Exercise 7:
Cross Sections for WIMS




NJOY is most valuable when it makes data for code systems used for practical application calculations. As an example, we will use one of the NJOY standard test problems to prepare a WIMS library for Pu-239 from ENDF/B-IV. Here is a copy of the core part of in11 from the NJOY distribution: 
      moder
      20 -21
      reconr
      -21 -22
      'pendf tape for pu-238 from endf/b-iv tape 404'/
      1050 3/
      .005/
      '94-pu-238 from endf/b tape t404'/
      'processed by the njoy nuclear data processing system'/
      'see original endf/b-iv tape for details of evaluation'/
      0/
      broadr
      -21 -22 -23
      1050 3 0 1 0 /
      .005/
      300. 900. 2100.
      0/
      unresr
      -21 -23 -24
      1050 3 7 1
      300 900 2100
      1.e10 1.e5 1.e4 1000. 100. 10. 1
      0/
      thermr
      0 -24 -25
      0 1050 8 3 1 0 1 221 0
      300. 900. 2100.
      .05 4.2
      groupr
      -21 -25 0 -26
      1050 9 0 5 3 3 7 1
      '94-pu-238'/
      300. 900. 2100.
      1.e10 1.e5 1.e4 1000. 100. 10. 1
      3 1 'total'/
      3 2 'elastic'/
      3 16 'n2n'/
      3 17 'n3n'/
      3 18 'fission'/
      3 102 'capture'/
      3 221 'free gas thermal'/
      6 2 'elastic'/
      6 16 'n2n'/
      6 17 'n,3n'/
      6 18 'fission'/
      6 51 'discrete inelastic'/
      6 -59 'continued'/
      6 91 'continuum inelastic'/
      6 221 'free gas thermal'/
      0/
      3 1 'total'/
      3 2 'elastic'/
      3 18 'fission'/
      3 102 'capture'/
      3 221 'free gas thermal'/
      6 2 'elastic'/
      6 221 'free gas thermal'/
      0/
      3 1 'total'/
      3 2 'elastic'/
      3 18 'fission'/
      3 102 'capture'/
      3 221 'free gas thermal'/
      6 2 'elastic'/
      6 221 'free gas thermal'/
      0/
      0/
      wimsr
      -26 27
      1/
      1050 1 1050./
      3 7 1e10 3 10.890 221 0/
      1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1./
      stop

Note that the THERMR module is run. Because WIMS is a thermal-reactor lattice code, it is important to include proper thermal cross sections. Also note that GROUPR requests self shielding. A careful treatment of resonance absorption is very important for doing good lattice calculations.

Run the job using a deck appropriate for your system.

Open the output file and scroll down to "wimsr". You will see a modified version of the GROUPR output. First, remember that WIMSR uses the conventional group ordering with group 1 being the high-energy group; GROUPR works backwards with group 1 being the low energy group. Since WIMS uses 69 groups, you can convert the group indices by subtracting from 70. You will find some other transformations: absorption is capture plus fission, and the scattering matrices have all the channels added together.

The "potential scattering cross section" of 10.89 in the input deck is obtained as 4πa2, where a is the scattering length from File 2 on the ENDF tape. It is important to include a nonzero value for this entry for resonance absorbers.


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23 January 2013 T-2 Nuclear Information Service ryxm@lanl.gov