**5.1 Sub-channels analysis**

The CORTH program utilizes the uniform flow model of four equations to process two-phase flow in the reactor core. The energy conservative equation could be abbreviated as the manner of structured grids, which is easy to use in finite difference and form the linear system. For the description, CORTH could be divided as components in **Table 4**.

The features of the parallel algorithm are listed summarily to make the program refinement practical.


The bottleneck of the parallel algorithm design is related to the components of the coefficient update, linear system solve, and mesh matching. Parallel scheme was realized in NAC4R with the features mentioned above. The ACP1000 model (177 fuel assemblies, there are 264 fuels and 25 guide tubes in each assembly, axial segments are 54, and there are 50868 sub-channels in radial direction) is used for performance validation. Cluster processors are chosen as 4, 6, 18, 27, 36, 54, and 108, which is the divisor of 50868 as the Aztec solver needs that the number of processes matches the tasks. The expected performance of strong measurability is improved by more than five times.

Performance conclusion: The trend of processor change shows that using about 30 processors, it has met the expectation in **Figure 2**, with the speedup ration about eight times. Not only the conservation system can be constructed in parallel, but the Aztec solver can also solve the matrix with the size of 50868 efficiently.
