Abstract: With rapid development of computer and super computation, coupling for multi-physics, multi-scale and multi-process has become possible. Some lone process are integrated together. Some approximates from experience will be removed after all of the processes to be considered. This work is based on a virtual reactor. The goal is to improve precision by improvement of modeling and high fidelity computation. At present, study measures are changed from experiment and engineer dependent to theory analysis and numerical simulation. Numerical simulation will become more and more important. In this paper, CASL and NURESAFE are introduced. Then, several challenges, which include key technologies of software, are put forward in development of nuclear energy. Finally, suggestions are given for numerical reactor. It is only for reference and discussion.