# Investigation of Point-Contact Strategies for CFD Simulations of Pebble-Bed Reactor [PERSON]
## 4 Discussion  
This study numerically investigated the effects of various contact strategies on the flow and heat transfer behavior within a structured bed of 100 explicitly modeled pebbles. The pebbles were modeled as perfect spheres, and each had an inner spherical fuel region and an outer spherical shell fuel-free region. Four contact strategies and two thermal hydraulic conditions were considered. The strategies to avoid point-contact singularities included decreasing and increasing the pebble diameter, along with capping and bridging the pebble surfaces near the contact region. The two thermal hydraulic conditions were full-power operation (Case 1) and pressurized loss of forced cooling (Case 2). The most desirable contact strategies are those that prioritize minimizing the porosity error as much as possible while providing a thermal connection to enable specifying an effective point-contact thermal resistance. Therefore, the recommended contact strategies and sizing selection are expected to hold for unstructured beds. Also, a study investigating externally heated unstructured beds reached similar conclusions as this effort with internally heated pebbles regarding the opportunity cost of each strategy and sizing recommendations [1]. Strategy 3a, bridging with a cylinder equal to 10% of the pebble diameter, was selected as the baseline strategy because it addressed the contact singularity while minimizing the geometric changes that affect the bed porosity. For Case 1 (full power), the hydraulic behavior was sensitive to the various contact strategies, with a 30% difference in the predicted pressure drop (3% GCI) and a 4% difference for the volume-averaged velocity magnitude (0.1% GCI). The thermal behaviors within the pebbles and fluid were not sensitive to these changes as a difference of less than 0.5% (0.6% GCI) was observed across the strategies. The strategy most similar to the baseline (Strategy 3a, bridge 10%) was Strategy 1a, with a pebble diametral reduction of 0.1%. However, the small fluid volumes created from the diametral reduction required an additional +12% of the volumetric elements to adequately resolve the RANS flow field. A benefit of Strategy 3 is the simplification of specifying the interfacial thermal contact resistance between pebbles. This is because the bridges can be kept as separate regions during the model-building process. However, this bridge building is not trivial for large and unstructured beds. The contact patch definitions required for conformal interfaces become increasingly difficult for large-bed model building with bridging. These can be avoided with diametral reduction (Strategy 1) and chamfering (Strategy 4). For Case 2 (PLOFC), large differences between the contact strategies were observed in the predicted pebble temperature when only considering thermal conduction. The difference in the maximum pebble temperatures for various strategies was 23% in Case 2a. When including radiation in Case 2b, the difference decreased to 2%. When adding 7% flow in Case 2c, the thermal differences decreased further to 0.3% for the various strategies. The hydraulic behavior's sensitivity to the contact strategies in Case 2c was similar to the sensitivity observed in Case 1, where Strategy 1a was in close agreement with the baseline. If the objective is to accurately predict the pressure drop and velocity distribution, Strategies 1a, 3, and 4a are recommended. Any larger diametral changes or large bridge sizes have too large of an impact on the bed porosity (>2% porosity error for global strategies). To accurately predict the temperature distribution within the fuel pebbles, any of the studied point-contact strategies are suitable when neglecting the thermal contact resistance. Strategy 3 simplifies the implementation of the interfacial thermal contact resistance between pebbles. Nevertheless, the geometry modifications of Strategy 1a can be achieved significantly faster for large and unstructured pebble beds.