MCNP Guide
Surface Cards in MCNP
Creating the geometric boundaries that define your model
The Role of Surfaces
Surface cards create the geometric boundaries that divide space into regions. Each surface acts like a mathematical fence, separating space into "inside" and "outside" regions. These regions become the building blocks for your cells, allowing you to create complex three-dimensional models from simple mathematical shapes.
Surface Format
surface_number type parameters $ comment
1 px 5.0 $ Plane at x = 5
2 cz 0.5 $ Cylinder on z-axis, radius 0.5
3 so 10.0 $ Sphere at origin, radius 10Each surface begins with a unique number that you'll reference in your cell definitions. The surface type (px, cz, so) determines the mathematical shape, while parameters specify its size and position. Comments help document your geometric choices.
Essential Surface Types
Planes: The Basic Building Blocks
Planes are the workhorses of MCNP geometry. They can slice space, create boundaries, and form the faces of complex shapes. The three basic plane types (px, py, pz) create surfaces perpendicular to the coordinate axes, while the general plane (p) can create surfaces at any orientation.
c Basic planes
1 px 5.0 $ Vertical plane at x = 5
2 py -2.0 $ Vertical plane at y = -2
3 pz 0.0 $ Horizontal plane at z = 0
4 p 1 1 0 5 $ Diagonal plane: x + y = 5Think of these planes as infinitely thin sheets cutting through space. The px plane at x=5 divides space into regions where x<5 (inside) and x>5 (outside). This inside/outside concept is fundamental to how you'll combine surfaces to create cells.
Cylinders: Perfect for Nuclear Applications
Cylindrical surfaces are essential in nuclear engineering because fuel pins, control rods, and coolant channels are typically cylindrical. MCNP provides several cylinder types to handle different orientations and positions.
c Cylindrical surfaces
10 cz 0.5 $ Cylinder on z-axis, radius 0.5
11 cx 2.0 $ Cylinder on x-axis, radius 2.0
12 c/y 3 1 0.8 $ Cylinder parallel to y-axis at x=3,z=1The cz surface creates an infinite cylinder centered on the z-axis, perfect for vertical fuel pins. The c/y variant creates a cylinder parallel to the y-axis but displaced to pass through the point (3,0,1). These displaced cylinders are useful for modeling off-center components.
Spheres: Modeling Sources and Detectors
Spherical surfaces excel at modeling point sources, spherical detectors, and problems with spherical symmetry. They're also useful for creating curved boundaries that approximate more complex shapes.
c Spherical surfaces
20 so 5.0 $ Sphere at origin, radius 5
21 s 2 3 1 2.5 $ Sphere at (2,3,1), radius 2.5
22 sx 10.0 3.0 $ Sphere on x-axis at x=10, radius 3The so surface centers a sphere at the origin, while the general s surface allows placement anywhere in space. The sx variant creates a sphere centered on the x-axis, which is convenient for problems with cylindrical symmetry where you need spherical boundaries at specific locations.
Building Real Geometries
Fuel Pin Assembly
Let's build a complete fuel pin with proper geometric boundaries. This example shows how surfaces work together to create a realistic nuclear component.
c Fuel pin surfaces
1 cz 0.4096 $ Fuel pellet radius
2 cz 0.4180 $ Gap inner radius
3 cz 0.4750 $ Cladding outer radius
4 cz 0.6350 $ Unit cell boundary
5 pz 0.0 $ Bottom of active fuel
6 pz 365.76 $ Top of active fuel
7 pz -10.0 $ Bottom reflector
8 pz 375.76 $ Top reflectorThis surface set creates a realistic fuel pin with fuel pellets, gas gap, cladding, and reflector regions. The concentric cylinders define radial boundaries, while the horizontal planes create axial zones. Notice how the dimensions progress logically from smallest (fuel) to largest (unit cell boundary).
Shielding Configuration
For radiation protection problems, you often need rectangular boundaries with internal structures. Here's how to set up a typical shielding calculation.
c Shielding problem surfaces
10 px -100.0 $ Left boundary
11 px 100.0 $ Right boundary
12 py -100.0 $ Front boundary
13 py 100.0 $ Back boundary
14 pz 0.0 $ Ground level
15 pz 200.0 $ Top boundary
20 so 5.0 $ Source sphere
21 BOX 20 -30 0 40 0 0 0 60 0 0 0 150 $ Concrete shieldThe six planes create a large rectangular boundary ensuring particles don't escape the problem. The spherical source provides a simple radiation emitter, while the BOX macrobody creates a concrete shield wall. This combination demonstrates how different surface types work together in practical problems.
Advanced Techniques
Using Macrobodies
Macrobodies combine multiple surfaces into predefined shapes, saving time and reducing errors. They're particularly useful for common geometric shapes that would otherwise require multiple surface definitions.
c Macrobody examples
30 RCC 0 0 0 0 0 10 2.5 $ Right circular cylinder
31 HEX 0 0 0 0 0 20 3.0 $ Hexagonal prism
32 RPP -5 5 -5 5 0 10 $ Rectangular parallelepipedThe RCC macrobody creates a finite cylinder with end caps, perfect for fuel pellets or control rods. HEX creates hexagonal prisms common in reactor lattices, while RPP forms rectangular boxes. These macrobodies automatically handle surface intersections and provide clean, unambiguous geometry definitions.
Practical Guidelines
Successful surface modeling requires systematic organization and careful attention to detail. Start by sketching your geometry on paper, identifying the key boundaries and dimensions. Group related surfaces using logical numbering schemes - for example, use 1-10 for fuel regions, 11-20 for structural components, and 21-30 for boundaries.
Always include descriptive comments that explain each surface's physical meaning. Comments like "fuel outer radius" or "containment wall" are much more helpful than just "cylinder" or "plane." This documentation becomes invaluable when debugging geometry errors or modifying your model later.
Use MCNP's plotting capabilities to verify your geometry before running calculations. The PLOT command can show cross-sections of your model, helping you catch errors in surface definitions or unintended gaps between regions. Remember that all dimensions are in centimeters and that surface intersections must be handled carefully to avoid ambiguous geometries.