Positive void coefficient of reactivity
During the operation of the reactor through an active zone is pumped the water used as coolant. Inside the reactor, it boils and partially converted into steam. Neutron-physical state of the reactor depends on the density of boiling in the reactor coolant. This dependence was obtained in the project using the program BPM, developed at the Institute of Atomic Energy and was used to develop control systems and power systems, emergency reactor shutdown. The peculiarity of this dependence was a positive value of the steam coefficient of reactivity in the small steam quality and negative - and large. The combined effect of the reactivity of dehydration of the core (ie, reactivity, introduced into the reactor at full dehydration of the core), it turns negative. In addition, rapid cardinality reactivity coefficient is also negative, in line with the normative documents meet the requirements for safety. However, a more thorough analysis done after the accident showed that the technique used to measure the steam coefficient of reactivity, gives incorrect result in the low steam quality and steam quality coefficient of reactivity is positive in the whole range of steam quality. Moreover, the specific conditions created immediately before the experiment (low heat content in the reactor core, as well as the low value of the operational reactivity margin, the ISP) could lead to an additional increase in the steam coefficient of reactivity. As a result, rapid cardinality reactivity coefficient was also positive, which meant that the increase in capacity contributes to the further acceleration of the reactor (without the work of control and protection system, that is, without moving the absorbing rods), and determined the possibility of catastrophic dispersal reactor.
Post-accident analysis in IAE using improved Monte Carlo method showed that the effect of dehydration of the reactor core, instead of negative values can reach from +4 to +5 β, which was experimentally confirmed in late 1986 when the physical launch of the Chernobyl and Smolensk NPP.