The main purpose of this work is to realize the internal coupling mode between the Monte Carlo neutronics transport code RMC and sub-channel thermal-hydraulic (TH) code CTF. The coupling was implemented with the coupling interfaces of RMC and CTF. And it features to use memory rather than files to transfer data from each other, which has a lot of advantages. With the internal coupling mode, power distribution calculated from RMC can be precisely provided to CTF, instead of utilizing the approximate method adopted by the external coupling mode. In addition, using memory to transfer data between those two codes can reduce the total calculation time significantly. The percentage of time reduction can be as large as 10%. Moreover, we have realized the parallel execution of CTF in the internal coupling mode and this saves a lot of time during the TH calculation.
A modified Virtual Environment for Reactor Application (VERA) Problem #6 assembly and a 4-assembly structure have been used to test the accuracy and efficiency of internal calculation mode to external calculation mode. The results show that internal coupling can give a very close solution to the external one but with 10% time reduce, and can come to an ideal convergence state with only a few iterations.