MCNP Syllabus

1
MCNP Syllabus

• Introduction
• Input File Basics
• Geometry Definition
• Source Definition
• Tally Definition
• Variance Reduction
• Criticality

2
MCNP Geometry

• Geometry Basics
• Quick Start
• Surfaces
• Combining Surfaces
• Macrobodies
• Cell properties
• Examples

3
Learning Objectives Geometry

• Understand how to define all four classes of
  surfaces
• Understand how to create cells from surfaces
• Understand the detailed definition of macrobodies
  
• Understand how to use surface transformations
• Understand when to use special surfaces

4
Geometry Basics

• The universe is divided into regions of
  different materials and properties
• All parts of the entire infinite universe must be
  included in geometric model
• The basic unit of a geometry is a cell
• All cells are defined by bounding surfaces
• All surfaces divide the universe into two regions
• left vs right OR inside vs outside

5
Quick StartSurfaces

• Equation surfaces
• Surfaces defined by providing the equation of the
  surface and the parameters (Table 3.1)
• examples
• A sphere located at the origin with radius Rj
  so R
• A cylinder parallel to the y-axis at X,Z with
  radius Rj c/y X Z R
• A plane normal to the z-axis at Zj pz Z

6
Quick StartCombining Surfaces into Cells

• Interior points of cell are related to a surface
  by the sense of the cell or -
• surfaces divide universe into 2 half-regions
• Boolean operators
• Combine half-regions to create cells/objects
• Intersection
• Union
• Complement

7
Quick Start Combining Surfaces into CellsSense

• All the points in a cell are related to the
  surfaces that define the cell by the sense
  which side of a surface the points of a cell are
  on
• (positive sense)
• For open surfaces (planes), points in positive
  direction from surface
• For closed surfaces (spheres, cylinders, etc),
  outside the surface
• - (negative sense)
• For open surfaces, points in negative direction
  from surface
• For closed surfaces, inside the surface

8
Quick Start Combining Surfaces into
CellsIntersection

• Space where both senses are true
• Input syntax a ltspacegt between two surface
  numbers
• 2_-1 represents only the region of space with
  sense 2 and sense 1

Surface 2
Surface 1
9
Quick Start Combining Surfaces into CellsUnion

• Space where either sense is true
• Input syntax a ltcolongt between two surface
  numbers
• 2-1 represents both the region of space with
  sense 2 and the region with sense 1

Surface 2
Surface 1
10
Quick Start Combining Surfaces into
CellsComplement

• Space outside another cell
• Input syntax a ltpoundgt symbol before a cell
  number
• 5 represents the region outside cell 5
• This region can then be intersected or united
  with other regions
• -2 5 represents the only the region outside
  cell 5 and inside surface 2

Cell 5
Cell 5
Cell 5
Surface 2
11
Quick StartCells

• C ell Mat Dens Surface combinations
• 1 1 -1 -1
• 2 2 -5 -2
• 3 0 1 2 -3
• Input cards for cells have 3 main parts
• Cell number
• Between 1 and 99999
• Cell contents
• Material number
• Material density
• gt0 number density
• lt0 mass density
• Surface combinations

qs1
12
Four Classes of MCNP Surfaces

• Analytic equations
• Planes, Spheres, Cylinders, Cones, Tori,
  Arbitrary Quadratics
• Macrobodies
• Combinatorial geometry based on closed primitives
• Axisymmetric sets of points
• Planes Linear or Quadratic surfaces
• Three point in a plane
• General Planes

13
Surface Equations

• Basic format(Chapter 3, Section III.A, p 3-12,
  Table 3.1)
• j n a list
• j surface number 1 ? j ? 99999
• n absent or 0 for no coordinate transform
• gt0 transform this surface with card TRn
• lt0 surface j is a periodic boundary with
  surface n
• a equation mnemonic (code)
• list data for equation a

14
Suface Equations Cones

• The equation for a cone defines two sheets

-

• Extra entry in parameters to specify positive
  (1) sheet or negative (-1) sheet
• Only valid for cones parallel to axes

15
Defining Macrobodies

• Surfaces of finite building blocks(Chapter 3,
  Section III.D, pg 3-19)
• BOX arbitrarily oriented orthogonal box
• RPP rectangular parallelepiped
• all surfaces normal to respective axes
• SPH sphere
• identical to equation for sphere
• RCC right circular cylinder
• axis orthogonal to base, but arbitrarily oriented
• RHP(HEX) right hexagonal prism
• like RCC but with arbitrary hexagonal base

16
Using Macrobodies

• Sense the same as other closed surfaces
• Positive outside the surface
• Negative inside the surface
• Can be combined with other kinds of surfaces
• Built from facets that can be referenced
  individually

17
Macrobody Facets

• Facets are sequentially numbered(see page 3-21)
• Referenced using macrobody number and facet
  number
• Cylindrical surface of RCC macrobody j5
• 5.1
• Plane at ymax of RPP macrobody j10
• 10.3

18
Points for Axisymmetric Surface

• One to three points on surface of revolution
  around either the x, y, or z axis(see page 3-16)
• Points are defined by coordinate pairs

• first coordinate distance along axis

• second coordinate radius from axis

19
Sample Axisymmetric Surfaces

• 1 Point planar

• 2 Points planar or linear (cylinder/cone)

• 3 Points planar, linear or quadratic (sphere,
  general quadratic)

• All points must be on the same sheet

• All sheets must be definable as planar, linear
  or quadratic

20
Three Points for a General Plane

• Any three points define a plane
• Sense?
• Origin is negative sense,
• (0,0,8) is positive sense,
• (0,8,0) is positive sense,
• (8,0,0) is positive sense,
• Fatal error

21
Surface Transformations

• TRn Data Card (see page 3-30)
• TRn Ox Oy Oz Bxx Byx Bzx Bxy Byy Bzy Bxz
  Byz Bzz M
• n
• Transformation number, matches surface card
• Ox Oy Oz
• Displacement of origin
• Bxx . . . Bzz
• Transformation matrix (cosines or angles)
• M
• 1 Ox,y,z define new origin in main coordinate
  system (default)
• -1 Ox,y,z define main origin in new coordinate
  system

22
Why Surface Transformations

• Easier to transform a standard surface than
  define a complicated surface
• Simple example Cylinder with axis parallel to
  (1,1,0)
• What is equation for this (GQ)?
• Instead
• Define cylinder on x-axis
• Transform by 45

qs2
23
MCNP CellsMinimum Cell Properties

• Cells are more than just geometry
• Materials
• Define the cross-sections to be used for
  transport and interactions in that cell
• Importance
• Minimum usage separate the universe from the
  physical model
• Power usage improve statistical results of
  problem

24
MCNP CellsMaterial Definitions

• Mn - in data card section of input file (see pg
  3-108)
• Provide a list of isotopes in material with
  atom/weight fractions
• Mn zaid1 frac1 zaid2 frac2 zaidN fracN
• n material number, matches cell card entry
• zaid isotope ID based on Z and A
• In most cases, zaid Z1000A
• A0 for all element with natural abundances of
  all isotopes
• Optional entry for specific cross-section
  table - zaid.xsid
• frac atom() or weight(-) fraction of this
  isotope
• MCNP will renormalize automatically

25
MCNP CellsSample Material Definitions

• M1 92235 4.5 92238 95.5 8016 13.5
• Uranium oxide (nuclear fuel) with 4.5 235U
  enrichment
• M25 7000 78 8000 21
• Approximation to air
• M12 1001 0.5 8016 0.25 6012 0.25
• (Poor?) approximation to tissue
• Small impurities are generally unimportant
• This steel has 23 different elements
• M4 26000 88.8 24000 9 74000 2 25000 0.5
  14000 0.25 6000 0.1

26
MCNP CellsImportance

• Every cell must have an importance
• Typical importance is 1
• Different values are used for variance reduction
• If importance is 0, particles are not tracked in
  that cell
• Terminate particle history
• Rest-of-Universe typically has importance0

27
MCNP CellsDefining Importance

• Different importance for different particles
• IMPn, IMPp, IMPe, IMPn,p, etc.
• Two options for defining importance
• On Cell definition cards, after surface lists
• 1 3 -8 -1 2 (-3 5) IMPn1
• 4 0 1 -4 5 IMPn0
• As a Data card with one number per cell
• IMPn 1 1 1 0 for 4 cells

28
Quickstart 2The Whole Story

• Source Definition
• REQUIRED With no source, there are no particles
• Problem Cut-off
• OPTIONAL With no cut-off, the problem will run
  forever
• Tallies
• OPTIONAL With no tallies, you wont know any
  results

29
Quickstart 2Source Definition

• Source Card SDEF, defines the following
• Where a particle is created
• Cell, Surface, (X,Y,Z)
• When a particle is created
• Energy direction of particle
• Weight of particle
• Type of particle
• Default (no arguments)
• Created at origin, at time zero, E14 MeV,
  isotropic, weight1

30
Quickstart 2Cut-off Cards

• Two main cut-offs
• Number of particles, NPS
• Computer time, CTME, in computer minutes
• Other cut-off for individual particles
• CUT defines maximum time, minimum energy, etc,
  for each particle
• ELPT defines minimum energy in each cell

31
Quickstart 2Tallies

• Tallies provide estimates to physical quantities
• Fna j1 j2 j3 jN T
• n tally number where last digit defines type
• a tally particle type n, p, e
• ji tally locations
• T optional total/average over all N locations

32
Quickstart 2Tally Types

• Surface Tallies
• 1 current integrated over surface
• 2 flux averaged over surface
• Cell Tallies
• 4 flux averaged over a cell
• 6 energy deposition averaged over a cell
• 7 cell-averaged fission energy deposition
• Special Tallies
• 5 point or ring detector
• 8 pulse height detector

qs3
33
MCNP Geometry Summary

• Four classes of surfaces
• Equations, macrobodies, general planes ,
  axisymmetric rotations
• Boolean operations combine surfaces into cells
• Intersection, union, complement
• Cells need materials and importance
• Macrobodies are formed from facets
• Facets are internally represented as other
  surfaces
• Surfaces can be arbitrarily transformed