Several inches of submarine hull should do the trick. In any case, any leak would start "small" and it is easy to detect harmless quantities of radiation. And I'm sure the ONR will have audited the location of, and records from, the installed sensors.
Indeed I have. Having been there in CEGB days when you needed a hand-written pass and much rigmarole as an "outsider" to BNFL and UKAEA, it was a bit of a shock when Magnox became part of BNFL just to have my car waved onto site after showing any Magnox pass. I was relieved to be visiting Calder Hall, so it was not too difficult to navigate to my location "by sight". It is a pretty complicated site. But even last time I was there (more than 10 years ago), there was a lot of "security within security" and I don't doubt that it has got tighter. And last time I was at Devonport, again quite a few years ago, I had to walk past a marine with an SA80 to get to my destination. That was in the days when it was rare to see security people with visible firearms in the UK.
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