I've posted this link before. The short version is that it is something to do with the decay of the tritium which is integral to the fusion reaction.
https://books.google.co.uk/books?id=rot6QG6G6bgC&pg=PA27&lpg=PA27&dq=old+age+can+kill+the+bomb&source=bl&ots=aH_k9_qLut&sig=J6b7zNKc6-qSOv5tYXekHjeKc4M&hl=en&sa=X&ved=0CBwQ6AEwAGoVChMImavQpqmiyAIVx9UUCh37BAZE#v=onepage&q=old%20age%20can%20kill%20the%20bomb&f=false
That article was published in 1984 and appears to suggest that renewal (of the warheads) is required every 20 years or so. I assume there is a rolling programme to keep them in working condition, going on in the background. The current debate is, however, about the cost of replacing the submarine fleet.
I'm not sure if Corbyn grasps the point that the use of submarines means that an opponent does nothing to protect its own cities by flinging missiles at us first, in the vain hope of destroying a launch site. The point of having them at sea is that they cannot be found.
Whatever words went into the question, I think what his brain thought he'd heard was "would he *initiate* a massive nuclear attack?" It is the leader who initiates a nuclear exchange who is responsible for destroying the planet and saying he wouldn't want to be that leader is quite understandable, IMHO.
Wittingly or not, in the eyes of the public, he has answered the (hypothetical) question "will you despatch our reflex strikes in the event of an incoming attack" with a "no". So he will allow UK to be burnt to a crisp and will not even retaliate against the perpetrator(s).
I have distorted what he was actually asked on purpose here because I don't think Ms Kuensberg's question was phrased in the right way to extract the required level of detail in his response. His answer is consistent with his desire to scrap Trident but his stance could conflict with the wishes of the British public. If elections or a referendum says we keep it then we need to know whether he would attempt to subvert it, in a moment of crisis.
An entirely hypothetical eventuality but we need to know, nevertheless. It is part of the PM's job; he has now stated he would not perform the task, therefore he cannot be allowed to win the next election.
Principles are fine things but they will prevent you gaining power, in such cases. Having no power means you cannot exert your principles, in which case, what's the point in having them?