Exercise 5:
Resonance Integrals

This exercise will make use of the RECONR and BROADR modules to reconstruct the resonance cross sections for U-235 and broaden them to 293.6K (.0253 eV). This will allow us to look at the standard thermal quantities computed by BROADR.

Copy t511 from the NJOY distribution to tape20. This file contains the ENDF/B-V version of the U-235 cross sections. Create a file named in5 containing the following lines (as usual, you can leave off the comments to the right of the slash symbol):

           20 21
           'exercise 5'/    new tape ID title
           1395 1/          MAT
           .002/            fractional tolerance
           '92-U-235'/      descriptive card for new tape
           20 21 22
           1395 1/          MAT, one temperature
           .002/            tolerance
           293.6/           temperature

Run the NJOY job, and then look at the resulting output file. Find the table labeled "thermal quantities at 293.6 K = .0253 eV." NJOY is often run at temperatures different than the one corresponding to .0253 eV, and this table is capable of showing values corresponding to both the temperature used (e.g., 300K) and the standard value of .0253 eV. The table contains the simple fission and capture cross sections as evaluated at "tev" and .0253 eV, and it also contains the Maxwellian-weighted average of these cross sections ("thermal capture integral", "thermal fission integral"). For 1/v cross section shapes, these two numbers are proportional, and the "g-factor" listed is unity. For more general shapes, g-factors different from unity give a integral measure of how seriously the cross sections differ from the 1/v shape.

The "capture resonance integral" and the "fission resonance integral" give the integrated effect of the higher energy data.

The alpha, eta, and K1 integrals are commonly used in thermal reactor analysis.


23 January 1998 T-2 Nuclear Information Service ryxm@lanl.gov