MCNP Guide
Cell Cards in MCNP
Define physical volumes and their properties
Cell Card Format
Cell cards form the foundation of MCNP geometry modeling. Each cell represents a unique physical volume in your model, defined by its material composition, density, and geometric boundaries. The cell definition tells MCNP what material fills the space and how particles should interact with it.
Basic Structure
cell_number material density geometry parameters $ comment
1 1 -10.4 -1 imp:n=1 $ Fuel region
2 2 -6.55 1 -2 imp:n=1 $ Cladding
3 3 -1.0 2 -3 imp:n=1 $ Water
4 0 3 imp:n=0 $ Outside voidEach cell card begins with a unique number (1-99999) that identifies the cell throughout your input file. The material number references a material definition in the data cards section, with 0 indicating a void region. For material-filled cells, the density specifies the material's physical density - negative values indicate g/cm³ (mass density), while positive values represent atoms/barn-cm (atom density).
The geometry field defines the cell's boundaries using surface numbers and boolean operations. Additional parameters like particle importances (imp:n=1) control how MCNP handles particle tracking in the cell. Comments after the dollar sign help document your choices.
Defining Cell Boundaries
Surface Sense
When defining cell boundaries, the sign before a surface number determines which side of the surface you're using. A negative sign means "inside" or "below" the surface, while a positive sign (or no sign) means "outside" or "above". For example, -1 specifies the region inside surface 1, while 1 specifies the region outside it.
This concept is particularly important when defining nested structures like fuel pins, where you need to specify regions between surfaces. The expression 1 -2 defines the region that is outside surface 1 AND inside surface 2.
Combining Regions
MCNP provides three boolean operators for combining regions. Spaces between surface specifications create intersections - all conditions must be true. The colon operator (:) creates unions, where a point must satisfy either condition. The pound operator (#) creates complements, useful for defining regions outside other cells.
c Simple and complex cell definitions
1 1 -10.4 -1 $ Inside sphere 1
2 2 -6.55 1 -2 $ Between spheres 1 and 2
3 3 -7.8 (-3 4 -5):(6 -7 8) $ Two separate regions
4 0 #1 #2 #3 $ Outside all other cellsThe example above shows how these operators work together. The first two cells use simple intersections. Cell 3 demonstrates a union of two regions, each defined by multiple surfaces. Cell 4 uses the complement operator to define everything outside the other cells.
Cell Parameters
Particle Importance
The importance parameter (imp) controls particle tracking in each cell. A value of 1 enables normal tracking, while 0 kills any particle entering the cell. You can set different importances for different particle types (imp:n for neutrons, imp:p for photons). This parameter is crucial for defining problem boundaries and implementing variance reduction techniques.
Universe and Fill
For repeated structures like fuel pin arrays, MCNP provides universe and fill parameters. The universe parameter (u=) assigns a cell to a collection that can be reused. The fill parameter places that universe into another cell, creating hierarchical geometries.
c Universe definition
10 1 -10.4 -1 u=1 $ Define universe 1
c Using the universe
20 0 -10 fill=1 $ Fill region with universe 1Practical Example: Complete Fuel Pin
Let's examine a complete fuel pin model that demonstrates proper cell definitions, surface relationships, and parameter usage:
c Fuel Pin Cell Definitions
c mat dens surfaces params
1 1 -10.4 -1 imp:n=1 $ UO2 fuel
2 2 -6.55 1 -2 imp:n=1 $ Zircaloy clad
3 3 -1.0 2 -3 imp:n=1 $ Water moderator
4 0 3 imp:n=0 $ External void
c Surface Cards (referenced above)
1 cz 0.4096 $ Fuel radius
2 cz 0.4750 $ Clad outer radius
3 cz 0.6617 $ Water boundaryThis model creates a cylindrical fuel pin with concentric regions of fuel, cladding, and moderator. Each cell builds on the previous one, sharing surfaces to ensure a complete and consistent geometry. The cells progress logically from the innermost fuel region outward to the problem boundary.
Notice how the surface numbers maintain the natural ordering of the physical structure, making the input file easier to understand and modify. The comments provide essential documentation about each component's purpose and properties.
Common Mistakes and Solutions
When working with cell cards, several common issues can arise. Unbounded geometries occur when you haven't fully enclosed your problem space, leaving paths for particles to escape. Always ensure your outer boundary completely surrounds your model and has appropriate importance settings.
Overlapping cells create ambiguity about material properties and particle tracking. MCNP requires that each point in space belong to exactly one cell. When combining surfaces with boolean operators, carefully consider how regions interact to avoid unintended overlaps.
Surface sense errors often appear in complex geometries. Remember that the negative sign means "inside" for any type of surface. When working with multiple surfaces, sketch the geometry and verify that your surface sense definitions create the intended regions.