This book is a state-of-the-art presentation in the area of nuclear simulation. In this area the mathematical modelling is the most important part. Then the question is: how is this model solved to correlate in a satisfactory way with reality (i.e. real life data)? An answer to this question is given for several reactors and their related systems for the first time. In these cases, software tools are very important, especially when they have to solve a safety problem related to a reactor. One must show the reliability of these tools in order to certify the reactor to the nuclear regulatory authority. The plant analyser is the tool needed to learn about the behaviour of a "total" reactor in order to understand the mechanism of the process. The second main subject is the reactor core. After Cernobyl this is self explanatory. Here we are able to point out that the reader can find several answers to new problems in the nuclear industry. This includes a new approach to nuclear mathematical modelling.