Nuclear pulse propulsion (Project Orion) is a different concept to nuclear thermal. Pulse works by using bombs to generate your energetic reaction mass in burst

Nuclear pulse propulsion (Project Orion) is a different concept to nuclear thermal. Pulse works by using bombs to generate your energetic reaction mass in bursts, whereas thermal uses a more normal fission reactor to superheat some reaction mass that then passes through an ordinary rocket nozzle. You're right that both are worrying until they're out of atmosphere.