"Limit effect"
The specific status of the reactor plant, as it turned out after the accident, created the conditions for the manifestation of "end effect" - the positive deceleration reactivity at the beginning of immersion absorbing control rods into the core.
The existence of the end effect was observed at Chernobyl in 1983 during a physical start-up unit. Performed at the same time research has shown that end-effect was observed when immersed in an active zone of single rods from the upper limit switch, in case of mass input bars (more bars 15-18 PP) end effect was absent.
Studies were conducted on the reactor, which was more than 200 PD. What will be the "end effect" on the faded reactor without the DP has not been experimentally verified. To avoid the end effect Organisation chief designer suggested a number of measures. Unfortunately, at the time of the accident, these proposals were not implemented.
Thus, at the time of the accident the essence of "end effect" and its causes and conditions for implementation are known. The analysis conducted immediately after the accident (as before tragedy methods) showed that to achieve the end effect requires a strong bias field (3 times). However, analysis of data recorded by the PRISMA program immediately before the experiment, indicated that such a strong bias before the accident was not.
A more thorough study of the "end effect" showed that some factors affecting the feasibility of "end effect", have been underestimated. In particular, the possibility of introducing positive reactivity occurred at M-shaped form of the neutron field at a height of reactor. Completed assessment showed that the positive effect of dehydration 4-5 β, only the end effect does not cause a catastrophic increase reactivity. At the same time, the analysis of changes within the measurement error type field (adapted to the most unfavorable form) showed the feasibility of the accident.
Thus, the end effect could contribute to the development of a catastrophic accident at Chernobyl 26 April 1986. Of the reported data it is known that immediately prior to the disaster reactor was unacceptably low operating reactivity margin, and thus, the majority of control rods were in the top trailer. In this case, the mass input control rods into the core could lead to the positive input of uncompensated reactivity (according to different estimates from 0,3 to 1,1 β). Either way, end effect prevented silencing of the reactor control rods during the first seconds (5-6) after the formation of the appropriate command.