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Transactions, SMiRT-22 * :: "
I San Francisco, California, USA - August 18-23, 2013 IAS R
I Division III A iT
A COMPARISON OF THERMOMECHANICS COUPLING STRATEGIES
IN FUEL PIN AND PRESSURE VESSEL SIMULATIONS
S. R. Novascone1, B. W. Spencer, R. L. Williamson, D. Andrsi, J. D. Hales, and D. M. Perez
'Idaho National Laboratory, Idaho Falls, ID, (email@example.com)
Nuclear fuel operates in an extreme environment that induces complex nonlinear multiphysics
phenomena. This multiphysics behavior is often tightly coupled, a well-known example being the
thermomechanical interaction as the gap between fuel and clad closes in LWR fuel rods. Fuel
performance simulation codes must obtain solutions for the thermal and solid mechanics physics and the
coupling between them. We define solution strategies for solving systems of coupled equations as
loosely-coupled, where the individual physics are solved separately, keeping the solutions for the other
physics fixed at each iteration, and tightly coupled, where the nonlinear solver simultaneously drives
down the residual for each physics, taking into account the coupling between the physics in each
nonlinear iteration. In this paper, we compare the computational performance and results of loosely and
tightly coupled solution algorithms for a single fuel pin including thermal and mechanical contact, which,
due to the relationship between gap size and fuel centerline temperature, is the source for strong
interdependence between thermal and mechanics solutions. The results of these comparisons indicate that
loosely coupled simulations require significantly more nonlinear iterations and may lead to convergence
issues as the gap between fuel and clad closes. We also show how loosely coupled solution strategies
perform better in problems that do not have a strong two-way connection between thermal and
mechanical response using a comparison of tightly and loosely-coupled thermomechanics simulations of a
reactor pressure vessel.
To understand the behavior of the nuclear fuel/cladding system in light water reactors (LWRs), it
is important to be able to accurately model the key physics involved, including interactions between the
individual physics. For modeling the performance of fuel at the engineering scale, the main physics of
interest are heat conduction and solid mechanics. These physics can have a strong influence on each
other, particularly during closure of the gap between the fuel and the cladding.
Numerical methods for the implicit solution of the partial differential equations that describe
physical phenomena typically lead to the solution of a system of discretized equations. If multiple
coupled physics are included in a model, the set of equations to be solved includes degrees of freedom
from all of these physics.
The strategies used to solve coupled sets of physics equations can be generally categorized as
loose coupling and tight coupling. In loose coupling, the individual physics in a coupled problem are
solved individually, keeping the solutions for the other physics fixed. After a solution is obtained for an
individual physics, it is transferred to other physics that depend on it, and solutions are obtained for those
These fixed-point iterations are repeated until convergence is obtained. If there is not a strong
two-way feedback between the physics involved, convergence can be obtained quickly with a minimal
number of loose-coupling iterations. An advantage of this approach is that it allows for independent codes
to be coupled with relatively minor modifications to those codes, and they can each use their own
solution strategies that are tailored for their solution domain. The disadvantage of loose coupling is that if
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Novascone, S. R.; Andrs, D.; Spencer, B. W. & Ha, J. D. A COMPARISON OF THERMOMECHANICS COUPLING STRATEGIE, article, August 1, 2013; [Idaho Falls, Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc863298/m1/2/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.