# 47-group cross sections and then merging them into 2-group cross sections using the get\_condensed\_library function

**URL:** <https://openmc.discourse.group/t/47-group-cross-sections-and-then-merging-them-into-2-group-cross-sections-using-the-get-condensed-library-function/6014>\
**Category:** User Support\
**Created:** [September 21, 2025, 8:41am UTC](https://openmc.discourse.group/t/47-group-cross-sections-and-then-merging-them-into-2-group-cross-sections-using-the-get-condensed-library-function/6014 "2025-09-21T08:41:33Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Lishuaishuai](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/lishuaishuai/32/4219_2.png) [@Lishuaishuai](https://openmc.discourse.group/u/Lishuaishuai)\
**Post date:** [September 21, 2025, 8:41am UTC](https://openmc.discourse.group/t/47-group-cross-sections-and-then-merging-them-into-2-group-cross-sections-using-the-get-condensed-library-function/6014/1 "2025-09-21T08:41:33Z")

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Hello everyone, I have a question to ask.  
I am currently using openmc to output 47-group cross sections and then merging them into 2-group cross sections using the get\_condensed\_library function. When outputting on a per-assmbly basis, the 2-group cross sections yield the same results as the continuous ones. However, when outputting on a per-material basis, there is a significant discrepancy between the 2-group results and the continuous ones. Can you please explain why this occurs?  
This assmbly comprises fuel, cladding, water, and poison materials.

 ![plot_4](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/0/096dde79ec235b12cf86059e39621df3c96da170.png)