Suppressing axial oscillations of the energy distribution in a VVÉR-1000 reactor without half-length control rods

Abstract

Nonequilibrium redistribution of the concentration of 135Xe nuclei in the active zone of the VVI~R-1000 reactor may lead to xenon oscillations, that is, relatively powerful axial oscillations of the energy distribution, the prevention and suppression of which is one of the goals of reactor control [1, 2].Xenon oscillations are described by means of the axial offset (AO); this quantity, which characterizes the nonuniformity of the energy distribution, expresses the difference in the rates of energy liberation in the upper and lower halves of the active zone as a percentage of the total rate of energy liberation. In the ascending phase of the xenon oscillations, energy liberation is redistributed from the lower to the upper half of the active zone; the offset increases. In the descending phase, conversely, energy liberation is redistributed from the upper to the lower half, and the offset decreases.After the first VVI~R-1000 units went into operation in 1980-1985, a series of experiments were conducted on the xenon oscillations, and algorithms for their suppression were developed [3][4]. The algorithms are of discrete type and usually assume the use only of the working group of control rods.An exception is the algorithm for suppressing the descending phase of the xenon oscillations when control by means of the working group is not totally effective or is limited (at the end of the working period) by the low productivity of the boron-regulation system. In this case, an additional group of control rods is introduced; these rods are half as long as the absorbing rods. Recompensation of the reactivity on introducing and removing the additional group of control rods is ensured by moving the working group, which means that this control system is independent of the boron-regulation system. To suppress the descending phase of the xenon oscillations, the additional group of control rods is dropped to the limiting position, with a positive change in the offset, and then removed after a certain period. Safety concerns rule out an intermediate position of the half-length rods, and so the algorithm is of discrete type.

Adoption of Improved Algorithms for Controlling the Energy Release of a VVER-1000 Core at the Khmel’nitskii Nuclear Power Plant

“…When the reactor operates at constant power, the controlling groups can be loaded into the top half of the reactor no lower than the prescribed maximum admissable positions, which are determined from the operating safety conditions, in order to suppress the ascending phase of the oscillations (offset increases). When the descending phase is suppressed (offset decreases), the motion of all 10 groups can be compensated by lowering the central regulating organ to the bottom limit of the core [5].…”