Recently browsing the youtube videos I found an interesting interview film where the vice chief engineer of the Chernobyl Nuclear Power Plant Anatoly Stepanovich Dyatlov is explaining what happened during such scary night in 26 of April 1986. Anatoly Stepanovich Dyatlov supervised a test of freewheel stop mode of the turbine at Reactor 4 of the nuclear plant. Some information about him is availabe at wikipedia.
Previously, I have never seen this video. The publish date is (according to youtube) Apr 24, 2016 uploaded by KruchinaFILM. The video is filemed on Betacam tape in 1994 the year before the Anatoly Stepanovich had passed away. As far as I understand, the interview consists of 2 parts i.e tape 1 and tape 2. There are available only digitized version of the 1st tape. According to the autor the location of the 2nd tape is unknown. Another thing and quite strange thing the video was filmed in 1994, but published only in 2016.
Of cause, Anatoly Stepanovich was interviewed in Russian language and to translate this interview to English requires some effort, because interview lasts about one hour!
But, speaking shortly, in the interview Anatoly Stepanovish is explaing what happed befor the accident, at the moment of accident and what he and his colleagues has performed to avoid/prevent the accident worsening. In other words, according to the interview, during the experiment no one from the group of people working in this period expected that by pressing a single button which triggering emergency protection (AZ-5) will cause a termination of the reactor's active zone. The use of the AZ-5 has been agreed in advance before the experiment started, in other words it was declared in the program of the test. The accedent happed so fast, approximatelly after 7 seconds after the AZ-5 was triggered.
If you ask me, what I am thinking about him and this interview, I say: I don't know. I do not have any reason not to believe him because we know that the construction of the reactor core had serious design drawbacks with control (absorbent) rods of the System of Control and Protection Effect expl in Russian. The operators of the nuclear plant did not suspect this serious drawback. Other problems like lack of the process control feedback on the operators control desck and poor performance of the computer system used in plant has played its role too.