The results of a numerical simulation of an experiment on measurement of the steam coefficient of the reactivity of a RBMK reactor are presented. The "inverted variation of the reactivity" effect is explained. The computational results show that this effect is due to the nonuniformity of the load and coolant flow rate in the channels in the core.The steam coefficient of the reactivity is an important parameter for assessing the state of a RBMK reactor. It is given by the relation α ϕ = ∂ρ/∂ϕ, where ∂ρ is a small change in the reactivity and ∂ϕ is a small change in the volume fraction of the steam content. According to the standard method for measuring the steam coefficient of the reactivity, the flow rate of the feed water changes, which changes the steam content in the reactor. The steam content increases as the flow rate of the feed water decreases, which introduces positive reactivity and, conversely, the steam content decreases as the flow rate of the feed water increases, which introduces negative reactivity. Often, "inverted variation" of the reactivity is observed during the measurements. For example, immediately after the flow rate of the feed water increases (introduction of negative reactivity) the reactor power increases for several seconds and only then starts to decrease. This effect is also observed in computational simulation of this experiment using the nonstationary version of the computer code STEPAN/KOBRA [1]. The purpose of the present work is to explain the inverted variation of the reactivity.The fuel present in the channels differs by the degree of burnup. As the burnup increases, the steam coefficient of reactivity of a cell increases. It is well known that the fuel cells in an infinite uniform lattice, which contain low-burnup fuel with an absorber (2.6% U + 0.41% Er and 2.8% U + 0.6% Er), have a negative steam coefficient of reactivity (Fig. 1). The calculations were performed for an infinite cell using the computer code WIMS-D4 [2].In reactor calculations, the inverted variation of the reactivity becomes even stronger because an increase of the coolant density decreases the neutron leakage from the high-power channels. As a result of this effect, the steam effect for any fuel channel with fresh fuel (even without erbium) is negative. At the present time, only uranium-erbium fuel is loaded into RBMK reactors. Ordinarily, the high-power channels are the channels with fresh fuel. Thus, the channels with the maximum power are the channels with relatively fresh fuel, which have a large negative effect of dehydration.The coolant flow rate is high (6-8 kg/sec) in channels with the maximum power. The effect is that coolant velocity in these channels reaches its maximum value. Thus, high-power channels are more rapidly filled with cold coolant, which has a high density (with increasing flow rate of the feed water). The result is an initial growth of the reactivity and power of the reactor. Figure 2 shows the time dependences of the reactivity and power initially, obtained in simulations of the m...
After the accident in the No. 4 reactor of the Chernobyl nuclear power plant, in all RBMK channel-type uraniumgraphite reactors measures have been taken to improve the safety level, the major goal of which has been to reduce the steam coefficient of reactivity and to eliminate the design disadvantages in the RR type manual regulating rods in the control and safety rod assembly.A disadvantage which cannot be eliminated is the large positive reactivity effect from loss of water in the coolant loop of the control and safety rods in the operational state (p = 4/3"), due to the fact that the ordinary water filling the coolant loop of the control and safety rods has a relatively large neutron absorption cross section.Therefore upon loss of water in elements of the rods such as telescopic units (actually, columns of water) and plungers, a significant positive reactivity is observed. Upon loss of water, the efficiency of the absorbers in the control and safety rod assembly increases. However, due to the relatively small number of inserted rods in the operational state (43-48 RR manual regulating rods), the absolute value of the negative effect is significantly less than the positive effect. In the subcritical state, for a large number of inserted rods these effects of opposite sign counterbalance each other.The possibilities for elimination of such a disadvantage in the control and safety rod assembly by changing their design are limited. Accordingly, we consider a variant replacing ordinary water in the control and safety rod loop with D20, whose neutron absorption cross section is significantly lower.Using the WIMS-D4 program, we calculated the neutron physical constants of elements of the control and safety rod assembly with the coolant loop of the assembly filled with DzO for different states of the core. Then using the program POLY (a version of the program STEPAN), we calculated the nuclear safety system characteristics of the RBMK-100 with the control and safety rod loop filled with D20 compared with the standard situation. The composition of the polycell corresponds to the loading of actual RBMK-1000 reactors. When replacing light water by heavy water, the position of the control and safety rods were not altered; the volume distribution of fuel burnup for both cases was determined as a result of iteration calculations in the POLY program. The calculation results are presented in Table 1.The analysis shows that the main goal of going to heavy water is certainly achieved. The effect of loss of water in the coolant loop of the control and safety rod assembly in the operational state changes from a large positive effect down to almost zero. The efficiency of the RR manual regulating rods increases by 25 % in the operational state, and by 70 % in the shutdown state. The subcriticality changed especially significantly. Its increase by 3 % along with the increase in the efficiency of the rods is determined to a large degree by the change in such reactivity effects as the density effect of the water and the reactor cooli...
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