>is there any real chance ,possibility,of there-being a nuclear war within 20-40 years?
Yes, but you won't think so unless you are following everything going on. US hegemony made sure nuclear proliferation was out-of-reach, but several countries have already started developing nuclear for "energy needs" in such a way that it can quickly be turned around for bomb production, and nuclear bombs are far easier to produce than most modern guided or large-blast unguided weaponry. Russia's foreign policy is "everyone should have nukes and it is up to their neighbors to stop them if they don't like that" while China's foreign policy is "fuck everybody I want it" which is rapidly radicalizing the rest of Asia towards nuclear proliferation. As America becomes utterly incapable of preventing their enemies from getting nukes, their allies will start developing them, and unless Russia and China get together and agree on a "world council" to replace the UN as they become the dominant economic order (unlikely), someone, somewhere, is gonna get a nuke and use it.
Is it going to look like mass-scale ICBMs around the world? Probably not. It will, however, look like a smaller country being invaded by a larger country and having no choice but to use a nuclear deterrent to wipe out a large amount of the enemy all at once to try and collapse the enemy's morale, and at that point it will escalate to countries firing back and forth at each other assuming the world doesn't get boots on the ground to try and stop them.