Monte Carlo NParticle Transport Code System
MCNP is a generalpurpose, continuousenergy, generalized geometry, timedependent, coupled neutronphotonelectron Monte Carlo transport code system.
METHOD OF SOLUTION
MCNP treats an arbitrary threedimensional configuration of materials in geometric cells (see for instance Figure 1) bounded by first and seconddegree surfaces and some special fourthdegree surfaces.
Pointwise continuousenergy cross section data are used, although multigroup data may also be used. Fixed source adjoint calculations may be made with the multigroup data option. For neutrons, all reactions in a particular crosssection evaluation are accounted for. Both free gas and S(a ,b ) thermal treatments are used. Criticality sources as well as fixed and surface sources are available. The tallies have extensive statistical analysis of convergence. Rapid convergence is enabled by a wide variety of variance reduction methods. 
Energy ranges are 060 MeV for neutrons (data generally only available up to 20 MeV) and 1 keV  1 GeV for photons and electrons.
APPLICATIONS
Currently MCNP4C is being extensively used to perform full scale analysis of e.g. void reactivity coefficients for complex reactor systems for transmutation
Both 2D and 3D calculations have been performed studying overall and local effects. Extensive uncertainties analyses have also been performed investigating the effect of the application of different cross section data models/treatments (see Figure 2) i.e.:
 Inclusion of delayed neutrons
 Free gas treatment for hydrogen
 Thermal data for hydrogen in water
 Thermal data for hydrogen in zirconium hydride
 Inclusion of probability tables to describe the unresolved resonance region.


Furthermore different cross sections libraries (JEF2.2, JENDL3.2, ENDF/BVI.2, and ENDF/BVI.7) were used in order to compare the differences among them (see Table 1). 
Data library that replaces ENDF/B6.7 
ENDF/B6.2 
JEF2.2 
JENDL3.2 

keff variation 
0.01114 
0.01225 
0.01015 
Seed void variation ($) 
1.03 
0.90 
0.77 
Total void variation ($) 
0.67 
0.26 
1.02 
This work proved to be extremely useful to outline the differences among cross section data files and of different cross section treatments, and to point out new areas for future research.