Nuclear thermal or pulse, fission or fusion, aren't enough for interstellar relativistic speeds. They have way too low specific impulse. They don't even have enough for constant acceleration inside the solar system unless the acceleration is really low or the fuel ratio is huge.
> unless the acceleration is really low or the fuel ratio is huge
You’d need kilotons of nuclear fuel and a 10^3 fuel ratio. But that’s plausible.
The economically-plausible answer is antimatter, where the ratio stays in the single digits and starts permitting deceleration. But I wouldn’t call that technologically plausible at this time.
