# Nuclide O-18 is missing from official ENDF7.1 Library **URL:** \ **Category:** User Support\ **Created:** [June 12, 2020, 5:45pm UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617 "2020-06-12T17:45:29Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Muhammad\_Imron](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@Muhammad\_Imron](https://openmc.discourse.group/u/Muhammad_Imron)\ **Post date:** [June 12, 2020, 5:45pm UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/1 "2020-06-12T17:45:29Z")
Hi All, I am a new openMC user. I tried to run some problems in the provided examples using official ENDF7.1 Library and found that the data for nuclide for O-18 is missing (after downloading for long time). Is the library obsolete or should I use other libs? This problem seems quite trivial so I believe it must have been addressed before, but I couldn't find this information using search feature in this group. Thanks. ---
**Author:** ![Pranto](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/pranto/32/489_2.png) [@Pranto](https://openmc.discourse.group/u/Pranto)\ **Post date:** [June 12, 2020, 6:28pm UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/2 "2020-06-12T18:28:13Z")
O18 is not included in the official ENDF 7.1 data library. See the attached file… O18.h5 [from tendl-2019 ACE file to HDF5 file using NJOY2016, temperatures = 250 K, 293.6 K, 600 K, 900 K, 1200 K, and 2500 K] Or, you can use convert\_lib80x.py script [O18, C13 included] from [data](https://github.com/openmc-dev/data). Hope this helps [O18.h5](https://openmc.discourse.group/uploads/short-url/8JS0rchLlEeL1fRALCLwO0wCr8t.h5) (541 KB) [convert\_lib80x.py](https://openmc.discourse.group/uploads/short-url/vusGC0vU5S6XhZ3dM4BgbeJnGxn.py) (2.96 KB) ---
**Author:** ![Muhammad\_Imron](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@Muhammad\_Imron](https://openmc.discourse.group/u/Muhammad_Imron)\ **Post date:** [June 13, 2020, 2:09am UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/3 "2020-06-13T02:09:07Z")
Thanks Ariful. That is really helpful. I will just simply use the data from TENDL. Btw, I am wondering why O-18 is not included in the official OpenMC data lib? Is it missing from original ENDF7.1? Or because O-18 content is not significant in typical light water? ---
**Author:** ![Pranto](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/pranto/32/489_2.png) [@Pranto](https://openmc.discourse.group/u/Pranto)\ **Post date:** [June 13, 2020, 3:27am UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/4 "2020-06-13T03:27:11Z")
Hi Muhammad, One thing you need to know that TENDL did not include any thermal scattering evaluation data. If you want to use it then you need to pick it up from another data library ( ENDF-7.1, ENDF-8, JEFF-3.3 ). [NNDC](https://www.nndc.bnl.gov/endf/b7.1/download.html), [LANL](https://t2.lanl.gov/nis/data/endf/index.html) did not include neutron reaction sublibrary file for O18, C13. That’s why these nuclides are not in the official OpenMC data library ( endf 7.1 ). ---
**Author:** ![Muhammad\_Imron](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@Muhammad\_Imron](https://openmc.discourse.group/u/Muhammad_Imron)\ **Post date:** [June 14, 2020, 5:04am UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/5 "2020-06-14T05:04:32Z")
Thanks Ariful. ---
**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\ **Post date:** [June 16, 2020, 12:03pm UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/6 "2020-06-16T12:03:59Z")
One more thing to keep in mind is that when you are generating inputs from the Python API, it will look at your OPENMC\_CROSS\_SECTIONS environment variable to see what nuclides are available. So, if you run: mat.add\_element(‘O’, …) and your OPENMC\_CROSS\_SECTIONS environment variable points to a file that does not include O18, it will not be included in your material (the density of O18 in this case [gets replaced with O16](https://github.com/openmc-dev/openmc/blob/develop/openmc/element.py#L185-L186)) ---
**Author:** ![Muhammad\_Imron](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@Muhammad\_Imron](https://openmc.discourse.group/u/Muhammad_Imron)\ **Post date:** [June 16, 2020, 1:51pm UTC](https://openmc.discourse.group/t/nuclide-o-18-is-missing-from-official-endf7-1-library/617/7 "2020-06-16T13:51:45Z")
Thanks Paul and Jonathan (his email was sent personally).