# Investigation of Point-Contact Strategies for CFD Simulations of Pebble-Bed Reactor [PERSON]
## 1 Introduction
Therefore, the optimal contact strategy may not be the same for all the reactor conditions where the dominant mode of heat transfer changes from forced convection (steady-state full-power operation) to either conduction- or radiation-dominant. In this study, point-contact strategies were investigated for full-power operation at a typical pebble power density and for a PLOFC with a reactor trip event. Each contact strategy is illustrated in Figure 1.  
* Strategy 1 (a-b-c): Decreasing the pebble diameter by 0.1%, 0.5%, and 1%. This is a simple model-building approach of slightly decreasing the pebble diameter. Decreasing the pebble by 0.1% was the most computationally expensive approach due to the very small gaps between the pebbles. * Strategy 2 (a-b-c): Increasing the pebble diameter by 0.1%, 0.5%, and 1%. * Strategy 3 (a-b-c): Bridging--that is, chamfering contact pebbles with a given diameter (10%, 15%, and 20% in this study) and filling the volume with pebble material. This approach was considered the most similar to the true bed configuration when comparing its porosity to the true value, and thus it was selected as the baseline for comparison. * Strategy 4 (a-b-c): Capping--that is, chamfering contact pebbles with a given diameter (10%, 15%, and 20% in this study). Two reactor operating conditions were investigated for each contact strategy: (Case 1) full power and (Case 2) PLOFC. The Case 1 power density of \(1\times 10^{7}\) W m\({}^{-3}\) is prototypical of full-power operation for TRISO fuel forms using dispHALEUhigh-assay low-enriched uranium and helium gas as the heat transport fluid [9]. The associated full-power values were specified as boundary conditions, including a 3 m s\({}^{-1}\) fluid inlet velocity, a 250 \({}^{\circ}\)C inlet temperature, and an outlet pressure of 6 MPa. Case 2 roughly corresponds to the PLOFC with a reactor trip event, and this case was motivated by an objective to better understand the contact strategy under the separate effects of each heat transfer mode of conduction, convection, and radiation. For this case, the pebbles were assumed to have a temporally constant power density equal to 7% of the full-power density, which corresponds to approximately 20 s after the reactor trip [10]. Instead of temporally resolving the PLOFC transient, the event was divided into three unique and steady subcases that correspond to the distinct scenarios of the event. Case 2a represented a phase of the PLOFC transient when the heat transport fluid reaches a near-zero velocity during the expected flow reversal phenomena after circulator pump coastdown and control rod scram [11]. Case 2a only considered conduction as a means of energy transport to study how different the temperature and heat flux fields would be between contact strategies that result in pebble overlaps (Strategies 2 and 3) or pebble gaps (Strategies 1 and 4). In Case 2a, the pebble surface temperatures reached sufficiently high temperatures that would make thermal radiation a non-negligible mode of energy transport. Therefore, Case 2b included radiative heat transfer. Case 2c represented a late phase of the PLOFC transient after the establishment of the buoyancy-driven natural circulation of the heat transfer fluid. This case captured the contact strategy behavior when conduction, convection, and radiation occur simultaneously. In this work, the pore-scale hydraulic diameter (\(d_{h}\)) and pore Reynolds number (\(Re_{p}\)) are defined as  
\[d_{h}=d_{p}\frac{e_{i}}{1-e_{i}}, \tag{1}\]  
Figure 1: (**a**) Computational domain consisting of a structured bed of 100 explicitly modeled pebbles with (**b**) pebble dimensions. Strategies for point contacts with adjacent pebbles consist of (**c**) capping, (**d**) bridging, (**e**) increasing the diameter, and (**f**) decreasing the diameter. (**g**) A mesh of the bridging strategy.