There's a popular Wordpress website, WWII after WWII which has an entry on the SU-152's used during cleanup after the nuclear disaster at Pripyat. In passing, the author states that before the Germans occupied Chernobyl the Soviets removed anything of value, killed the city jail's inmates and blew the bridge over the Dnieper River. Before the 1986 disaster there was seemingly nothing remarkable about the city and I would imagine that the Soviets didn't keep high profile, political prisoners there. Who were the inmates they executed and why did they just kill him? Was August 1941 an awful time to get picked up for being drunk in public in Chernobyl?
After the launch of Operation Barbarossa in the summer of 1941, the NKVD killed tens of thousands of prisoners in western Ukraine before the German advance reached that area. Scholarly estimates vary from 10,000 - 40,000. The exact number is unknown as records do not exist for all local Ukrainian holding centers, and the executions took place in a hurried manner amidst the chaos of invasion. Many of the victims were Ukrainian nationalists who had been imprisoned for "Anti-Soviet activity" (aka, Ukrainian nationalism or suspicions thereof) in 1940-1941. Across the entire USSR, from January to June 1941, 90% of all arrests for "anti-Soviet conspiracy" were made in Western Ukraine.
Besides Ukrainians, many of these victims were ethnic Poles, and their killings can be seen as an extension of the Polish operation of the NKVD launched 1937-1938.
The bodies were generally left to rot in the prisons, dumped in ditches nearby, or otherwise strewn about, which presented a horrific sight to the local townspeople and advancing German troops. Some of the (surviving) Ukrainian nationalists allied themselves with the invading German forces, seeing an opportunity to throw off Soviet rule in Ukraine (but creating an independent Ukraine was not part of the Germans' plans, of course). Nazi-allied Ukrainian nationalists and the Germans presented the dead NKVD prisoners as victims of fictitious Jewish atrocities and attempted to use these killings to stir up local antisemitic sentiment. Similar things happened in the Baltic states where the NKVD also liquidated the prisoners in 1941 before retreating. This mass-killing of prisoners is not as well known compared to the Holocaust or the massive carnage of Soviet civilians and soldiers on the Eastern front.
The nuclear power station was built in the 1970s, and has no connection to the war-time killings.
Soruces: Black Earth: The Holocaust as History and Warning by Timothy Snyder. The Great West Ukrainian Prison Massacre of 1941: A Sourcebook Kiebuzinski & Motyl, ed. https://assets.ctfassets.net/4wrp2um278k7/49qXJfjUqkGkaYAwmcKqCo/893093588735e3c4cd3a08568e57df39/9789048526826_ToC___Intro.pdf