Evaluation of communication-avoiding GMRES in multi-phase thermal-hydraulic problem

Mayumi, Akie ; Idomura, Yasuhiro   ; Yamada, Susumu  ; Ina, Takuya; Yamashita, Susumu   

In order to simulate molten core relocation behavior in the accident of the Fukushima Daiichi Nuclear Power Plant, JAEA has been developed the JUPITER code, which analyzes a multi-phase thermal-hydraulic problem. Although the current simulation can analyze a fraction of a single fuel assembly, Exa-scale machines are needed to simulate whole part of the molten core. The JUPITER code is based on a incompressible fluid model, and thus, the main part of the computational cost comes from an iterative solver for the Poisson equation. Since the communication cost in the Poisson solver is a bottleneck in improving scalability, avoiding the communication is of critical importance. In this work, we apply the Communication-Avoiding GMRES algorithm to the Poisson solver and evaluate its utility by investigating its convergence property in real problems.