Development of an MCNP6 - ANSYS FLUENT multiphysics coupling capability

Gurecky, William Ladd
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This thesis presents a novel core multiphysics coupling method and its application to geometries and thermal hydraulic operating conditions typical of U.S. PWRs. Monte Carlo based radiation transport from the MCNP6 package and finite volume thermal hydraulic (TH) packages provided by ANSYS-FLUENT are combined to produce results with intra-pin resolved spatial resolution equivalent to state-of-the-art reactor physics and multi-physics suites. The Virtual Environment for Reactor Applications (VERA) whose development is spearheaded at Oak Ridge National Laboratory is one such example package. Benchmark and validation tasks performed as an integral part of the development of VERA demand intra-pin resolved pin power distributions as well as finely spatially resolved fuel burnups. This level of detail is not provided by most other lattice physics code packages. Intra-pin powers, for example, are reconstructed from lower fidelity model results using empirically derived shape functions. In addition, data sets from operating PWRs are sparse, resolved only at the inter-pin level, and prone to experimental error. With the proposed MCNP-FLUENT model, it is possible to provide within-pin/channel resolved power, temperature and moderator density field data. MCNP-FLUENT iteratively solves for multiple physical fields: flow velocity, temperature, energy deposition rate, and neutron flux. It does so by repeatedly passing information between dedicated solvers which independently handle the neutron transport and thermal hydraulic physics. The codes are linked by a Picard iteration scheme. Doppler and moderator density feedbacks are explicitly treated. In contrast to preceding generations of MCNP-FLUENT coupling implementations, the coupling framework described employs the latest unstructured mesh capabilities of the MCNP v6.1 code to achieve a new level of geometric and mesh tally generation flexibility. The coupling is demonstrated by a suite of test cases spanning planar 2D geometry, singe pin and a 3x3 assembly at hot full power with TH feedbacks. Good power and eigenvalue agreement (+/-4%, 340[pcm] respectively) is achieved for the hot full power single pin case. Qualitative agreement in the predicted power profiles and fuel temperature distributions is seen in the 3x3 pin geometry.