V. I. Borysenko, V. V. Goranchuk
Institute for Safety Problems of Nuclear Power Plants,
NAS of Ukraine, 12, Lysogirska st., Kyiv, 03028, Ukraine
Analytical studies have been performed on the model of a reactivity accident at the 4th Unit of the Chornobyl NPP with an RBMK-1000-type reactor. The model of the RBMK-1000 reactor was developed based on the equations of nuclear reactor kinetics. In the model, the reactivity changes as a result of external influences (the movement of control rods; changes in the coolant temperature at the reactor inlet), as well as a result of feedback on reactivity effects (changes in fuel temperature, coolant density, 135Xe concentration). The change in the coolant density takes into account the formation of steam in the reactor core, and the coolant pressure is introduced into the model as an external factor, according to the results of the registered data during the accident on 26.04.1986. There is a high sensitivity of the RMBK-1000 reactivity model to the absolute values of reactivity coefficients that have occurred on the eve of the accident (negative reactivity coefficient by fuel temperature, positive steam coefficient of reactivity). Therefore, the study is conducted for different combinations of values of the efficiency of the control rods, reactivity coefficients, as well as other factors affecting the course of the accident — emergency protection triggering time, and reactor power level before the accident. Considering that the main stage of the accident lasted less than 10 s, fuel destruction is possible when the critical value of fuel enthalpy is reached, at which the fuel dispersion process begins. The results of modeling of reactivity accident on RBMK-1000 with the values of parameters of reactivity effects, which best correspond to the chronology of recorded events as well as to the recorded values of technological parameters, are presented.
Keywords: RBMK-1000, reactivity, reactivity coefficient by fuel temperature, reactivity coefficient by coolant density, fuel enthalpy, end effect, graphite displacer.
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