The role of the operational reactivity margin
In the analysis of the Chernobyl accident a large emphasis on operational reactivity margin (ISP). This parameter indicates the value of reactivity caused by the rods in the reactor control and protection system. The high value of the operational reactivity margin or increasing the proportion of neutrons absorbed by the absorbing rods, which is unfavorable in terms of their use, since these neutrons could implement fission and produce energy. Besides the increased value of the ISP and is a potential danger, because it means a sufficiently high value of reactivity, which can be brought into the reactor because of the erroneous extraction CPS.
At the same time, the RBMK reactors low value of the ISP led to a reduction of the spatial stability of the reactor and increase the positive steam reactivity coefficient, which was described in the project materials. In addition, as it turned out after the accident, created conditions for increasing power in the first seconds after actuation of emergency protection because of the "end effect" of rods.
The technological regulation 4 blocks of Chernobyl, there are two limitations to work with low ISP. The first is 26 effective manual control rods (RR), allow operation of the reactor with the permission of the chief engineer of the station. Second - 15 eff. rods PP demanded the immediate silencing reactor.
Since the loss of capacity from 100% to 50% began poisoning core xenon, which resulted in the reduction of operational staff registered ISP. Operationally, the journal was stated permission chief engineer to work with the value of the ISP below 26 st. PP. However, after the stabilization of power at 50% of the ISP continued its decline, reaching a few hours values ≈ 13 st. RR (later been suggested that the reason for such small values were failures in the program PRISM).
Notwithstanding the technical regulations demand the immediate silencing of the reactor, the reactor had not been drowned out - during the 25 th of the peak poisoning has been passed and the ISP has increased to values in the range of 15-26 century. PP.
Further reduction in power up to 700-500 MW on April 26, short stop, and follow-up to 200 again increased the poisoning, which significantly reduced the ISP. Increased coolant flow through the reactor as well reduces the operational reactivity margin. As a result immediately before the experiment (that is, 1:22:30 for 2 minutes before the start of the experiment) the value ISP, calculated from the "standard" field of energy was ≈ 2 eff. Stud PP (≈ 7 eff. rod PP when calculating the field, reconstructed from the data recorded shortly before the accident).