The United Kingdom’s supply of nuclear missiles are stored at their purpose-built home in Faslane.
The majority of Scots support a no-nukes Scotland.
All of this adds up to the surety that when Scotland becomes independent, and Faslane ceases to be a UK military base, the nuclear missiles must go.
But the removal of Trident is always going to be the biggest problem the Westminster government/the UK’s Ministry of Defence has with Scottish independence, because not only is there nowhere else for it to go and it would take a couple of decades to build an alternative site, there isn’t a realistic alternate site in the rest of the UK for deep-water nuclear submarines.
Devonport is physically possible but is a political impossibility, certainly for any Conservative government (and in a twenty-year construction plan there will likely be at least one Conservative government): while Scots feel uncomfortable about how near Faslane is to Glasgow, Devonport is literally in the middle of Plymouth. Pembrokeshire is a technically feasible location, but building an entirely new military depot for nuclear weapons on the coast of Wales creates a whole new political problem for rUK after iScotland has voted Yes and departed.