MCNP Guide
Basic Shapes in MCNP
Building blocks for all MCNP geometries
What you'll learn
- Apply the surface sign convention: negative is inside or behind, positive is outside or in front.
- Bound a rectangular region with
px,py, andpzplanes. - Place spheres with
soands, and cylinders withcx,cy,cz, and thec/xfamily. - Read the parameter lists for cone (
kz,k/z) and torus (tz) cards, including the cone sheet selector. - Assemble a PWR fuel pin from concentric cylinders and two axial planes.
Before you start
Surface-Based Geometry
MCNP builds all geometries from mathematical surfaces. Each surface divides space into two regions: positive (outside) and negative (inside). By combining surfaces with Boolean operations, you create the cells that define your model.
The Sign Convention
The key to MCNP geometry is understanding surface orientation. When you reference a surface in a cell definition:
Negative (-)
Inside or behind the surface
Positive (+)
Outside or in front of the surface
This convention applies to all surface types and is essential for creating correct cell definitions.
Planes: The Foundation
Planes are the most fundamental surfaces in MCNP. They create flat boundaries and are essential for building rectangular geometries like rooms, boxes, and slabs.
Axis-Aligned Planes
c Simple plane definitions
1 px 5.0 $ Plane perpendicular to x-axis at x=5
2 py -2.0 $ Plane perpendicular to y-axis at y=-2
3 pz 0.0 $ Plane at z=0 (xy-plane)
c Concrete shield wall (50 cm thick)
10 px 0 $ Inner surface of wall
11 px 50 $ Outer surface of wallThe px, py, and pz cards create planes normal to the coordinate axes. The number specifies where the plane intersects its axis. These are perfect for creating rectangular boundaries and slab geometries.
Building a Simple Room
c Laboratory room (4m × 3m × 2.5m)
1 px -200 $ Left wall
2 px 200 $ Right wall
3 py -150 $ Back wall
4 py 150 $ Front wall
5 pz 0 $ Floor
6 pz 250 $ Ceiling
c Room interior cell
1 0 1 -2 3 -4 5 -6 $ Void inside room (use mat for air transport)
2 1 -2.3 -1:2:-3:4:-5:6 $ Concrete walls (density 2.3 g/cm³)
c NOTE: as written cell 2 extends to infinity; a real model bounds the
c concrete with outer surfaces and adds a graveyard cell with imp:n=0This example creates a simple room using six planes. Cell 1 defines the air inside by intersecting the positive side of the left/back/floor surfaces with the negative side of the right/front/ceiling surfaces. Cell 2 uses a union of the remaining half-spaces to create the surrounding concrete.
Spheres: Radial Symmetry
Spheres are ideal for point sources, detectors, and any geometry with radial symmetry. They're mathematically simple but powerful for modeling many nuclear applications.
Sphere Definitions
c Basic spheres
1 so 5.0 $ Centered at origin, radius 5 cm
2 s 10 0 0 2.5 $ Center at (10,0,0), radius 2.5 cm
c Neutron source and shield
10 so 1.0 $ Pu-Be source (1 cm radius)
11 so 11.0 $ Lead shield (10 cm thick)
12 so 21.0 $ Concrete biological shieldThe 'so' card creates origin-centered spheres, while 's' allows arbitrary positioning. Concentric spheres are perfect for modeling shielded sources or multi-layer detectors.
Detector Modeling
c NaI scintillation detector
1 so 3.81 $ NaI crystal (3-inch diameter)
2 so 3.91 $ Aluminum housing (1mm thick)
3 so 4.01 $ Air gap
4 so 4.21 $ Steel outer shell
c Detector cells
1 1 -3.67 -1 $ NaI crystal
2 2 -2.70 1 -2 $ Aluminum
3 0 2 -3 $ Air gap
4 3 -7.87 3 -4 $ Steel housing
5 0 4 $ Surrounding airThis realistic detector model uses concentric spheres to represent each material layer. The cell definitions use the sphere surfaces to create the proper material regions with correct thicknesses.
Cylinders: Rods and Pipes
Cylinders are essential for nuclear applications - fuel pins, control rods, coolant channels, and piping systems all require cylindrical geometry.
Basic Cylinder Types
c Infinite cylinders along axes
1 cx 2.0 $ Along x-axis, radius 2 cm
2 cy 1.5 $ Along y-axis, radius 1.5 cm
3 cz 0.5 $ Along z-axis, radius 0.5 cm
c Positioned cylinders
10 c/x 5 3 2.0 $ Parallel to x-axis at y=5, z=3
11 c/y 2 4 1.0 $ Parallel to y-axis at x=2, z=4
12 c/z 1 2 0.8 $ Parallel to z-axis at x=1, y=2The cx, cy, cz cards create infinite cylinders along coordinate axes. The c/x, c/y, c/z cards create cylinders parallel to axes but at arbitrary positions. These are the building blocks for complex piping and rod geometries.
PWR Fuel Pin
c Fuel pin geometry (standard PWR dimensions)
1 cz 0.4096 $ UO2 fuel pellet
2 cz 0.4178 $ Inner cladding (gap)
3 cz 0.4750 $ Outer cladding surface
4 pz 0.0 $ Bottom of active fuel
5 pz 366.0 $ Top of active fuel
c Fuel pin cells
1 1 -10.4 -1 4 -5 $ UO2 fuel
2 0 1 -2 4 -5 $ Helium gap
3 2 -6.56 2 -3 4 -5 $ Zircaloy cladding
4 3 -0.714 3 4 -5 $ Water coolant around pin
5 3 -0.714 -4:5 $ Water above/below fuelThis realistic fuel pin uses cylinders for radial boundaries and planes for axial limits. The cell definitions create the fuel, gap, cladding, and coolant regions with proper material assignments and densities.
Cones and Tori
While less common, cones and tori are useful for specialized geometries like beam collimators, vessel heads, and complex piping systems.
c Cone surfaces
c KZ format: z0 t^2 [+/-1] (z0=apex, t^2=tan^2(half-angle))
1 kz 0 1.0 1 $ Cone apex at z=0, 45-deg half-angle, upper sheet
2 k/z 5 5 0 1.0 $ Cone at (x=5,y=5), apex at z=0, 45-deg
c Torus (donut shape)
c TZ format: x y z A B C (center, major radius, two minor radii)
3 tz 0 0 0 10 5 5 $ Torus centered at origin, R_major=10, R_minor=5
c Beam collimator using cone
10 kz 0 0.031 1 $ Cone apex at z=0, ~10-deg half-angle, upper sheet
11 pz 0 $ Entrance plane
12 pz 50 $ Exit planeThe KZ card takes the apex z-coordinate, the square of the tangent of the half-angle (t²), and an optional sheet selector (+1 for upper, -1 for lower). K/Z adds x,y offsets. Tori (TZ, TX, TY) require six parameters: center coordinates, major radius, and two minor radii. These surfaces require careful attention to parameter counts and orientation.
Practical Guidelines
Surface Numbering Strategy
Use a logical numbering system to keep your geometry organized. Consider reserving number ranges for different purposes:
1-9: Major boundaries (room walls, vessel surfaces)
10-99: Primary components (fuel pins, detectors)
100-999: Secondary features (supports, instrumentation)
1000+: Lattice and universe surfaces
Always include descriptive comments for each surface. Good comments explain both the physical object and its role in the overall geometry.
Visualization and Verification
Always verify your surfaces using MCNP's plotting capabilities before building complex geometries. The plot command helps catch orientation errors and dimension mistakes early in the modeling process.
Remember that all MCNP dimensions are in centimeters. When working with engineering drawings in other units, convert carefully and document your conversions in comments.
Try It Yourself
Macrobodies are the shortcut worth knowing, and also the easiest place to guess a keyword that does not exist. MCNP defines ten: BOX, RPP, SPH, RCC, RHP (also spelled HEX), REC, TRC, ELL, WED, and ARB. Each takes a fixed number of entries, and a body with the wrong count is a different body.
RPP wants xmin xmax ymin ymax zmin zmax. The hexagonal prism is already correct.RHP and HEX are the same card; the manual heads the section with both spellings and uses each in its examples. It takes nine entries for a regular hexagon — vertex, height vector, first facet vector — or fifteen when you supply the second and third facet vectors to describe an irregular one.
Card semantics on this page follow MCNP6.3.1 Theory & User Manual (LA-UR-24-24602 Rev. 1), §5.3.1 Surfaces Cards, Defined by Equations and §5.3.4 Surfaces Defined by Macrobodies.
Full reference list on the attribution page.
Check yourself
- Apply the surface sign convention without stopping to check it?
- Bound a rectangular region with
px,py, andpzplanes? - Place spheres and cylinders, including the off-axis
c/xfamily? - Read a cone card, sheet selector included, and a torus parameter list?
- Assemble a PWR fuel pin from concentric cylinders and two axial planes?