Hello paolo, well my suggestion for you will be to develope a career in software engineering and reach out for the very best in your present vocation, rather than trying to go back to nuclear engineering which i think is not very marketable nowadays,simply go for membership for software engineers and all other stuff relating to your chosen career,so you have a career on for you,wish all the very best LOYE
Sorry, I can't give you any sound advise, but I would like to say I know what you are going through - I'll soon be 40, and I' ve had a career break, and I am a foreigner - just keep on trying - if you are worth something it will get recognised earlier or later - don't give up.
If you can consider carrying on being a s/w engineer have a look at the web site of the company I am working for - www.eldon.co.uk. It is nothing to do with Nuclear Engineering, but they are not bothered about one's age - more about one's qualifications and natural abilities - and they love the mixture of software development experience and wider engineering skills.
Have you tried the Health and Safety Executive? We are currently recruiting nuclear engineers to be Government Health and Safety Inspectors in the nuclear industry and 40 is considered young for a H&S Inspector!