# Neutronic Activity for Development of the Promising Alternative Water-Cooled DEMO Concepts
## 2 WCLL-db Blanket Study
### CAD Geometry Models for WCLL-db Blankets
#### 2.1.2 CAD Model for the Neutronic Simulations  
The CAD geometry model of the WCLL-db blanket suitable for the neutronic calculations should include all essential elements affecting particle transport simulations and providing finally high fidelity nuclear responses that can be directly compared to those obtained within the WCLL blanket R&D project. Therefore, this CAD geometry model includes simplifications of the same degree that were adopted in the WCLL CAD blanket model, and it also encloses enough heterogeneity in the geometry resulting in such representation of the layout that adequately reflects the original WCLL-db blanket design.  
The approach adopted in this work assumes a decomposition of the original CAD blanket model in several main parts: the first wall, the cooling pipes with the stiffening plates, and the manifold, Figure 1. Initially, all geometry parts include some complexities preventing their direct use for the model's conversions into the MCNP geometry representations: the CAD models enclose spline geometry surfaces, geometry cells are not grouped according to materials assignments, and these models appeared to be not free from geometry errors, for example, overlapping of the cells. Therefore, significant efforts were spent on replacing all spline surfaces found in the models with surfaces of the first and the second order, such as plane, cylinder, sphere, cone, and torus. Special attention was paid to the precise fitting of the original surfaces with the new ones.  
As mentioned above, specific simplifications were introduced in the CAD models to make possible the elaboration of the geometry for the neutronic calculations. Excessive fine geometry details left in the CAD blanket models could result in a complex input deck that significantly increases the probability of failure during geometry conversion and then in the particle transport calculations. Shown in Figure 2 is the simplification of the FW introduced in the CAD models of the WCLL-db blankets. Similar to the WCLL model for neutronic simulations, the modified FW does not include chamfers in the corners. This simplification slightly increases the BZ volume and the TBR, respectively. This effect is addressed in Section 2.3.1. The double-bundle tubes having originally long free-body shapes were replaced with cylinders following very closely the original cooling tube shapes, Figure 2.  
Figure 1: The layout of the WCLL-db breeder blanket.  
The original CAD model provided for the current work did not include definitions of the materials or the substances filling various pipes, the manifolds, and the BZ itself. In the approach assumed in the original CAD models, the filler of these channels or big cells should be defined during geometry modeling for the neutronic simulations. To this end, these geometry cells filling all pipes, tubes, channels, etc., were newly introduced in the CAD models. The geometry cells that belong to the same material are grouped together separately in the models.  
The DEMO baseline 2017 [6] includes 16 toroidal segments (22.5\({}^{\circ}\)); each segment contains two IB blankets, two lateral and one central OB blanket. Initially provided for the current work was the CAD model for the OB lateral blanket. The CAD geometry models for the IB and OB central blankets were developed using patterns, i.e., the same geometry solutions implemented in the OB lateral blanket model.  
The U-shaped first wall of the WCLL-db blanket, Figure 3, holds water cooling channels of 7 \(\times\) 7 mm size with 13.5 mm pitch. This layout was accepted from the reference WCLL design. Because such a dense arrangement of the cooling channels was already proven excessive [3], the layout with the six channels per 135 mm height was adopted in the WCLL-db project, similar to the WCLL-db one.  
Figure 3: The layout of the FW in the WCLL-db blanket.  
Figure 2: Geometry simplifications adopted in the WCLL-db CAD model: first wall (**left**) and the cooling bundles (**right**).