Kyshtym disaster

The Soviets learned that uranium 235 was required to cause a nuclear explosion.

An excerpt from the article 42 facts about Kyshtym disaster

It accounted for 0.7% of natural uranium, and the required purity of uranium 235 in a nuclear bomb had to reach at least 80%.

To create a nuclear bomb, scientists would have to enrich uranium, but this was both difficult and expensive.