# Simulation freezes when using weight windows (version 0.14) **URL:** \ **Category:** User Support\ **Created:** [January 24, 2024, 2:00pm UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861 "2024-01-24T14:00:31Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![M\_Zammataro](https://avatars.discourse-cdn.com/v4/letter/m/46a35a/32.png) [@M\_Zammataro](https://openmc.discourse.group/u/M_Zammataro)\ **Post date:** [January 24, 2024, 2:00pm UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/1 "2024-01-24T14:00:31Z")
Dear Community, When running a simulation with weight windows in OpenMC version 0.14, the simulation freezes after a certain number of batches. Changing the seed also changes the batch where the simulation stops. The same simulation runs fine in version 0.13. Any suggestions? Thanks in advance! ---
**Author:** ![kingyue](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/kingyue/32/507_2.png) [@kingyue](https://openmc.discourse.group/u/kingyue)\ **Post date:** [March 14, 2024, 3:35am UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/2 "2024-03-14T03:35:04Z")
I have met the same issue. I have 22 threads. The simulation froze in a certain batch and only one CPU ran at that time. In 0.13 I also met this issue in some of my models containing wws. No idea what’s the cause. ---
**Author:** ![kingyue](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/kingyue/32/507_2.png) [@kingyue](https://openmc.discourse.group/u/kingyue)\ **Post date:** [March 14, 2024, 3:51am UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/3 "2024-03-14T03:51:16Z")
Maybe particles at one thread split a lot but particles on other threads have already reached the end of life, so finally only one thread is running which seems stuck? Is it possible to redistribute split particles to other threads? ---
**Author:** ![kingyue](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/kingyue/32/507_2.png) [@kingyue](https://openmc.discourse.group/u/kingyue)\ **Post date:** [March 15, 2024, 3:17am UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/4 "2024-03-15T03:17:59Z")
Installing from source solves my issue. This probably relates to [this bug](https://github.com/openmc-dev/openmc/issues/2810) which has been fixed in develop branch. ---
**Author:** ![M\_Zammataro](https://avatars.discourse-cdn.com/v4/letter/m/46a35a/32.png) [@M\_Zammataro](https://openmc.discourse.group/u/M_Zammataro)\ **Post date:** [March 21, 2024, 2:00pm UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/5 "2024-03-21T14:00:58Z")
Thanks for your answers Kingyue. Unfortunately, even after installing the very latest version of the code, the problem still exists. I experienced the same situation as you: out of 48 cores used to run the simulation, only 2 were actually working when the simulation froze. I’ll let you know what I find out. ---
**Author:** ![cp-pete](https://avatars.discourse-cdn.com/v4/letter/c/9e8a1a/32.png) [@cp-pete](https://openmc.discourse.group/u/cp-pete)\ **Post date:** [April 30, 2025, 8:37am UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/6 "2025-04-30T08:37:22Z")
Hi folks, was there any update on how you solved this? I’m currently experiencing this on the latest version. ---
**Author:** ![cp-pete](https://avatars.discourse-cdn.com/v4/letter/c/9e8a1a/32.png) [@cp-pete](https://openmc.discourse.group/u/cp-pete)\ **Post date:** [April 30, 2025, 9:17am UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/7 "2025-04-30T09:17:29Z")
To update everyone, after some testing… I’ve discovered that to solve this you should set the update interval to something higher than 1. more commonly 5-10. Then you can set the amount of realizations to do, I did 3 to test, but have done with 5-10 again and had no problem ---
**Author:** ![jeickman\_RF](https://avatars.discourse-cdn.com/v4/letter/j/c68b51/32.png) [@jeickman\_RF](https://openmc.discourse.group/u/jeickman_RF)\ **Post date:** [October 6, 2025, 1:28pm UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/8 "2025-10-06T13:28:38Z")
I’m reviving this discussion since I’m currently running into the same issues. I’m using version 0.15.2 but have noticed that using weight windows often causes my simulations to freeze as described in this thread. This occurs for me when I’m generating the weight windows or if I’m simply using a weight windows file that was generated in a previous run. This is an issue for using previously-generated weight windows because I can not use the solution detailed above since I don’t have an update interval. This tends to be more likely for larger amounts of particles and batches, but I can’t figure out how to fix it. I set settings.max\_history\_splits to 1,000 but that has not solved the problem. ---
**Author:** ![jeickman\_RF](https://avatars.discourse-cdn.com/v4/letter/j/c68b51/32.png) [@jeickman\_RF](https://openmc.discourse.group/u/jeickman_RF)\ **Post date:** [October 10, 2025, 3:17pm UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/9 "2025-10-10T15:17:57Z")
Update: I reviewed a few other posts with similar issues and came across this one. [Particle lost without warning/error or being killed, stalling the run - #4 by openMC\_guy](https://openmc.discourse.group/t/particle-lost-without-warning-error-or-being-killed-stalling-the-run/3098/4) One solution that worked in that case was changing the seed. I tried this in my case as well (settings.seed = 2) and now my simulation does not freeze. I don’t know why this works, but it is a good enough solution for now. ---
**Author:** ![Juan\_GaliciaA](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@Juan\_GaliciaA](https://openmc.discourse.group/u/Juan_GaliciaA)\ **Post date:** [May 21, 2026, 9:57am UTC](https://openmc.discourse.group/t/simulation-freezes-when-using-weight-windows-version-0-14/3861/10 "2026-05-21T09:57:08Z")
Hi everyone. I am now having the same issue with version 0.15.3. When using 2E+06 particles and 50 batches (1E+08 histories), the simulation (employing a WW file) just get stuck in batch 1. Changing to 20,000 particles and 5000 batches to get the same number of histories of 1E+08, the simulation ran. I do not know exactly why this happens. I was trying to run the case with the solutions you provided before (update interval other than 1 and changing the number of seed to 2 for example) but none of those worked. I generated the WW file using Weight Windows generator with Magic method.