The IMDB entry for Chernobyl: Chronicle of Difficult Weeks describes it as:
The first film made following the nuclear meltdown accident at the Chernobyl Nuclear Power Plant, near Pripyat, Ukrainian SSR, Soviet Union, on the 26 April 1986, focuses on the immediate aftermath of the disaster and the cleanup effort.
Like a lot of movie descriptions, this doesn't quite do it justice. Susan Schuppli makes a much more compelling argument for watching this documentary when she describes it in her book MATERIAL WITNESS: Media, Forensics, Evidence, published by MIT Press:
Shevchenko's film, "Chernobyl: Chronicle of Difficult Weeks," provides us with an intimate view into the space of disaster. And while its pictorial mediation allows us to remain at a safe and objective distance from the hazard, the sudden distortion of the documentary's sound and images, and the Geiger-like interference of radiation, inaugurates a sense of dread that what we are witnessing on film is in fact the unholy representation of the real: an amorphous and evil contagion that continues to release its lethal discharges into the present and future yet to come.
The contaminated film footage thus complicates the conventional partitioning of time by hurling us unwittingly back into the contact zone of the event — not merely as viewers but also as witnesses to an event whose time has not yet passed. Even when I am watching a safe VHS copy of this film, I am reminded of the transgressive agency of the nuclear to contravene the material borders that traditionally maintain the integrity between human and nonhuman entities, between bodies and images, between past and present.
Given what we know about the radical chemistry and anarchic temporality of nuclear materials, it is impossible to fully distance ourselves from this fallout on film, regardless of how far removed we believe ourselves to be from the event in both space and time.
In other words: the radioactive fallout from the Chernobyl devastation was so bad that it literally irradiated and distorted the celluloid tape.
(Presumably the YouTube upload is safe to watch though)
You can learn a helluva lot more from this excerpt over at MIT Press.
The Most Dangerous Film in the World [Susan Schuppli / MIT Press]