Monte Carlo Libraries

In these days of inexpensive fast computers with huge memories, people are making more and more use of detailed Monte Carlo transport calculations. This approach is much more expensive than the multigroup approach, but it has two great advantages:
  • detailed cross sections don't have to be approximated, and
  • simplified models of the geometry don't have to be used.

There are Monte Carlo codes (e.g., MORSE) that use multigroup data, and NJOY's multigroup data can be reformatted for these codes, but the most faithful physics modeling can be obtained by using a full continuous-energy Monte Carlo code like MCNP from Los Alamos. NJOY can produce libraries for the MCNP code by using its ACER module. Here is an example of how we normally do this using a unix script:

         echo "ACE production and testing run:"
         echo "getting files"
         cfs get tape20:/endf/6u/neutron/c/nata   [This is an existing ENDF tape]
         cfs get tape21:/pendf/6u/c/nata          [This is an existing PENDF tape}
         cat input EOF
          20 21 0 31 32
              1 0 1/ fast data, no listing, type-1 format (ASCII)
          '6-c-nata from endf-vi.1'/
          600 296.3/ MAT and temperature
          .01/ thin distributions to 1%
          0 31 33 34 35 / file33 is plotting output
          7 1 2/ read type-1 file, make listing, write type-2 file (binary)
          '6-c-nata from endf-vi.1'/
          33 36/ convert the acer plot output to postscript
         echo "running njoy"
         xnjoy input
         echo "saving njoy output"
         cfs store dir=/acer/6u/f/c output:natao tape31:nata tape32:natax
         cfs store dir=/acer/6u/f/c tape33:natav tape36:natap
         echo "creating local xsdir file for material test ..."
         cfs get /acer/xsdir2
         xsdir2  EOF
         echo "creating mcnp inp file for material test ..."
         cat inp EOF
          material test
         1 1 .05 -1
         2 0 1

         1 so 1

         imp:n 1 0
         fcl:n 1 0
         mode n p
         fcl:p 1 0
         sdef erg=d1
         si1 1.e-8 .000001 .0001 .01 .1 1 18i 20
         sp1 0 1 23r
         nps 100000
         phys:p j 1
         m1 6000 1
         echo "running mcnp ..."
         echo "cleaning up ..."
         cfs store outp:/acer/6u/f/c/natar

The run uses a previously prepared PENDF tape. The first ACER section generates the ACE library and a file containing a line for a new "XSDIR" file, MCNP's index file for data. The second ACER section reads and prints this ACE file as an integrity check. It also makes a number of consistency checks on the file, and it prepares an extensive series of plots showing the cross sections and the distributions of secondary particles. The VIEWR section prepares these plots in Postscript for printing or for viewing on your screen. Finally, the script makes a quick MCNP run on the new file as a final quality check. The "cfs" lines read and write from a Los Alamos archival storage system, and they can be changed to satisfy local requirements.


23 January 2013 T-2 Nuclear Information Service