MCNP Guide

❮ Universes & LatticesNuclear Data & Cross Sections ❯

MCNP Example: PWR Fuel Assembly Model

Problem Description

This example models a 17×17 PWR fuel assembly using the MCNP universe and lattice system. Three universes are defined — fuel pin, guide tube, and lattice — then placed in a global assembly container with a thin water reflector.

  • 17×17 lattice with fuel pins and guide tube water columns
  • Assembly pitch 21.42 cm (17 × 1.26 cm), pin pitch 1.26 cm
  • Thin water reflector (0.79 cm) on all four sides
  • F4 cell flux and FMESH pin-by-pin power map tallies

Complete Input File

Hover over any highlighted block to see how each part of the assembly model works.

mcnp — hover sections to explore
c PWR 17x17 Fuel Assembly Model
c ---- Universe 1: Fuel pin ----
1    1  -10.4   -1      imp:n=1  u=1   $ UO2 fuel
2    2  -0.0001  1 -2   imp:n=1  u=1   $ Helium gap
3    3  -6.56    2 -3   imp:n=1  u=1   $ Zircaloy-4 cladding
4    4  -0.998   3      imp:n=1  u=1   $ Water (fills to lattice boundary)
 
c ---- Universe 2: Guide tube (water column) ----
11   4  -0.998  -5      imp:n=1  u=2
12   4  -0.998   5      imp:n=1  u=2   $ Water (fills to lattice boundary)
 
c ---- Universe 3: 17×17 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 water 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
 
c ---- Surface Cards ----
c Fuel pin surfaces
1    cz  0.4095
2    cz  0.4178
3    cz  0.4750
 
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 (17 x 1.26 = 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
 
c ---- Data Cards ----
m1   92235.70c   0.04     $ 4% enriched UO2 (atom fractions)
     92238.70c   0.96
     8016.70c    2.0
m2   2004.70c    1.0      $ Helium fill gas
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             $ S(a,b) 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                  $ Track-length flux in 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
 
fmesh14:n geom=xyz
          origin=-10.71 -10.71 0
          imesh=10.71  jmesh=10.71  kmesh=1
          iints=17     jints=17     kints=1

Annotated MCNP Input

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●Universe 1 — fuel pin cells
●Universe 2 — guide tube
●Universe 3 — lattice + fill map
●Assembly container + reflector
●Pin + guide tube surfaces
●Lattice pitch planes
●Assembly boundary
●Reflector boundary
●UO₂ fuel material
●Helium fill gas
●Zircaloy-4 cladding
●Water + S(α,β) scattering
●KCODE + KSRC settings
●F4/E4 cell flux tally
●FMESH14 power map tally

Important Considerations

  • Verify the lattice fill map carefully — each row must have exactly 17 entries
  • Check for geometry errors at universe/global-level boundaries before production runs
  • Use multiple source points (ksrc) for better fission source convergence across the assembly
  • Monitor the pin-by-pin power symmetry in the FMESH tally to detect modelling errors
  • Large lattice arrays can consume significant memory — use the MCNP geometry checker first

Expected Results

  • k-effective ≈ 1.0–1.15 for a single fresh assembly with a thin reflector
  • Peripheral fuel pins show higher thermal flux due to proximity to the reflector
  • Thermal flux peaks near water-filled guide tube positions
  • FMESH power map should exhibit fourfold symmetry for this guide tube pattern

Model Extensions

Additional features

  • Burnable poison rods (replace water in selected guide tubes)
  • Multiple enrichment zones for a more realistic loading pattern
  • Instrument thimble at the central position
  • Axial variation in materials using u= assignment per z-slice

Multi-assembly modelling

  • Define a fourth universe for the full core lattice of assemblies
  • Assign different enrichments per assembly universe
  • Add structural components (grid spacers, nozzles)
  • Use DXTRAN or weight windows for peripheral tally accuracy

Related Topics

❮ Universes & LatticesNuclear Data & Cross Sections ❯