Well who is to know the size of the warhead. Given that we must take some liberties with the show and suspend disbelief for 42mins then one could easily summerize a few things that could make sense.

Armor Plating.

We have seen Galactica take many hits from many different weapon types and hold up pretty well. The armor the ship must have would be have to be extrodinary to take the stresses of the jump to FTL so it's not too much of s stretch to think that it could withstand a nuke. We do see craters all over the ship from hits, so there is damage.

Space itself.

The how and why's of the behaviour of a nuke blast in a vacum aside (i'm sure someone will post something thoughtful on it) it's reasonable to assume that the blast is nothing like what we expect on Earth.

The Warhead.

Perhaps the term "nukes" hold a slightly different meaning in BSG. It is of course meant to be very similar if not the same as what we know, but it's up to us the audience to decide for ourselves the exact meanings of such things.Maybe the warheads are just not that powerful.

The size of the ship.

It may have suffered damage but the size of the ship relative to the damage taken may make it look like there was little effect. Galactica is meant to be around 1400m long and over 500m wide so when looked at in scale a small warhead may not do so much visable damage from a near miss.

For whatever reason some structures hold up well to nuke blasts. After WWII the German Heavy Cruiser Prinz Eugen was awarded to the US Navy and was subjected to two nuke tests. It survived both. It did spring some leaks after the second test but was deemed to eradiated to allow workers to safely complete repairs.

If a WWII warship can survive two blasts (though not direct) and not be a hulk of melting steel, then i'm happy to just believe that Galactica has stong enough armor to withstand direct nuke hits.

Sure, though the amount of gas needed would be rather large i guess. Far more than one would hope to contain within the confines of a missle.

Though, one could also assume that the compression techiniques used would be far greater than what we know of today. That coupled with superior pressure vessels it seems like it could be a solution.

Which, given the radition found in space that Galactica would be subjected to in everyday use, that's a reasonable assumption.

hmmm. i thought nukes always just emitted EM Radiation in space(no air to explode). maybe the nuke was designed to punch a hole in the hull before detonating.

At the end of the miniseries, when they led the fleet out of the nebula and the Cylons were waiting.

Two things.  First, insofar as size is concerned we know that the Cylons had nukes at least up to 50 Megatons (the size of some of the largest nukes produced by us and far larger than the normal American nuclear warhead used for countervalue targets, which averages between 200 and 400 kilotons).  Second, it is quite possible that the Galactica's armor plating could consist of several types of armor, to include a high tech "skirt" style, reactive armor, or a combination of these to to produce armor in depth.  Remember that armor doesn't necessarily mean (and in our military today often isn't) simply a hard plate.

I would say that an EMP wouldn't be a concern, being a warship built to withstand nukes it's easy to assume some form of shielding for all components.

In fact would a EMP behave the same in a vacum as it does in the atmosphere? I'll admit i'm not bothered to look up the physics of it but i suspect the properties of the pulse would be vaslty different in space.

Radiation, again would be shielded just for every day use of the ship in space. Though of course any fracture of the hull would allow that radiation in.

As for the size of the Nuke. Yeah 50M was what was used on the colonies, there is no way a raider carries that size of a nuke. Yes, i'm sure in BSG they have considerably more smaller warheads in size with a large yeild compared to us, but they differ not so much in weaponary so i think that, yes it could be bigger than a few hundred kilotons but i doubt that they can cram a 50megaton warhead into a raider, or basestar launched missle.

I'm sure the info is out there somewhere if someone is willing to sift for it.

Just on the 50megaton warhead. That's actually the biggest detonated warhead ever. The Soviets deployed it once in 1961 and it was so large that the shock waves were measurable after 3 passages around the world.

Over 6km of fireball and its affects were felt and heard over 1000k/m away....frightining stuff..

But i digress....

I had thought that before myself. Worse though is how does what only ever seems to be maybe 2 dozen vipers actually hold off the swarms of raiders we see. At time there must be over 100 raiders, yet the vipers always manage to hold their own.

Not complaining, just always found that curious.

I really enjoyed BSG's take on space warfare. It was a welcome change to see "big guns" as main weaponry instead of some laser/phaser/torpedo.

Why nukes were not used more often though i don't know. Remember when Lee shows up in Peaguses and fires a large salvo of nukes at near point blank range and rips apart the basestar?

Great scene, i loved it, but made me think why it wasn't used more often....

Anyway like you say, it's a movie/show where we have to fill in the blanks and just for a little while believe that the good guys are that invunerable.

Wasn't hard to do....

Ahh good explanation. I guess it's a little hard to manufacture more nuke warheads in the machine shop!!

If you didn't see the miniseries, Galactica was going to be retired, so it is safe to assume that the Galactica has been around for awhile.  With this, that would also mean that we can assume that the armor of the ship has been upgraded a lot and i think it also mentions this in the miniseries or series.  With this upgraded armor, the ship would be able to withstand near misses from nukes and also a direct hit or two that i saw in the miniseries.

yeah...i'll wear that, to a degree. In the end the Raiders are still machines. they would have programming that would surpass the knowledge of any nugget.

Plus sheer weight of numbers would still offset lack of skill, but it's a reasonable assumption still.

I've seen it all. But again thats another good explantaion. Though the counter to that though would be that it is still a 40 yr old ship. No matter what upgrades you did, it could still be vunerable to a direct nuke hit, espeacially sinve the colonial fleet would have been upgrading with little knowledge of what the Cylons were now capable of.

c'mon...radiation bomb only does what? few hundred hull points?

That's assuming the builders' knowledge surpasses the knowledge of a trained viper pilot. Which it wouldn't The programming would still be limited to basic physics and "this is how you control space flight". What do the skinjobs know about dogfighting?

Imagine a complete rookie chess player trying to program an AI to play chess. The rookie knows how the board works, but beyond that..