How we need to characterize materials at atomic level in order to understand their macroscopic behaviour. Some components of nuclear reactors, such as steam generators, can weigh over 300 tonnes (4m diameter and 20m tall) and are expected to be safely in service for over 20 years. However, it is not unusual that they suffer from environmental degradation and develop cracks that can ultimately force their replacement (which is very costly to the nuclear industry). In this talk, I will show how we currently need to characterize materials at atomic level in order to understand their macroscopic behaviour. So, in another words, to explain how a crack propagates in a 300 tonne structure, we need to look at the atoms right ahead of the crack tip.