Continued Verification of MOOSE Structural Mechanics Tools for Modeling Core Bowing Phenomena in Fast Reactors

Abstract

Under the U.S. Department of Energy Office of Nuclear Energy's Advanced Modeling and Simulation (NEAMS) Program, an integrated multiphysics approach is being developed to model the core bowing phenomena important to liquid metal-cooled fast reactors. Core bowing is an important passive safety mechanism whereby increased power (which leads to temperature and flux gradients) influences the core to bow into less reactive configurations when the restraint system is properly designed. The phenomenon includes a complex interplay of radiation transport, duct temperature calculations involving fluid flow and heat transfer, and thermo-mechanical responses to the induced temperature and flux gradients. Structural material properties are also important to determining inelastic response to longer term flux gradients which cause irradiation creep and swelling. While core bowing provides a strong negative reactivity feedback when the restraint system is designed properly, it also results in additional forces between assemblies which increase the loads required to extricate them during refueling or control rod movement. Therefore, the restraint system must be designed with these tradeoffs in mind.The first stage of the work, which commenced in FY21 and continues through FY22, assesses thermo-mechanical modeling tools for producing core bowing predictions consistent with conventional tools. The Multiphysics Object Oriented Simulation Environment (MOOSE) Tensor Mechanics and Contact Modules are employed. This status report describes work on additional thermo-mechanical benchmark verification problems with increased complexity from the examples demonstrated in FY21.Several benchmark verification examples were selected from the IAEA verification and validation report. These examples involve clusters of ducts representative of a sector of a hexagonal reactor core which bow into each other and cause contact and load pad elevations, as well as single ducts subjected to irradiation fields undergoing swelling and subsequent bowing. The MOOSE-based results were compared to both IAEA benchmark participants' results, analytic equations as available, and NUBOW-3D, a beam model code developed by Argonne National Laboratory. In every case, the MOOSE results agreed with other simulations results, providing additional verification basis of the tools for this particular physics application.

FY23 Status Report on MOOSE-Based Approaches to Modeling Core Bowing in Fast Reactors

“…The benchmark problems selected for FY23 work continued to exercise more complex examples from FY22 work [9], by modeling full sector versions of the symmetry examples, including a limited free-bow example with restraint ring around the core periphery, and irradiation creep and swelling in a multiple assembly simulation. The MOOSE simulation results are compared with the IAEA participants' results, which are based on results from legacy software with beam element formulation to calculation displacement and contact.…”

“…IAEA Verification Problem 3B examined a 120° sector of a core with the center duct thermally bowing into the sector and a free boundary around the sector, Figure 2-1. This problem was modeled previously with a symmetry boundary condition to reduce computation effort [9]. The work was continued to model the full sector without symmetry boundary to assess whether the deflected results and contact forces were affected by cutting the contact faces in half along the symmetry line.…”

“…This shows that even small displacements can propagate quickly within the reactor causing multiple assemblies to bow from contact alone. The deflections are compared with the calculation results from the symmetric model results [9]. The values of the displacements at the ACLP and TLP for the symmetric and sector models along with the percentage difference are shown in Table 2-2.…”

“…The MOOSE deflection results fit between most of the IAEA results for all three ducts at the load pad locations, however there is a bit of overestimation in the mid-core deflection results. This is most likely attributed to incorrect implementation of the creep part of the damage dose field effects, since the irradiation swelling effects were previously investigated with good results [9]. Further investigation will take place to determine the best method for implementing the creep behavior to determine the incremental strain values.…”

“…While performing the benchmarks, some necessary improvements were made to the Contact Module by NEAMS code developers: 1) Allowing multiple sidesets to be defined in multiple contact sets and 2) automatic contact detection. Both of these improvements were assessed in this work, using the VP3A example, which was previously assessed for both symmetric and full sector models in [9]. As a reminder, VP3A models a single duct at the center of the core bowing into the top 60° sector of the core, with free boundaries along the sector edge, shown in Figure 2…”