Stealth in space really doesn't work well because of a few details that are quite foreign. The background temperature of space is about 3 kelvin or -455 degrees ferenheit. In order to not stick out like a sore thumb you would need to cool all parts of your ship to this temperature that are oriented towards a sensor. Another detail often misunderstood is that there is very little convection. On earth we are surrounded by an atmosphere which allows for heat to be transferred from heat source to the gasses around it. This does not take place well in a vacuum because of low density. This is the same principle that make liquid cooling of a computer core more effective than air cooling. Liquid nitrogen and other liquified gasses function because the liquid uses heat energy to convert from liquid to gas, thus "taking" energy causing cooling. A closed system by which liquid helium is used to cool the skin of a ship to 3 kelvin and is then recompressed to liquid would require its own cooling until a scale of magnitude larger than the the system used to cool the ship. This is why perpetual motion doens't work (You would need a 3rd system to cool 2nd and so on). You could eject the heated medium behind your stealth ship you would suffer a very large weight penalty. Also, heated materials vibrate which would cause dispersal of the heated medium meaning it would soon escape the shadow of your ship and be detectable. On a side note nuclear weapons function very differently without an atmosphere. There is no pressure and therefor no over pressure or fireball. A nuclear explosion in space is a massive release of radiation and would have to be quite close to an object to do much more than irradiate the target.