Hiroshima and Nagasaki is currently livable because the bomb was detonated in the sky, the radiation disappates quickly.
In constrast, Chernobyl had much more fuel and since the power plant was on the ground, it contaminated a lot of the soil, therefore, it's gonna take much much longer before Chernobyl is ever livable again.
A tactical nuke is a bomb that will detonate in the air, and since its "tactical", its gonna have much less yield. Its gonna be become livable again even quickly than Hiroshima and Nagasaki.