# Hexagonal Lattice Troubles

**URL:** <https://openmc.discourse.group/t/hexagonal-lattice-troubles/5213>\
**Category:** User Support\
**Created:** [February 2, 2025, 12:03am UTC](https://openmc.discourse.group/t/hexagonal-lattice-troubles/5213 "2025-02-02T00:03:42Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Daedalus](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/daedalus/32/2928_2.png) [@Daedalus](https://openmc.discourse.group/u/Daedalus)\
**Post date:** [February 2, 2025, 12:03am UTC](https://openmc.discourse.group/t/hexagonal-lattice-troubles/5213/1 "2025-02-02T00:03:42Z")

</div>

Need to do hexagonal lattices for a reactor I’m working on, but I’m having a hard time getting the definition right. How do I make this thing smooth instead of bumpy?

 ![plot_5](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/c/c8ff7033475b88848cd68973493675308dee97ea.png)

---

<div class="post-metadata">

**Author:** ![wahidluthfi](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/wahidluthfi/32/3375_2.png) [@wahidluthfi](https://openmc.discourse.group/u/wahidluthfi)\
**Post date:** [February 2, 2025, 2:37pm UTC](https://openmc.discourse.group/t/hexagonal-lattice-troubles/5213/2 "2025-02-02T14:37:35Z")

</div>

Hi Daedalus, welcome back to the openmc community.  
I think you want to declare the universe filling the outside of your hexagonal lattice, you can use the outer feature from your hex lattice. Here is the example, i.e. you named the hexagonal lattice as fuel\_universe and the gray material is graphite, then

```auto
fuel_universe.outer = openmc.Universe(cells=[openmc.Cell(fill= graphite)], name='graphite surrounding fuel lattice')

```

---

<div class="post-metadata">

**Author:** ![Daedalus](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/daedalus/32/2928_2.png) [@Daedalus](https://openmc.discourse.group/u/Daedalus)\
**Post date:** [February 2, 2025, 3:26pm UTC](https://openmc.discourse.group/t/hexagonal-lattice-troubles/5213/3 "2025-02-02T15:26:11Z")

</div>

Solved. For future note, I had the void universe already defined and filled with graphite, but you have to remove the region definition from it in order for it to work.