Thanks MARINW for the recommendation to watch CITY 40 on Netflix which I did tonight and it was really informative. I learned so much about Closed Cities of which I knew nothing even though I knew Mercury, NV, existed in the United States; I just didn't realize that it was a Closed City. Hopefully, the Russian Human Rights Specialist, Nadezhda Kutepova who had to flee to France after she was accused by the Russian government of "Industrial Espionage" in 2015 will not suffer the same fate that Sergei Skripal and his daughter Yulia when they were poisoned by the Russians.
Here's a good article about Nadezhda Kutepova which details some of the things discussed and shown in the documentary.
I see Netflix also has the PBS documentary, Building Chernobyl's Megatomb which I've never seen; that's next on my viewing list!