The Mayak plant, which basically is the Soviet Hanford, neglected any environmental concerns and was the place of the second-worst nuclear incident: https://en.wikipedia.org/wiki/Kyshtym_disaster

>Initially Mayak dumped high-level radioactive waste into a nearby river, which flowed to the river Ob, flowing farther downstream to the Arctic Ocean. All six reactors were on Lake Kyzyltash and used an open-cycle cooling system, discharging contaminated water directly back into the lake. When Lake Kyzyltash quickly became contaminated, Lake Karachay was used for open-air storage, keeping the contamination a slight distance from the reactors but soon making Lake Karachay the "most-polluted spot on Earth".

In the former GDR(DDR), too[0]

[0] https://apnews.com/article/b771e863afa629be5139b65784b21aba