Integral and axial xenon oscillations superposition and vver-1000 core energy-release stability

Abstract

The effects arising from the interaction between the xenon oscillations of the power and the axial distribution of the energy release in the core as well as the role of these effects in the stability of energy release in VVER≠1000 are examined. It is shown that the reactor poses self-regulation properties. The superposition of the integral and axial xenon oscillations combined with the operation of the automatic power regulator prevents the appearance of diverging xenon oscillations and suppresses low-amplitude oscillations without operator intervention.It is known that transient xenon processes in VVER-1000 can give rise to integral and spatial oscillations, i.e., the oscillations of the total power and the spatial distribution of the energy release in the reactor core volume, respectively [1-3]. The present article examines the effects of the interaction of integral and spatial xenon oscillations and their role in the stability of energy release. Figure 1 shows the distribution of the energy release rate (ω) and the heating of the coolant (ΔT) over the core height for cases with maximum energy release in the top (curve 1) and bottom (curve 2) parts of the core with constant total reactor power. The coolant temperature on the end faces of the core is the same in both cases; this follows from the condition that the reactor power is fixed. In the middle layers, the temperature in the second case is higher, since the power maximum in the bottom part of the core gives more rapid heating of the coolant as it flows from bottom to top. Thus, on the descending phase the axial xenon oscillations, when the maximum of the energy release moves from the top to the bottom part of the core, the average temperature of the coolant increases, which is accompanied by the introduction of negative reactivity; similarly, on the ascending phase the temperature decreases and as a result the positive reactivity is introduced. Moreover, the additional reactivity with an increase of the off-set is introduced as a result of the nonuniformity of the fuel burnup (burnup is higher in the bottom than top part of the core). Figure 2 displays the results of computational modeling of the axial xenon oscillations excited in a VVER-1000 core at the initial moment of the displacement (extraction) of the working group of control rods with the reactor operating at nominal power. The extraction of the group of control rods was compensated by an increase of the boric acid concentration in the coolant; the subsequent calculation was performed without searching for criticality. The computational data obtained using the IR code [4] under the conditions of a typical stationary fuel load are used here and below. The obvious correlation of the oscillations of the axial off-set (AO), the temperature of the coolant (δT is the deviation of the average temperature from the stationary temperature), and the reactivity (ρ) confirms the mechanism of excitation of reactivity oscillations under the action of axial xenon oscillations.In the course of the axial xenon osc...

Investigation of Algorithms for Suppressing Xenon Oscillations in a VVER-1200 Reactor

“…При снижении мощности группы органов регулирования двигаются вниз, а при повышении мощности -вверх, и тем самым влияют на аксиальный офсет нейтронной мощности. При этом указанное движение групп и его влияние на аксиальный офсет происходит таким образом, что выполняется и другая немаловажная функция управления -повышение устойчивости реактора по отношению к высотным колебаниям локальной мощности при сохранении интегральной мощности [13].…”

Analysis of the influence of methods for controlling power unit with a pressurized water reactor for axial offset

“…Во время работы реактора в стационарном режиме в АКЗ присутствуют отравитель 135 Xe, который обеспечивает постоянную отрицательную реактивность. Любое изменение мощности от стационарного значения приводит к изменению концентрации 135 Xe, а, следовательно, к изменению реактивности вызванная этим изменением, что сказывается на нестабильности и надежности реактора в целом [1,2].…”