MCNP Guide

❮ Basic Shapes & SurfacesComplex Geometries ❯

MCNP Example: PWR Fuel Pin Model

Problem Description

This example models a typical PWR fuel pin cell: a UO₂ fuel pellet, helium-filled gap, Zircaloy-4 cladding, and water moderator — all enclosed by a reflective equivalent cylinder that simulates the infinite lattice environment.

Key Dimensions

Radial dimensions:

  • Fuel outer radius: 0.4095 cm
  • Gap outer radius: 0.4178 cm
  • Clad outer radius: 0.4750 cm
  • Equivalent cell radius: 0.7108 cm

Material densities:

  • UO₂ fuel: 10.4 g/cm³
  • Helium gap: 0.0001 g/cm³
  • Zircaloy-4: 6.56 g/cm³
  • Water: 0.998 g/cm³

Complete Input File

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mcnp — hover sections to explore
PWR Fuel Pin Cell Model
c ---- Cell Cards ----
c cell mat  density  surfaces        params
1  1  -10.4     -1      imp:n=1  $ UO2 Fuel
2  2  -0.0001    1 -2   imp:n=1  $ Helium Gap
3  3  -6.56      2 -3   imp:n=1  $ Zircaloy-4 Cladding
4  4  -0.998     3 -4   imp:n=1  $ Water Moderator
 
c ---- Surface Cards ----
1  cz  0.4095    $ Fuel Radius
2  cz  0.4178    $ Gap Outer Radius
3  cz  0.4750    $ Clad Outer Radius
*4 cz  0.7108    $ Reflective boundary (* = reflecting)
 
c ---- Data Cards ----
c Materials  (positive fractions = atom, negative = weight)
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 (atom ratios)
     8016.70c   1.0
mt4  lwtr.10t             $ S(a,b) thermal scattering
 
c kcode: neutrons/cycle  k-guess  skip  total
kcode 5000 1.0 50 250
ksrc  0 0 0
 
c Tallies
f4:n  1                   $ Track-length flux in 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   $ Energy bin upper bounds (MeV)
f7:n  1                   $ Fission energy deposition in fuel
 
c Output control
prdmp  j  300  1  2

Annotated MCNP Input

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●Cell cards (4 regions)
●Cylindrical surfaces (1–3)
●Reflective boundary (*4)
●UO₂ fuel material
●Helium fill gas
●Zircaloy-4 cladding
●Water + S(α,β) scattering
●KCODE settings
●KSRC initial source
●F4/E4 flux spectrum tally
●F7 fission power tally
●Output control

Expected Results

Physics

  • k∞ typically 1.3–1.5 for fresh 4% fuel with this H/HM ratio
  • Thermal flux peaks in water, dips in fuel (self-shielding)
  • F7 fission heat concentrated in the fuel pellet

Statistics

  • k-effective uncertainty: ~0.0002–0.0005
  • F4 flux spectrum: good statistics in thermal/epithermal bins
  • All statistical checks should pass with 200 active cycles

Analysis Considerations

  • The cylindrical outer boundary is an approximation of the actual square pin cell geometry
  • The reflective BC gives k∞ — not keff of a finite reactor
  • Room-temperature cross sections (.70c) are not suitable for hot operating conditions
  • Thermal scattering (mt4 lwtr.10t) is essential for accurate k-effective

Model Variations

Design Studies

  • Vary enrichment (2–5%) for reactivity coefficients
  • Adjust pin pitch to study H/HM effects
  • Change water density to model void coefficient

Advanced Analysis

  • Add burnable absorber coating on fuel surface
  • Apply temperature-dependent cross sections (TMP card)
  • Use BURN card for depletion calculation

Related Topics

❮ Basic Shapes & SurfacesComplex Geometries ❯