We need to know how much energy some amount of TNT produces.
Fortunately, I was willing to go and search that number on Floogle.
The energy density of TNT is taken to be
4.184 million Joules per kilogram
I also learned that the "kiloton" and "megaton" mentioned in connection with nuclear weapons is the metric ton, equal to 1,000 kilograms.
So NOW we have something to work with.
(450 kilotons) x (1,000,000 kg/kiloton) x (4.184 x 10⁶ joule/kg) =
(450 x 10⁶ x 4.184 x 10⁶) (kiloton - kg - joule / kiloton-kg) =
(450 x 4.184 x 10¹²) joules =
1.8828 x 10¹⁵ Joules released by the 450-kiloton bomb.
How long does it take the 1-Gw power plant to produce this much energy ? Well, let's see:
1 Gw = 10⁹ joules per second
(1.8828 x 10¹⁵) joules / (10⁹ joules/sec) =
1.8828 x 10⁶ seconds .
Crunching this period of time some more . . .
(1.8828 x 10⁶ sec) / (86,400 sec/day) =
21 days 19 hours <=== this many days
Note: If I made a mistake, then my answer will probably be wrong by some factor of ten. I mean, the digits are probably OK, but the correct number of days might be 1/100, or 1/10, or 10 times, or 100 times as much as what I said in my answer. I KNOW you're going to check my work very carefully, so if I made a mistake, I'm sure you'll find it.