Kyshtym disaster

The Soviets decided to produce plutonium 239, with which they wanted to replace uranium 235.

An excerpt from the article 42 facts about Kyshtym disaster

It takes one ton of uranium to produce 500 grams of weapons-grade plutonium (plutonium 239), and 5-10 kilograms of plutonium are required to make one nuclear bomb. The USSR in 1945 did not have such large uranium resources.

To this end, in 1945, they received orders from Stalin to build a suitable nuclear infrastructure to refine and process weapons-grade plutonium - the Mayak combine.