MCNP Guide

❮ Universes & LatticesNuclear Data & Cross Sections ❯

MCNP Example: PWR Fuel Assembly Model

Problem Description

This example demonstrates how to model a 17x17 PWR fuel assembly with the following features:

  • 264 fuel pins
  • 25 guide tubes
  • Assembly pitch of 21.42 cm
  • Pin pitch of 1.26 cm
  • Water moderator and reflector

Universe Definitions

mcnp
c Universe 1: Fuel pin (u=1)
1    1  -10.4   -1      imp:n=1  u=1
2    2  -0.0001  1 -2   imp:n=1  u=1
3    3  -6.56    2 -3   imp:n=1  u=1
4    4  -0.998   3 -4   imp:n=1  u=1

c Universe 2: Guide tube (water column)
11   4  -0.998  -5      imp:n=1  u=2
12   4  -0.998   5 -4   imp:n=1  u=2

c Universe 3: 17x17 assembly lattice
21   0  -21 22 -23 24  lat=1  u=3  imp:n=1
     fill=-8:8 -8:8 0:0
     1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
     1 1 1 1 2 1 1 1 2 1 1 1 2 1 1 1 1
     1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 2 1
     1 1 2 1 1 1 1 2 1 2 1 1 1 1 2 1 1
     1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1
     1 1 1 1 1 2 1 1 1 1 1 1 1 2 1 1 1
     1 1 1 1 1 1 1 1 2 1 2 1 1 1 1 1 1
     1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1
     1 2 1 1 1 1 2 1 2 1 2 1 1 1 1 2 1
     1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
     1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 2 1
     1 1 1 1 1 2 1 1 1 1 1 1 1 2 1 1 1
     1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1
     1 1 2 1 1 1 1 2 1 2 1 1 1 1 2 1 1
     1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 2 1
     1 1 1 1 2 1 1 1 2 1 1 1 2 1 1 1 1
     1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

c Assembly container and reflector
31   0          -31 32 -33 34          fill=3  imp:n=1
32   4  -0.998  (31:-32:33:-34)
                -35 36 -37 38          imp:n=1  $ Water reflector
33   0           35:-36:37:-38         imp:n=0  $ Outside void

In the lattice definition, the number 1 represents a standard fuel pin (universe 1) while2 selects the guide-tube universe. The 17×17 matrix above follows a typical Westinghouse pattern with 25 guide tubes placed symmetrically about the assembly center.

Surface Definitions

mcnp
c Fuel pin surfaces
1    cz  0.4095
2    cz  0.4178
3    cz  0.4750
4    cz  0.6617

c Guide tube surface
5    cz  0.6020

c Lattice pitch (P = 1.26 cm → 0.63 cm half pitch)
21   px   0.630
22   px  -0.630
23   py   0.630
24   py  -0.630

c Assembly boundary (21.42 cm pitch)
31   px   10.71
32   px  -10.71
33   py   10.71
34   py  -10.71

c Reflector boundary
35   px   11.50
36   px  -11.50
37   py   11.50
38   py  -11.50

Materials and Settings

mcnp
c Materials
m1   92235.70c   0.04     $ 4% enriched UO2 (atom fractions)
     92238.70c   0.96
     8016.70c    2.0
m2   2004.70c    1.0      $ Helium
m3   40090.70c  -0.5145   $ Zircaloy-4 (weight fractions)
     40091.70c  -0.1122
     40092.70c  -0.1715
     40094.70c  -0.1738
     40096.70c  -0.0280
m4   1001.70c    2.0      $ Light water
     8016.70c    1.0
mt4  lwtr.10t               $ Thermal scattering

c Criticality settings
kcode 10000 1.0 50 250
ksrc  0 0 0   2 2 0  -2 -2 0   2 -2 0  -2 2 0

c Tallies
f4:n  1                  $ Representative fuel cell
e4    1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2
      0.1 1 2 3 4 5 6 7 8 9 10

fmesh4:n origin=-10.71 -10.71 0
         imesh=10.71  jmesh=10.71  kmesh=1
         iints=17     jints=17     kints=1
         type=1

Important Considerations

  • Verify lattice fill pattern carefully
  • Check for geometry errors at boundaries
  • Use multiple source points for better convergence
  • Monitor power distribution symmetry
  • Consider memory usage for large arrays

Expected Results

  • k-effective ≈ 1.3-1.4 for fresh assembly
  • Higher power in peripheral pins
  • Flux depression near guide tubes
  • Symmetric power distribution

Variations

  • Add burnable poison rods
  • Model different enrichment zones
  • Include instrument tube
  • Add assembly grid spacers
  • Model partial length rods

Key Points to Remember

  • Use universe/lattice for efficient modeling
  • Verify all dimensions and positions
  • Check for geometry errors
  • Monitor power distribution
  • Document assembly configuration

Related Topics

❮ Universes & LatticesNuclear Data & Cross Sections ❯