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[[File:Chernobyl burning-aerial view of core.jpg|thumb|Steam plumes continued to be generated days after the initial explosion<ref>{{cite news |url=https://www.theguardian.com/environment/gallery/2011/apr/26/chernobyl-nuclear-disaster-in-pictures |title=Chernobyl nuclear disaster – in pictures |last=Kostin |first=Igor |author-link=Igor Kostin |date=26 April 2011 |newspaper=[[The Guardian]] |access-date=8 November 2018 |archive-url=https://web.archive.org/web/20181108184910/https://www.theguardian.com/environment/gallery/2011/apr/26/chernobyl-nuclear-disaster-in-pictures |archive-date=8 November 2018 |url-status=live }}</ref>]]
When the AZ-5 button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at {{convert|0.4|m/s|foot/s}}, so that the rods took 18 to 20 seconds to travel the full height of the [[nuclear reactor core|core]], about {{convert|7|m|ft}}. A bigger problem was the design of the [[RBMK#Control rods|RBMK control rods]], each of which had a graphite neutron moderator section attached to its end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor. That is, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with {{convert|1.25|m|ft}} columns of water above and below it.
Consequently, injecting a control rod downward into the reactor in a scram initially displaced [neutron-absorbing] water in the lower portion of the reactor with [neutron-moderating] graphite. Thus, an emergency scram could initially increase the reaction rate in the lower part of the core.<ref name=insag7/>{{rp|4}} This behaviour was discovered when the initial insertion of control rods in another RBMK reactor at [[Ignalina Nuclear Power Plant]] in 1983 induced a power spike. Procedural countermeasures were not implemented in response to Ignalina. The UKAEA investigative report INSAG-7 later stated, "Apparently, there was a widespread view that the conditions under which the positive scram effect would be important would never occur. However, they did appear in almost every detail in the course of the actions leading to the [Chernobyl] accident."<ref name=insag7/>{{rp|13}}
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