Let's be honest, we all think the Nuclear Weapon is cool. From the fact that it is radioactive, to the sheer thought of being able to pretty much annihilate an entire city, nuclear weapons rank on the Relaive Scale of Weapon Coolness somewhere between "Space Lasers" and "Stealth Aircraft." They also, when used right, can be the centerpiece of a great RP.
But, we all have been in that one RP when someone yells out "I n00k all of yor citiez lol I winz the rp!!!1!11!1111" without even giving a second thought to what that implies. This whole rant is trying to stop that.
Part One: Types of Nuclear Weapons
Before we go anywhere else, it is important to know the types of nuclear weapons. There are two main types: Fission and Fusion; and a few other... less conventional ones that won't be described here.
FISSION BOMBS: Use fissile materials (NOT "fissionable," though fissile materials is a subset of fissionable materials) to cause a chain reaction. This chain reaction is called nuclear fission, where a nucleus is split into two smaller nuclei, a few (most of the time 3) free neutrons, and a ton of energy plus some gamma rays. If radioactive, the smaller nuclei are known as nuclear fallout. The most common isotopes for fission are Uranium 238 and Plutonium 239, while there are a myriad of nuclear isotopes found in nuclear fallout. This is the same type of bomb used in Hiroshima and Nagasaki. They run from a handful of Kilotons to a megaton of power.
FUSION BOMBS: Heats up two different isotopes (Protium and Deuterium) Hydrogen to incredible temperatures and fuses them into Helium. This is the same process the sun uses to generate the heat and light that warms the earth. This is incredibly hard to do, since you basically [censored by the US government]. It generates a gigantic amount of energy, though. Remember Tsar Bomba? That was a Hydrogen Bomb, but you're first try is definitely not gonna get that big. They run from five hundred kilotons to... who knows?
OTHER TYPES: Besides the Fusion and Fission bombs, there are a few other types of nuclear weapons that don't get as much attention. Neutron or Hafnium bombs release copious amounts of free neutrons that can cause major damage to surrounding structures. The neutrons come from isotopes (usually Nuclear Isomers) that are relatively unstable and neutron rich, most commonly from Hafnium and Tantallium isotopes, especially 180Ta and178m2Hf. The process at work here is called Induced Gamma Emmision. These are usually a third as powerful as a nuclear weapon.
There are also other types, but they will not be discussed here.
DIRTY BOMBS: These are by far the easiest of the nuclear weapons to make. They are essentially conventional explosives with highly radioactive bits inside that disperse out when the bomb is detonated. They leave the area around it very radioactive (think nuclear blast with just fallout). These can leave an area radioactive for years, though you can't really run your troops through a radioactive area without serious problems emerging.
STRATEGIC NUKES: These are the really big ones that you use to wipe out cities. They are harder to build than tactical nukes (discussed below), but they cause much more damage. And fallout, which leads to the fact that it would be hard to occupy a territory that you just bursted a nuke over/on.
TACTICAL NUKES: These are the smaller ones that are used in battle. They don't do as much damage as their strategic cousins, but they have their advantages, one being that they are less likely to prompt retaliation than a strategic nuke. They also have the advantage that they do not disperse as much fallout, which means that occupation of that area would be practical.
Part Two: Making Your Bomb
OK, so you understand the kinds of nuclear weapons, and have chosen one for your crazy nation to make. But alas, grasshopper, you still have far to go before you reach nuclear enlightenment. You first have to make your bomb.
I'm not gonna go to far in depth on how to actually make a nuclear weapon, but there is one thing you need to know: to be able to build a nuclear weapon, you first have to get Enriched [isotope] (for the sakes of this demonstration, let's assume you're making a Uranium weapon). To enrich Uranium (and most other isotopes), you have to spin a sample of it around really fast. This is used to seperate U235, the isotope you need, from U238, the waste isotope. The waste isotope is called Depleted Uranium (to be discussed somewhere else). All in all, it is INCREDIBLY HARD to enrich the uranium to the levels you need for your nuke.
Next, you need to build the mechanism for your bomb. The allies, during WWII, spent almost four years trying to build the bomb. Hence, you should use a reasonable amount of time to make the weapon. It is advised that in an RP, you start as a nation that just began pursuing a nuclear weapon, and then go from there. Besides, that makes for a much more interesting RP than "I n00k u lol11!11!!!"
Part Three: Deploying your bomb
(Pointed out by Grenartia and Virana) Nuclear weapons come in all shapes in sizes, from tiny nukes that can be launched from tanks or even rocket launchers, to hulking bombs that can take out cities. There are many ways to actually get a nuke to another nation, from a plane drop, to an ICBM, to a truck. But remember, each way has advantages and disadvantages.
Then there is the difference between Air and Ground Bursts. Air bursts detonate nuclear weapons while they are still in the air. This distributes the energy and fallout around a wider area, though it doesn't have as much raw power. Ground bursts are more powerful, but they don't cover as wide of an area.
Then there's the fact that if you nuke "Lamastan," and Lamastan is right next to your borders, you're gonna have some bad problems with fallout. Fallout is intensely radioactive, and if you don't think that is a problem, you obviously are gonna have a problem with radiation sickness. Also, if you have just nuked a city and then run your troops through the area, you're troops are gonna drop like flies without protection. Plus, nuking someone really isn't a good way to make friends if you're new.
Then there's a fun phenomenon called "Nuclear Winter." This is where nuclear fallout blots out sunlight, and then the global temperatures plummet. Then there's the fact that if you set the atmosphere on fire, it will affect you too. Unless if you want to have an apocalypse on your hands, you should make your nuke strikes FEW and far between. I'd say 2 max before you start wishing you had more fossil fuels to heat up the atmosphere with.
Step Four: How not to ruin it
So, you've RP'd your nation building a nuclear weapon, and you've positioned it to attack Lamastan, there is still time to ruin it. One is if you use 9000 nukes at one time. First off, the resulting radiation doesn't leave you much to colonise, plus there is no way in hell that you're gonna be able to build more than 3 nukes at a time. Yes, I know that Russia has 16000 nukes, but you aren't Russia. I'd say, you should have a 100 nuke stockpile tops.
Then there's the fact that if you start pursuing a nuclear programme, Lamastan is too. You can either attack him conventionally, or send in spies to thwart him. (But with the spies, you aren't going to one shot destroy his nuclear programme.). Also, if you do that, Lamastan's gonna do it also, and it'll either turn into a gnarled mess or a fascinating RP, depending on how it's handled.
Also, be sure that both sides know that you have nuclear intentions. Among other things, bad communications in nuclear strikes can cause Mutually Assured Destruction when the other guy gets mad and nukes you, and the Ignore Cannon being used on you.
To conclude: Don't use nukes, but if you do, make it as limited and detailed in the making as possible.