Megaton: The power of the atom
Received from: Lucas Simms [Megaton]
Solution: The nuclear bomb can be found in the middle of Megaton and you'll find out about it during your first conversation with Lucas Simms. He's going to be the first person you'll meet after reaching Megaton. It would be wise to use your speech skill to demand a higher reward (100 bottle caps) for the job. You can also agree to do this for free if you desperately want to raise your karma. Before you decide to disarm the bomb, you should take your time and go to Megaton - Moriarty's Saloon. Find Mr Burke and listen to his solution to the problem. Just like with Lucas Simms, if you win a speech challenge you'll make him agree to a bigger reward. Remember that even if you listen to what Burke has to say, you don't necessarily have to become his puppet. You can always change your mind. There are two different (VERY different) ways to complete this quest.
Positive outcome - disarming the bomb and getting rid of Burke:
Before you decide to disarm the bomb, go back to Lucas Simms and tell him that Mr Burke has offered you to trigger the bomb. Now you can follow the sheriff to Megaton - Moriarty's Saloon. Once you've arrived, you will witness a confrontation between Simms and Burke. You should save your progress here, because Burke will try to kill Lucas. You can prevent this from happening by shooting at him when he tries to reach for his gun. You'll notice it when Lucas has turned his back on him and Burke is about to stand up (screen). I guess you may be surprised to find out that you'll raise your karma for killing Burke. You can also inspect his body, but the only valuable object he was carrying is a silenced 10mm pistol.
You may finally exit the saloon and go to the crater. Like I've already said, the crater is located in the center of Megaton. Make sure that you've raised your explosives skill to at least 25 points. If you didn't and you don't want to wait for the next level-up or you don't want to raise explosives, you can acquire and use mentats. In order to do that you must find Leo Stahl. You can find out more about him from the second chapter of my guide (description of Megaton). Approach the bomb, press the action key and choose the second option from the list (screen) to disarm the bomb. You'll be successful and you can go back to Lucas Simms (or - if he's dead - to his son Harden) to tell the good news. You will be rewarded with bottle caps (500 if you won the speech challenge) and you'll also receive a key to your house in Megaton. Cool reward, huh?
Negative outcome - destroying Megaton:
Once you've talked to Burke and agreed to help him out leave the saloon and approach the bomb. I have to remind you that your character must have at least 25 points spent on explosives, because otherwise he wouldn't be able to interact with the bomb. Don't forget that you find a workaround to this problems by buying drugs from Leo. Approach the bomb, press the action key and choose the last option from the list in order to arm the bomb. You may now exit Megaton. Your next objective is to get to Tenpenny Tower, because Burke is waiting for you there. Tenpenny Tower can be found in the lower left corner of the map. Once you've arrived listen to a short dialog between one of the guards and a ghoul who goes by the name of Roy Phillips. Approach the intercom (screen) and tell the guard that you're here to talk to Burke. Enter the building complex. Listen to what chief Gustavo has to say and proceed to the tower.
Once you're inside Tenpenny Tower proceed forward until you get to an elevator. Use the elevator to reach Penthouse suites. Once you're there, find the door that's protected by one of the guards. Talk to the guard and inform him that you're here to see Mr Burke. Wait for the guard to open the door for you, enter the apartment and locate a passageway to the top of Tenpenny Tower. Once you've arrived at your final destination, you can chat with Allistair Tenpenny or you can proceed directly to Burke.
Turn right once you've talked to Burke and you should notice a remote detonator on the table (screen). Activate the detonator and you'll be rewarded with an excellent nuclear explosion. Talk to Burke and he'll reward you with bottle caps (probably 1000 of them). You will also receive a key to your new apartment in Tenpenny Tower. You should notice that your karma has been dropped significantly. In addition to that, on your personal map Megaton has been replaced with Ruins of Megaton. If you decide to travel there you'll encounter Moira. Apparently she doesn't know that she's become a ghoul. :-)