Furthermore, Mr Yoon had made it clear that he expected to return home having made "tangible" progress.
Duyeon Kim, from the Centre for a New American Security, said it was a "big win" for South Korea to be involved in nuclear planning.
"Until now, tabletop exercises would end before Washington's decision to use nuclear weapons," said Ms Kim.
"The US had considered such information to be too classified to share, but it is important to practice and train for this scenario given the types of nuclear weapons North Korea is producing."
Read the full story and more from the BBC.