Investigation of xenon transient processes in VVER-1000 at the Tianwan nuclear power plant (China)

Abstract

The integral and spatial xenon transient processes in the No. 1 unit of the Tianwan nuclear power plant (China) have been studied experimentally. A measurement method which is unconventional for VVER-1000 was tested in the investigations of the integral processes: the course of the xenon process was recorded according to the variation of the critical concentration of boric acid in the reactor at the same time as the concentration was calculated in real-time. The spatial transient processes were studied for the diametric and axial free xenon oscillations of the energy release in the core. It was confirmed experimentally that axial deformations of the energy release affect the power of the reactor as well as the associated operational particularities of the automatic power regulator.Processes which are associated with a change of the 135 Xe concentration in the core have a large effect on VVER-1000 operation [1]. Xenon transient processes can be conventionally divided into integral -those involving a change of the average concentration of xenon in the core, and spatial -those involving a redistribution of the concentration within the interior volume of the core.Integral xenon processes result from a change of the power of the reactor. Their effect on the operation of the reactor is characterized by a reactivity increase or decrease and, correspondingly, a decrease or an increase of the xenon concentration. The following stages of the integral xenon processes are distinguished according to the character of the dominant effects: poisoning -xenon concentration increasing as a result of 135 I decay; unpoisoning -xenon concentration decreasing as a result of xenon decay; and xenon burnup -xenon concentration decrease as a result of neutron absorption by xenon.Calculations and experiments show that in the absence of control the xenon processes can be accompanied by power oscillations, which can be divergent and can result in reactor shutdown or power increasing above the admissible level [2]. In practice, ordinarily, the reactor power is kept at a prescribed level; the change of the xenon concentration is compensated by changing the 10 B concentration by moving the absorbing control rods and changing the boric acid concentration in the first-loop water.When the reactor shuts down, poisoning and subsequent unpoisoning occur in the core. In the process, correspondingly, the degree of subcriticality increases and then decreases. When the initial boric acid concentration in the first loop is

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

“…This is explained by the fact that traditionally the calculations were performed for free xenon oscillations, i.e., for constant reactor power, which was maintained by varying the boric acid concentration. Experimental studies performed under the same conditions, i.e., for fixed position of the control rods, confirmed the computational results [3]. However, in practice the reactor power was maintained at a constant level by using an APR, which makes it possible for the stabilizing effects described above to operate, thereby preventing the diverging oscillations from developing.…”

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

“…При колебаниях мощности, наведенных ксеноновыми процессами в активной зоне, движение рабочей группы совершается в противофазе с изменением офсета, что подав ляет колебания [14].…”

Investigation of the impact of steady-state VVER-1000 (1200) core characteristics on the reactor stability with respect to xenon oscillations

“…Under the condition of an asymmetric axial distribution of the neutron field, one more parameter appears that affects the reactor stability to xenon oscillations, i.e., the axial neutron power offset or the coefficient of nonuniformity of the axial distribution of energy releases. A mention of this and a description of the physical essence of this influence can be found in (Averyanova et al 2008, Andrushechko et al 2010. However, it is much more difficult to obtain final analytical expressions for the reactor stability criterion, taking into account an arbitrary axial neutron field distribution, and the authors are not aware of publications where such expressions could be presented.…”

“…In (Averyanova et al 2008), simple calculations are presented that show that, when the maximum energy release shifts to the upper half of the core, while a constant reactor power is maintained, the average water temperature in the reactor decreases, and the temperature coefficient of reactivity decreases modulo, which reduces the reactor stability in comparison with the option of an symmetric axial neutron field distribution and, even more so, in comparison with the option of the shift of the maximum energy release to the lower half of the core. But this paper lacks analytical expressions that reflect these findings.…”