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anomaly [uh-nom-uh-lee] (definition from

  1. a deviation from the common rule, type, arrangement, or form.
    Synonyms: abnormality, exception, peculiarity.
  2. an anomalous person or thing; one that is abnormal or does not fit in:
    With his quiet nature, he was an anomaly in his exuberant family.
    Synonyms: abnormality, exception, peculiarity.
  3. an odd, peculiar, or strange condition, situation, quality, etc.
  4. an incongruity or inconsistency.

At the request of workshop participants, we are publishing some of the most perplexing anomalies encountered by Randy in his years of consulting and teaching.  By making these available on this website, we hope to provide useful information and enable a larger discussion on the best practices for using MCNP.

  1. Wrong Answer in 6.1 - Fixed in 6.2: (8/14/2015) Same input file produces different keff value in MCNP5, MCNPX, and MCNP6Click here for more.
  2. Does Not Run in 6.1 - Fixed in 6.2: (8/14/2015) Lattice source runs in MCNPX and MCNP5 but not in MCNP6. Click here for more.
  3. Does Not Run in 6.1 - Fixed in 6.2: (8/14/2015) Zero Lattice Element Hit Error Occurs on a Rotated Lattice.  Click here for more.
  4. Does Not Run in 6.1 - Fixed in 6.2: (8/16/2015) Run terminates with lost particles on a Rotated Universe.  Click here for more.
  5. Does Not Run: (8/16/2015) General quadratic surface for sphere not recognized as identical to sphere surface.  Click here for more.
  6. Potential Wrong Answer (8/19/2015): Cell Rejection with Source Biasing gives wrong answer. Click here for more.