With nuclear energy related technologies, data to underpin operation is often difficult to obtain either because the controlling factor takes place on time scales or length scales that are challenging or the hazard is exceptionally demanding on experimental facilities. In these circumstances computer simulations can be especially useful. We can use the results of simulation in different ways. Most simply to provide property values for existing models and further analyze experimental values. Perhaps to check or test existing assumptions or to improve existing models by developing the physical models. Sometimes it is possible to develop totally new models, using the simulations to discover or identify the physics and chemistry behind the process. In this talk we will consider each of these in turn, to see where computer simulation can add value to the development of nuclear materials. We will also briefly touch upon the importance of scientific arguments behind political decisions as seen from our advisory experience with UK government.