Volume of Meshed object changed from CAD model

[Adambar24]

Hi All,

I have created a h5m model using the cad_to_dagmc conversion. It uses the DEMO model found in paramak. The layers of the blanket have been sliced into thin segments for better resolution of the neutronics in OpenMC. I have found a change in volume of the layers before and after the mesh that is up to 20% difference in some of the layers. I used CAD to give me the volumes of the objects before hand. I then converted the files to h5m and used OpenMC's stochastic volumes finder and found large differences between the two. There seems to be some issue when creating the mesh of the object. Any help or work around would be helpful. Below is the difference between the two measurements for each layer

OpenMC | FreeCAD | Percentage Diff

8832000 | 7985387 | 90.4142549818841
18238000 | 18305705 | 100.37123039807
20200000 | 18745010 | 92.7970792079208
19248000 | 19182706 | 99.6607751454697
21572000 | 19619631 | 90.9495225292045
24232000 | 20056649 | 82.7692679102014
24184000 | 20494116 | 84.7424578233543
23466000 | 20931891 | 89.2009332651496
23490000 | 21370217 | 90.9758067262665
25524000 | 21809467 | 85.446900955963
24600000 | 22249448 | 90.4449105691057
25080000 | 22690541 | 90.4726515151515
24934000 | 23133006 | 92.7769551616267
22920000 | 23577108 | 102.866963350785
22330000 | 24023068 | 107.582033139275
22372000 | 24470972 | 109.38213838727
22996000 | 24921086 | 108.371395025222

[shimwell]

Thanks for posting, I wonder if the min_mesh_size and max_mesh_size can be made smaller to get a better mesh, I think they default to 1 and 10 but perhaps try 0.1 and 0.2

[Adambar24]

Thanks Jonathan, I have tired this method out and confusingly got a larger deviation from the expected. There are no errors but in OpenMC the error was not at the magnitude to make a difference to the results. The previous results were completed with a max mesh size of 6 and the newer results with a max mesh size of 2. Both with a minimum of 1.

OpenMC	FreeCAD	Percentage Diff
7952200	7985387	100.417331053042
11224200	18305705	163.091400723437
14287200	18745010	131.201425051795
12628400	19182706	151.901317664946
14636200	19619631	134.04866700373
17770200	20056649	112.866760081485
17750800	20494116	115.454604862879
17316400	20931891	120.87899909912
16977600	21370217	125.873015031571
18126200	21809467	120.320127770851
16726200	22249448	133.021535076706
16534400	22690541	137.232321705051
14691600	23133006	157.457363391326
13121400	23577108	179.684393433628
12141400	24023068	197.860773881101
11732400	24470972	208.576011728206
11853800	24921086	210.2371054008

I have been attempting some workarounds which include:
Creating each part individually -> Struggling to input multiple h5m files into OpenMC with dagmc_universe
Converting to a single stp then converting to h5m -> Currently on going
Using Cubit and the dagmc plugin -> I cannot seem to get cubit to load the plugin it might be some compatibility issue , i am unsure

I think if I could get the cubit plugin working this would be the most robust

[shimwell]

cubit now ships with dagmc included and no longer needs the plugin installing