From watching the series I came away with:
1. They really didn't know the design of the reactor would result in the run away of the reaction and not much they did during the test would not have resulted in an explosion. The chief nuclear scientist, the one who hanged himself, indicated that no one thought about the situation they were faced with, hidden reports etc. Design with graphite, which accelerated reactions, ends on the control rods, which have been replaced)
2. He had promised that the test would be completed to his supervisor and he didn't want to have his reputation tarred by a failure to complete as promised.
3. He, by the series description, was a bit of an egotist and knew much more about the reactor than his very junior operators, even though they were going by the book.
4. From my work in the former USSR, the system had abundant fraud and dealt with lies to protect "your" job. For example in the oil field related activities. Water is pumped into wells to increase reservoir pressure back towards original, improving recovery percent. The individuals responsible for pumping water into wells were paid by cubic meters injected but there were no instruments to measure any amount and the employees lied about the quantity pumped(getting paid for the required amount even though perhaps 10% was injected). If workers dropped tools into wells, they went without pay while recovering the tools(so guess what, no tools were ever dropped
)
5. If the nuclear operations were anything like the oil work(I don't know). Safety was never a objective. Construction materials were notoriously below standard and no one to check the standards.
All in all the series was generally what I would have expected. Secrecy, not reporting the incident until stations in Scandinavia reported the radioactivity, was paramount. Same with the early rocketry secrecy, not reporting launch failures so that their technology would seem "superior".