Tootfinder

Experts: Stuxnet-linked Fast16 malware, designed to subvert nuclear weapons testing simulations, was likely part of a campaign to slow Iran's nuclear ambitions (Kim Zetter/ZERO DAY)
https://www.zetter-zeroday.com/experts-con

Experts Confirm the Fast16 Malware Was Sabotaging Nuclear Weapons Tests, Likely in Iran
Fast16 didn't predate Stuxnet but was contemporaneous with it. It also wasn't aimed at altering nuclear weapons but was simply feeding false data to engineers about the nuclear detonation tests they were conducting, in order to trick them into believing the tests were failing.