So the Convair NB-36H, a prototype US bomber from the 1950's that was designed to eventually be nuclear-powered, carried a 1 MW air-cooled reactor with a mass of 16 Mg (16,000 kg / 35,000 lbs). If we put the reactor from the NB-36H in an airship and use it to heat steam as the lifting gas (let's assume that the reactor has enough power to keep the steam heated), then the volume required for the lifting gas to lift the reactor alone would be 1 m^3 ÷ 6.26 N * 9.87 N/kg * 16,000 kg = 25,200 m^3. The Hindenburg, by comparison, contained 140,000 m^3 of lifting gas. Unfortunately the Wikipedia article for the Hindenburg doesn't give the mass of the airship minus engines, passengers, and cargo, so I can't guess at how much more mass would be required for the rest of our nuclear-powered airship, but let's assume for now that the machine would be light enough to leave the ground.
Unfortunately, there are other problems. First, if there are humans aboard, then they will require shielding. There is no way to make lightweight shielding; shielding inevitably requires mass. The NB-36H had 11 tons of shielding for five crew.
But it gets worse: the airship would leave a trail of radioactive gas everywhere it went, since the reactor is air-cooled. This might have been tolerated in the 1950s, but things are very different now. In a crash, the reactor would break apart into highly-reactive pieces, including the uranium or plutonium fissile material.
To sum up, it may be technically feasible to design an airship that uses a nuclear reactor to heat steam as the lifting gas. However such an airship would be ridiculously expensive, and probably not be able to carry more than a small handful of passengers or crew. It would likely irradiate anything nearby in normal operation, it would leave a trail of radioactive gas wherever it went, and it would cause a major nuclear incident if the reactor were to malfunction, or if the airship were to crash. Such an airship would be highly impractical.