Monte Carlo Methods, Codes, & Applications Group

Los Alamos National Laboratory


MCNP is a general-purpose Monte Carlo N-Particle code that can be used for neutron, photon, electron, or coupled neutron/photon/electron transport. Specific areas of application include, but are not limited to, radiation protection and dosimetry, radiation shielding, radiography, medical physics, nuclear criticality safety, Detector Design and analysis, nuclear oil well logging, Accelerator target design, Fission and fusion reactor design, decontamination and decommissioning. The code treats an arbitrary three-dimensional configuration of materials in geometric cells bounded by first- and second-degree surfaces and fourth-degree elliptical tori.

Pointwise cross-section data typically are used, although group-wise data also are available. For neutrons, all reactions given in a particular cross-section evaluation (such as ENDF/B-VI) are accounted for. Thermal neutrons are described by both the free gas and S(alpha,beta) models. For photons, the code accounts for incoherent and coherent scattering, the possibility of fluorescent emission after photoelectric absorption, absorbtion in pair production with local emission of annihilation radiation, and bremsstrahlung. A continuous-slowing-down model is used for electron transport that includes positrons, k x-rays, and bremsstrahlung but does not include external or self-induced fields.

Important standard features that make MCNP very versatile and easy to use include a powerful general source, criticality source, and surface source; both geometry and output tally plotters; a rich collection of variance reduction techniques; a flexible tally structure; and an extensive collection of cross-section data.

MCNP contains numerous flexible tallies: surface current & flux, volume flux (track length), point or ring detectors, particle heating, fission heating, pulse height taly for energy or charge deposition, mesh tallies, and radiography tallies.


MCNP Highlights

See the MCNP NEWS tab for information on:

Special issue of Nuclear Technology journal

MCNP Site Support Activities at LANL

MCNP User Symposium Meeting

Additions to MCNP Reference Collection (2020-09-13)

MCNP6 SQA Plan & Requirements, LA-UR-20-26666

Additions to MCNP Reference Collection (2019-11-07)

About 40 reports & documents for criticality safety, unstructured mesh, NCS postdoc & student work, statistics.

Additions to MCNP Reference Collection (2018-02-05)

MCNP6.2 Release Notes

Additions to MCNP Reference Collection (2017-11-21)

MCNP6.2 Users Manual
How to Build MCNP6.2
MCNP6.2 Release Testing

Note on Nuclear Data for MCNP6 (2017-09-28)

As discussed in the references below, previous data released with MCNP6.1 & MCNP6.1.1 had these problems:
  (1) Missing (n,g) production data for hydrogen ACE files, 1001.80c - 1001.86c
  (2) Errors in SiO2 S(a,b) thermal scattering data, sio2.30t - sio2.36t
  (3) Errors in h-zr.27t S(a,b) thermal scattering data at 1200K

These data issues are discussed in reports on the MCNP website, in the Reference Collection, topic "Release Information for MCNP6.2 Release":
  * Release of MCNP6.2 & Whisper-1.1 - Guidance for NCS Users, LA-UR-17-24260
  * Whats New with MCNP6.2 & Whisper-1.1, LA-UR-17-27992
  * Verification of MCNP6.2 for Nuclear Criticality Safety Applications, LA-UR-17-24406
  * Verification of MCNP6.2 for Nuclear Criticality Safety Applications, LA-UR-17-23822.
  * Listing of Available ACE Data Tables, LA-UR-17-20709.
  * Data Changes for the MCNP6.2 Release, LA-UR-17-21486, LA-UR-17-20703

The MCNP6.2 release includes corrected data files. These are discussed in the references above & in the MCNP6.2 Release Notes.

MCNP® and Monte Carlo N-Particle® are registered trademarks owned by Triad National Security, LLC, manager and operator of Los Alamos National Laboratory. Any third party use of such registered marks should be properly attributed to Triad National Security, LLC, including the use of the ® designation as appropriate. Any questions regarding licensing, proper use, and/or proper attribution of Triad National Security, LLC marks should be directed to

Operated by Triad National Security, LLC for the U.S. Department of Energy's NNSA | © Copyright Triad National Security, LLC. All rights reserved