First television report on Soviet TV mentioning the Chernobyl nuclear disaster, April 1986, on Вре́мя (Vremya), the main news program of the First Programme of the Central Television of the USSR.
For more on the USSR, see http://nationalanthems.me/soviet-union-national-anthem-of-the-ussr.
For more on the incident and its 25th anniversary, see http://hauntingeurope.com/2011/04/vremya/.