The setting is Earth in 2021 OTL. An invisible space station of an arbitrarily advanced alien civilization is orbiting around Earth keeping watch on humanity. The aliens have determined that nuclear technology is something that humanity should not have at this stage of development. It's like children playing with fire, has the potential to create mutually assured destruction. Although the aliens have otherwise has a noninterference policy regarding the Earth, they see humanity as something too precious to waste. So in order to keep them from destroying themselves, the aliens decide to magically remove all nuclear everything from human civilization.
So instantaneously all radioactive elements and compounds including uranium, radium, plutonium, thorium, etc ... are replaced with pure silver. An atom of the radioactive element is literally replaced with an atom or ordinary silver, every single atom, affecting the mass of the object. This affects all such radioactive elements in nuclear weapons, power plants, toxic waste dumps, mines, under ground deposits, and just scattered throughout the environment.
It is worth noting that the aliens performed this replacement affecting under ground deposits only within the outer layer of the Earth, that humans can feasibly get to with modern or near future equipment. Any radioactive elements deep inside the Earth are untouched, so the Earth's magnetic fields would continue to work normally.
There are no visual effects or anything associated with the replacement, it is all silent. This occurs without any warning, and humans are initially unaware of what happened.
I do apologize as this is a multi-part question, but never the less I will ask it. I would like to know what happens after this instantaneous replacement takes place? Obviously all nuclear power plants will cease working because all radioactive elements will have been replaced with silver, but will they gradually cool down, or have a sudden failure? Would humans be able to detect what happened?
What will happen within a few hours of the great replacement? A few days? Weeks? Months? Years?
How soon might humans, such as employees of nuclear power plants notice that something is amiss? Would their equipment even tell them that? What about smaller reactors such as in nuclear powered ships and submarines? Would CERN be able to detect anything? And how soon would they come to the conclusion that all nuclear reactors around the world have been disabled?
Also, how soon would the militaries realize that their nuclear weapons no longer work as intended? Essentially they are now reduced to using conventional non-nuclear weapons. Specifically the main question is, how would this realization affect global geopolitics? I mean to say that there are some countries in the world who only wield the thread of nuclear weapons to scare their foes. Without nuclear weapons such countries wouldn't be able to stake their claims with their regular army and economy alone. It would be good if at least one of the answer would mention which are those countries in particular and what would happen to them after the great replacement?
As I understand in addition to carrying the threat of mutual destruction, nuclear weapons also keep the rival militaries scared of each other, and not bold enough to start a hot war. In short it also is a bad way of guaranteeing peace. Once the militaries find out that no one has nuclear weapons anymore, would they start a war? Would some countries try to conquer the territories of other countries, because their military is weak without having the nuclear weapons anymore?
So I am asking about both the short term, medium term, and long term consequences of the sudden great replacement of radioactive elements into pure silver at the same time everywhere on Earth.