Possibility, admissability, and desirability of implementing pulsed stochastic regimes in boiling-water reactors with natural coolant circulation

Abstract

A one-dimensional integral momentum model for the boiling-water channel and the point kinetic equations are used to study the conditions and mechanisms for the appearance of temporary pulses of stochastic regimes and regimes with pulsed turbulence of the neutron field in water-moderated water-cooled boiling-water reactors with natural circulation of the coolant. These self-excited oscillatory regimes occur as a result of a negative steam effect of reactivity. They consist of the fact that neutron bursts with random intensity occur in a reactors in random time intervals. Such regimes make it possible to increase substantially the power and size of boiling-water reactors. The methods proposed for controlling pulsed chaos make it possible to excite and suppress stochastic regimes and to regulate the amplitude of self-excited oscillations so that it does not exceed admissable limits. Small external perturbations have no effect on the characteristics of pulsed chaos. The results are obtained by numerical calculation of the self-excited oscillations.Self-excited oscillatory pulsed stochastic operating regimes in nuclear reactors were discovered in [1-3]. They consist in the fact that neutron bursts with random intensity occur in a reactor in random time intervals. Such regimes can occur in reactors with gaseous nuclear fuel [2, 4] and in water-moderated water-cooled boiling-water vessel reactors [2, 5] as a result of the negative density feedback.In the present paper, pulsed stochastic regimes in boiling water reactors with natural coolant circulation are investigated using a model which does not claim to give a detailed quantitative description of dynamical processes, but it does describe the mechanisms, scenarios, and qualitative features of pulsed chaos well.The reactor core with height H and the thrust section of height H 1 following the reactor are assumed to be a single equivalent channel. The coolant with fixed enthalpy at the entrance into the channel is heated, as it flows along the channel, by heat transferred from fuel elements located inside the channel. Assuming the coolant pressure drop along the channel to be small compared with the pressure drop at the exit, which is assumed to be constant, and no slipping of phases the energy conservation and continuity equations for an equilibrium steam-water mixture can be written in the form ∂i /∂t + u∂i/∂z = qν; i(0, t) = i 0 ;(1)where i, u, and ν are, respectively, the enthalpy, velocity, and specific volume of the steam-water mixture; q is the specific (per unit coolant volume) heat flux; i 0 is the coolant enthalpy at the entrance into the reactor, assumed to be constant; z and t are, respectively, the spatial coordinate and time; z ∈ [0, H + H 1 ]; the point H corresponds to the exit from the core and the start of the thrust section; the point H + H 1 is the exit from the thrust section, located at the level of the overflow ports.

Investigation of chaotic oscillations in boiling-water reactors

“…Chaotic oscillations of the density and velocity of two-phase flow in boiling-water heated channels, including in the channels of boiling-water reactors, are investigated in [1][2][3][4][5][6][7][8][9][10].…”

“…The properties of the mappings and the bifurcation of their stationary points are investigated.Chaotic oscillations of the density and velocity of two-phase flow in boiling-water heated channels, including in the channels of boiling-water reactors, are investigated in [1][2][3][4][5][6][7][8][9][10].A very simplified model of a boiling-water reactor is studied in [1, 2]; the model uses point equations of kinetics and a system of three ordinary differential equations to describe thermohydraulic processes in a reactor and the density feedback on reactivity. It is shown that chaotic oscillations during which the sign of the coolant velocity in the boiling channel changes occur in the case of strong feedback on steam content with the parameters of the boiling channel deep in the region of instability occur in boiling-water reactors with natural and forced circulation of the coolant.…”

Methods for controlling pulsed stochastic operating regimes of nuclear reactors

“…Novel reactor designs which admit a large increase in the core size have been proposed. The numerical calculations performed of self-excited oscillations on the basis of distributed models of a reactor [5,6] have shown that the methods proposed in the present work can be used effectively to control the temporal and spatial-time pulsed chaos in water-moderated water-cooled boiling-water reactors and in reactors with gaseous nuclear fuel.…”

“…It is shown in [4][5][6] that if a regime of temporal pulsed chaos arises in small reactors, then increasing the reactor dimensions leaving all other parameters unchanged results in the appearance of a regime with pulsed turbulence of the neutron field. In this regime, neutron pulses having random intensity and occurring in random time intervals appear in local regions, whose dimensions, shape, and arrangement in the core vary irregularly in time, of the reactor.…”

“…The possibility, admissibility, and desirability of implementing pulsed stochastic regimes in water-cooled water-moderated boiling-water reactors and in reactors with gaseous nuclear fuel were investigated in [5,6] by numerical study of self-excited oscillations in distributed models describing the dynamics of specific types of reactors. The physical mechanisms and conditions for the appearance of pulsed chaos have been determined for reactors of such types with compressible fuel or coolant.…”