Investigations of transitional and accident operating conditions in models of fast-reactor fuel-element bundles

In recent years it has become necessary to analyze transient regimes for coolant flow in fuel rod bundles in connection with radiation from both transitional regimes of reactor operation and accident situations, which are caused by failure of large elements of the coolant circulation loops. Two types of processes are distinguished: controllable transition regimes of a nuclear power plant safety system, and accident regimes which are accompanied by failure of the safety system. Much attention has been paid to examining hypothetical, that is extremely improbable, accidents. Thermal hydraulic calculations of similar regimes lead to the postulation of problems with combined heat transfer in the fuel element plus coolant system, and they are much more complicated that stationary calculations. Specific problems arise when the coolant boils in fuel element bundles. The nonuniformity of the coolant heating over the bundle cross section leads to boiling initially only in some of the fuel elements, while the rest of the flow remains single-phase. The goal of this work is to analyze the features of transitional and accident processes in fuel element bundles (up to the destruction of the fuel elements) and to determine the direction for further development of these efforts. It should be noted that the occurrence of a vapor phase in the core also effects its physical characteristics (the void effect on reactivity); therefore the problem is complex. Here attention is focused on the thermal physics side of the problem, because published investigations do not sufficiently reflect the actual state of the problem