Chernobyl is a powerful and very interesting show on HBO based on the famous accident of the same name. If you'd like to watch the Chernobyl series online, then here are a few options you can try (both free and paid options):
The entire series of Chernobyl is available on Amazon Video, and you can sign up for Amazon Prime Video completely free in 10 seconds and have access to Chernobyl immediately for free (Amazon Prime users get Amazon Video access too), plus a whole bunch of bonus content. Just cancel before the 7 days is over and pay nothing!
Hulu has the series and it can be watched with a free trial, but I'd always recommend using Amazon Prime Video instead because the trial is much easier to cancel.