Lower K_eff than expected

User Support
1

Hi, ive had this error for a while now and cant seem to fond the issue. My av K_eff from another simulation for the same model was found to be in the region of 1.3 to 1.4 however im getting values of 0.011. So clearly something is wrong. Any help would be appreciated, im new to Openmc and currently have no idea why this is happening.

Finished_Reactor_ (1).ipynb (74.6 KB)

2

Hi Aaron,
I think there was a problem with your core model since some cells are overlapping when I am doing geometry debug

openmc.run(geometry_debug=True)

at the beginning, Overlapping cells detected: 28, 17 on universe 8
since the geometry is quite complicated, then I only being able to know that cell 17 is the cell filled with your core configuration universe (hex_lat_2_cell) and cell 28 is your sheilding_inner_steel_cell.
I think you assume that openmc will not fill the space/region that has been declared before, but actually you need to make sure that your cell region does not overlap with other cell regions.
so, I am doing a modification just to check,

image
image1514Ă—780 60.6 KB

and after that, those cells are not reported as overlapping again, but geometry debug reports other cells overlapping i.e. ERROR: Overlapping cells detected: 48, 36 on Universe 9
So I recommend you check your model again, especially on the cell’s region definition.
FinishedReactorcheck.ipynb (803.7 KB)
Here is my small modification on your material definitions (small typo) and plotting your cell region to check, i.e. your fuel rod region overlaps with other cell regions in U1 such as the steel cladding, coolant, and graphite.
I am not a fan of RightCircularCylinder for surface definition since I feel that RCC is complicated so I usually use simple surfaces like cylinder and plane, but you have a cool core configuration right there. Great

3

thank you very much @wahidluthfi, i had originally assumed that openmc would reduce the size of a bigger cell if a smaller cell was placed over the top, neglecting the volume of the cell in which the new one is placed. if i understand what your saying then this is a core issue with my geometry since i defined most of it in this way.

4

yop, that’s the reason behind your small keff, because the way you declare your region makes your cell overlay the other cells. I think your fuel is stacked by cladding, water, and graphite cells, so when neutrons come into that coordinate, it doesn’t interact with the fuel but interacts with graphite instead.
When you declare that a fuel rod surface is A(cylinder with Ra and Za), and the cladding surface is (cylinder with Rb and Zb), then your cladding region should only cover the region between the outer side of the surface of A and the inner side of surface B. another cell might also slightly crossing your previously defined cell since its surface is crossing, that’s why you need to double check your surface while constructing cell’s region.
I am using openmc.run(geometry_debug=True) for checking the overlap cells and cells that didn’t crossed by any neutron during simulations.

5

I’ve worked back through your solution an this worked great, thanks !

1 Like