increasing amplitude of flow and temperature pulsations. Data on an emergency regime with pressure reduction in the circulation loop, initial superheating upon boiling-up of the liquid metal, heat transfer, and a cartogram of the two-phase flow regimes of liquid metals are presented. The boundaries of the regimes are described by analytical expressions.

One of the problems of accidents in NPP with VVER is the evaporation of the primary coolant in the containment. Boric acid H₃BO₃ can ingress into the containment together with steam and crystallize, independently or in the form of salts, on heat-exchange and other surfaces of equipment placed in the containment, thereby lowering the efficiency of passive systems removing heat from the containment and the removal of hydrogen. The results of an experimental investigation of the deposition of insoluble aerosols and salts of boric acid on heat-exchange surfaces of the passive heat-removal system and the surfaces of autocatalytic hydrogen recombiners during serious accidents in NPP are reported. It is shown that the impact of crystallization of boric acid and its salts on the operability of these systems is very small.

The problems of managing spent nuclear fuel stored in at-reactor repositories at the NRC Kurchatov Institute and current methods of inventorying the nuclear content of the repositories are discussed. Methods of identifying FA and leak-tight bottles with fuel rods by means of radiometric methods of measuring the radiation parameters of АF are discussed. The use of remote-controlled machines makes it possible to reduce the effect of high radiation fields on the workers and protect them in their work. The main problems arising during long-term storage of spent fuel are degradation of the protective barriers of the fuel rods and the lack of information on the radioactive content and the produced high-level wastes. Regular inventorying of the repositories ensures that the missing data are replaced, and in preparation for shipment attention is focused on restoring the outer casings of the packages for shipment to reprocessing facilities.

A calculation of the removal of radionuclides by groundwaters from a subsurface repository of solid low-level waste was performed. A two-dimensional model of the groundwater flow and advective-dispersive transport of radionuclides in a vertical section of a subsurface aquifer, along which radionuclide-contaminated groundwater moves from the repository to an open body of water, was studied. Data from many years of experimentation on the leaching of cemented low-level waste were used in the calculations. It was shown that the rocks of the aquifer effectively retain medium half-life radionuclides (⁶⁰Co, ¹³⁷Cs, ⁹⁰Sr). An engineered barrier – a 0.7 m thick layer of consolidated bentonite clay – can secure the safety of a repository for long-lived transuranium radionuclides (²³⁵U, ²³⁹Pu) for 20000 years.

Verification of the MRNK+KEDR-D software system being developed for calculating reactor dynamics is performed using the numerical benchmark-test PWR MOX/UO₂ Transient Benchmark. The nonstationary process leading to an abrupt change of power caused by injection of large positive reactivity ~1.1 ß in 0.1 sec is studied. Analysis and a comparison of the obtained results with similar calculations performed using different software showed that the compared parameters are in agreement with one another to within admissible deviations.