Chernobyl occurred prior to the warming in relations that resulted from Gorbachev's reforms, so it is understandable that the Soviets perceived a significant threat from admitting the accident occurred. Unlike Fukushima, Chernobyl slowly released radioactivity for days due to the fire. Initial Soviet assessments of the accident severity were optimistic. I'm not saying that they were right in delaying, but the fire was still burning and the atmospheric release ongoing when they publicly revealed it.