Preface: I'm aware many aspects of this question simply can't be definitively answered without access to UK and/or U.S. classified material. However, given the (rare) quality of most responses in this sub, where this is the case, I'd welcome answers based on well-informed speculation.
I'd also like to emphasise that this is a technical question about a hypothetical unlikely scenario, so I'm not asking anyone to assess political risk.
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I'm an active member of a political party in the UK which has consistently seen internal debate over the UK's nuclear deterrent. I have a particular interest in UK defence policy, so I'm very used to debunking false statements about the British Trident system's operational independence from the United States.
However, I recently took part in a debate where it was argued that the Trident system, and the UK's nuclear capability as a whole, would be unusable within a few years if a future U.S. administration or Congress chose to withdraw co-operation (specifically, the McMahon Act 1946 was referenced as precedent.)
I know that Trident missiles are maintained by LM at King's Bay, GA, as part of a common pool with the USN, and it's my understanding that the missiles become sovereign UK property as soon as each individual round is accepted into RN service.
I also know that AWE Aldermaston is responsible for the sovereign production and maintenance of UK nuclear warheads, though I understand that the new Astraea warhead has a lot of commonality with the W93 program on which the UK is an observer, as Holbrook did with the W76.
In the event of the U.S. ending military nuclear co-operation under the U.S.-UK MDA, here are the questions that come to mind:
- How long could the UK maintain its existing Trident missiles and warheads? Does the UK have sufficient access to Trident D5 technical information to maintain the missiles in its possession to the end of service life, or build complete replacements if necessary?
- Given that the UK's two most recent warhead designs are, apparently, significantly based on U.S. ones, and to my knowledge the last fully British-designed warhead was the WE.177 - does the UK DNO still likely have the expertise to design a warhead within modern weight and safety standards for Trident or a sovereign successor, without testing, or would this need to be rebuilt?
- Does the UK have sufficient stocks of weapons-grade fissile material (both HEI and plutonium) to maintain or moderately increase its warhead numbers without supply from the U.S. or other countries?
- I've heard it argued here and elsewhere that the U.S. and UK nuclear weapons programmes are practically inseparable. Is the flow of expertise and materiel one-way, or are there any areas of British contribution that are potential risks or pain points for U.S. deterrence if nuclear co-operation were to end?
- Are there any other questions that I should be asking in this scenario?
For these purposes, let's assume that maintaining the British nuclear deterrent remains an absolute national priority (i.e. budget considerations are a secondary consideration, within reasonable bounds).
Thanks in advance!