Can I directly start a French Masters 2 with a UK 4 year integrated masters? I'm currently in my fourth and final year of my UK masters degree in Physics. The course is accredited as an MSci, equivalent to doing a 3 year Bsc followed by a 1 year Msc.

I'd like to apply to study nuclear engineering at Universite Paris-Saclay in France. The masters available there consists of a Masters 1 and a Masters 2, 2 successive years of study. I'm trying to find out if it's possible to start directly in the Masters 2, but it's difficult to figure out what exactly my MSci will be equivalent to. The course states it requires a Masters 1 or equivalent, but with no information on what it considers sufficient.

Under the bologna process, the UK masters is equivalent to the French (nominally), so in theory I'll already be above the required level.

Does anyone know of a student starting a French masters in the second year?
Yes, your integrated masters from the UK counts as a "Masters Européen"in France.

Source: I directly began a PhD immediately after finishing my MPhys in the UK, at Paris-Saclay, last year. If I was able to do that, I have doubts (although it's possible) that you would have trouble doing a M2 here.

As others have said, and as the other answer said, you should contact the university / admissions team as a first contact, to verify this. During the admission process you'll have to send your transcripts, and so in the end, they'll decide this for themselves. If it's anything like my admissions process, you will also have to send your MPhys dissertation, because it proves that you have done M2-equivalent work and a project with a significant time commitment and research result.

However, I'd be very surprised if you had any problem.