Can someone give me advice on my writing?

Nicely written. I'm not sure what you're trying to convey in portraying Martin as a man who is relaxed when he could see a large fireball outside his window but if you can make sense out of it, it's an introduction I'd be eager to read more of.

Good writing should be written in active voice. Limit the use of verbs like was and were. Instead of saying He was looking out the window - say He gazed

Instead of saying had risen - say rose from the ground or hovered on the horizon.

Instead of Radiation would spread - Radiation had spread (if the event happened 18 months ago, millions would already be dead. Your first sentence should pull them into the story. Make the reader wonder and want to read more. These few sentences have no sense of urgency or terror or even fear. It seems this character is way to calm based on the fact that millions are dying.

Good luck.

suggest a bit more detail about the whisky and cigar. how about smokey, well aged Tennessee whiskey and one of the finest, hand rolled Havana cigars. [this would make him either quite well to do or knowing his end is nigh.]

Of course, if he can directly see the fireball, he is doomed to near term death from radiation poisoning. less than a month at most and likely only a few days. His end will be gruesome ...

If you need him for the story later, you'll have to alter the detail ... idea: he's seeing the fireball on a 62 inch curved Samsung tv/monitor via remote viewing over the Internet from a camera feed he set up days ago. The scene is being captured on hard disk so he can look at it over and over -- thus admiring his handiwork. It isn't a very long scene -- the wave of hard particles that are coming behind the light of the fireball will destroy the computer gear and camera -- which is all to the good since no one will, thereafter, try to dust the then radioactive gear for fingerprints.

you misspelled "knew".

Savannah's population is not as large as millions ... even the whole SMSA.

Your first sentence doesn't seem to have anything to do with the rest of the material -- how can you tie it together? [or else, invent new first sentence -- or even just leave it out.]

If it fits into the story line, add detail about how the atomic device [bomb] got to Savannah in the first place. My suggestion -- in a six meter container onboard an unremarkable cargo ship. The ship's captain and crew, now all dead of course, were completely innocent -- they had no idea that the legit bill of lading describing custom milling machinery from Germany had been altered at transshipment in Rotterdam, nor that the container matching the new ID number on the paperwork that they actually carried to Savannah wasn't carrying milling machinery. Rathbone's inside man in Rotterdam harbor's IT department had invisibly changed the computerized data, as had the forger who fixed the paperwork to match.

oh, do have fun writing this ... btw, you might expect a visit from Homeland Security [or other security apparatus in your country] after they get a hint from your publisher as to what's in it. {I suppose I might expect one too -- after NSA reads this. Mustn't go giving Fat Boy ideas about how to attack America.}