  The nuclear community has coupled several three-dimensional Computational
Fluid Dynamics (CFD) solvers with one-dimensional system thermal-hydraulic
(STH) codes. This work proposes to replace the CFD solver by a reduced order
model (ROM) to reduce the computational cost. The system code RELAP5-MOD3.3 and
a ROM of the finite volume CFD solver OpenFOAM are coupled by a partitioned
domain decomposition coupling algorithm using an implicit coupling scheme. The
velocity transported over a coupling boundary interface is imposed in the ROM
using a penalty method. The coupled models are evaluated on open and closed
pipe flow configurations. The results of the coupled simulations with the ROM
are close to those with the CFD solver. Also for new parameter sets, the
coupled RELAP5/ROM models are capable of predicting the coupled RELAP5/CFD
results with good accuracy. Finally, coupling with the ROM is 3-5 times faster
than coupling with the CFD solver.
