Applied online, did online tests and went to assessment centre. The first day of the centre was focused around group activities, generally well relaxed and not too taxing. I think as long as you make sure to contribute sensibly then this part will be passed. There was also an aptitude test. The second day, in which half the people made it through too, was just a technical interview. This started with general questions about nuclear reactors and them became a bit more focused, but didn't require much outside knowledge.