There were many studies in the 1960s, and a lesser number of later ones, for launch vehicles with nuclear-thermal upper stages. I have rarely seen mention of what emergency procedures were considered for launch failures.
Dangers of launching a nuclear thermal rocket suggests the hazard is not extreme - for the case of an NTR started in orbit. A nuclear upper stage has the possibility of running for some time but failing to reach orbit.
Even in the pre-Chernobyl era, there has to have been serious consideration of this. What studies were done, and is there any available summary of the results?