# Strange current behaviour at boundary

**URL:** <https://openmc.discourse.group/t/strange-current-behaviour-at-boundary/2618>\
**Category:** User Support\
**Created:** [January 13, 2023, 6:05pm UTC](https://openmc.discourse.group/t/strange-current-behaviour-at-boundary/2618 "2023-01-13T18:05:16Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![lorenzoloi](https://avatars.discourse-cdn.com/v4/letter/l/43a26b/32.png) [@lorenzoloi](https://openmc.discourse.group/u/lorenzoloi)\
**Post date:** [January 13, 2023, 6:05pm UTC](https://openmc.discourse.group/t/strange-current-behaviour-at-boundary/2618/1 "2023-01-13T18:05:16Z")

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Hello everyone,

I’m currently performing some simple calculations in multi-group mode, aiming to extract group `flux` and `current`.

I’m considering a 2D square made with only one material and vacuum boundary condition, with side 100 cm and a regular mesh 40x40. For the `current` score, as the documentation sugest, I’m using a `MeshSurfaceFilter`.

My question is related to the current value at the boundary: since `vacuum` BC is applied, it is expected a score of 0 particles incoming. However, the outgoing score (i.e., the number of particles that crosses the surface from the square to the vacuum) should not be zero.  
For all the test I performed, the outgoing score at the boundary is either 0 or has magnitude 1e-8, making it negligible with respect to its neighbour cell.

It seems that the MeshSurfaceFilter do not catch the crossing toward the vacuum. What do you think about that?

 ![CurrentAtBorder](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/3/382becd27e9a0103aeeb4c6d8b1c659288f8ab3c.png)

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**Author:** ![lorenzoloi](https://avatars.discourse-cdn.com/v4/letter/l/43a26b/32.png) [@lorenzoloi](https://openmc.discourse.group/u/lorenzoloi)\
**Post date:** [January 17, 2023, 3:07pm UTC](https://openmc.discourse.group/t/strange-current-behaviour-at-boundary/2618/2 "2023-01-17T15:07:22Z")

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Hello again,

I would like to share with you an update: I tried to run a similar case in 1D, adopting both `MeshSurfaceFilter` and `SurfaceFilter` for the `current` score.  
In order to compare the scores with the same mesh size, I have divided the entire geometry in as many cells as the mesh size, so that (if I understood correctly) the two filters should behave in the same way (apart from the fact that `MeshSurfaceFilter` gives the partial currents at the boundaries, while `SurfaceFilter` gives the net value at the surface).

To have a comparison with an analitical solution, me and my colleagues also calculated the expected neutron current from a diffusion approximation.

The diffusion result seems to be in agreement with the `SurfaceFilter` one. Is there a bug in your opinion?

 ![currentSurfaceFilterg1](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/0/078178235bb640724191c8170a30bc5678a02d23.png)