Chernobyl called in Polish Czarnobyl belonged to Poland or rather Polish-Lithuanian Kingdom from 1569 (Lublin's Union) until the Second partition of Poland in 1792. But only very few Poles - less than 10% lived there. The majority of people were of Rusyn, Ukrainian or Jewish origin. Jewish were usually innkeepers and they owned all the taverns in the town.
Chernobyl nuclear accident occurred at night April 25/26 1986 during the test run. Although many human errors lead to the accident, the IAEA (International Agency of Atomic Energy) concluded that the main reason of the explosion and a massive release of the radioactive materials was faulty construction of this reactor. Nuclear reactors like this one in Chernobyl, type RBMK, were only built in Russia and some other countries of Eastern Europe. They were cheap because they did not require a usage of heavy water (as a coolant or moderator) or enriched uranium (as a fuel) in order to run. This reactor was using light water for cooling and graphite for moderation, therefore it had so called "positive water coefficient", meaning that an increase in reactor temperature did not shut out chain reaction like in the other reactor types but enhanced it, so they were inherently unsafe to run.
The accident happened just a couple of days before May 1st manifestation but people in Kiev and other big towns of Ukraine and Belarus were not warned against participation in the street demonstrations although in that time it was known even in Soviet Union about the accident. The increased levels of radiation were found in Sweden and this is why the world learnt about the accident in the first place before Soviet Union admitted to the accident.
Many people believe that the accident in Chernobyl was a turning point in Soviet policy of secrecy. Soon after the accident Soviet leader, Michail Gorbachev initiated Perestroika, which changed the Soviet system and led to breakdown of Soviet Union.
The radiation contamination was distributed unevenly on the area of Poland - depending on the direction of the wind and rain deposit. The worst contamination with the short-lived radioisotopes and traces of radioactive plutonium was in northeastern part of Poland. The southwestern part (Opole region) was contaminated with long-lived cesium, thus the high contamination of cesium in some wild mushrooms from this area was found. After the accident some precautions were taken in Poland to prevent any side effects. All Polish children were given iodine solution to replace and block any intake of radioactive iodine.
You can read more following this off site link to: Chernobyl: Biking through the Dead Zone
I also recommend Chernobyl Record: The Definitive History of the Chernobyl Catastrophe by Richard F. Mould