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.

No abstract

No abstract

No abstract

We have carried out experiments on a critical P test bench [1] to investigate the effectiveness of samples of absorbing elements 100 mm long and models of absorbing elements 1250 mm long, made of different absorbing materials. The active zones are assembled from experimental VVER fuel elements [2] enriched up to 4.4% 235U with a hexagonal lattice with a pitch of 12.7 mm with a moderator temperature of about 18~The absorbing elements have the following characteristics: sample 0 was stainless steel, a tube diameter of 8.2 mm and a thickness of 0.6 mm (the standard cladding of the absorbing element), samples 1, 2 and 3 were boron carbide, dysprosium oxide, and titanium dioxide in a tube of diameter 8.2 mm and thickness 0.6 mm thick, sample 4 consisted of titanium dioxide in a tube 9.6 mm in diameter and 0.6 mm thick, samples 5 and 6 consisted of a hafnium bar with a diameter of 8.2 mm and 9.6 mm, respectively, samples 7-12 were hafnium tubes 8.2 mm in diameter and 1 mm thick, 9.6 mm in diameter and 2 mm thick, 12 mm in diameter and 2 mm thick, 11.5 mm in diameter and 1.5 mm thick, 10.5 mm in diameter and 1 mm thick, and 9.5 mm in diameter and 0.5 mm thick, respectively. The characteristics of the models of the absorbing elements were as follows: model 0 was a water cavity, model 1 was a hafnium bar, 8.2 mm in diameter, model 2 was a hafnium tube 8.2 mm in diameter and 1 mm thick, and models 3 and 4 were boron carbide and dysprosium oxide in a tube 8.2 mm in diameter and 0.6 mm thick.The method for the experimental determination of the absolute efficiency of the absorbing elements is based on a measurement of the critical levels of the moderator (Her) alld ao/cgh = f0a) [3] for the active zone when the absorbers were and were not present in it. From these results we calculated AO --the absolute effectiveness of the absorbing elements. The effectiveness of different absorbing elements was normalized to the effectiveness of an absorbing element of boron carbide and we thereby obtained their relative effectiveness and eliminated uncertainty connected with ~eff.In the experiments the critical height of the moderator was determined by adding moderator [2] in the active zone. We recorded the level of the moderator (tl) and the reactivity as a function of h in states close to critical. By extrapolating the function o(h) to o = 0 we obtained the critical height employed as the experimental value.The position of the moderator was measured by two UKTs-3 contact level meters and the reactivity was measured using two PIR-4 instruments, whose detectors are KNK-56 cameras. When processing the o0a) data we took average readings of the instruments, which measure both the level of the moderator and the reactivity. The effectiveness of the i-th type absorbing element with respect to the active zone without art absorbing element is estimated as /' /El" ckP,lt3 = I t~ -10p/Ohdh, HerO where Her 0 is the critical height of the moderator for the active zone without absorbing elements and Her i is the critical height of the moderator whe...