# How to include dissociation & recombination effects on tritium transport through 316 steel pipe

**URL:** <https://festim.discourse.group/t/how-to-include-dissociation-recombination-effects-on-tritium-transport-through-316-steel-pipe/178>\
**Category:** User Support\
**Created:** [August 6, 2026, 2:58pm UTC](https://festim.discourse.group/t/how-to-include-dissociation-recombination-effects-on-tritium-transport-through-316-steel-pipe/178 "2026-08-06T14:58:40Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![srichr](https://avatars.discourse-cdn.com/v4/letter/s/ecd19e/32.png) [@srichr](https://festim.discourse.group/u/srichr)\
**Post date:** [August 6, 2026, 2:58pm UTC](https://festim.discourse.group/t/how-to-include-dissociation-recombination-effects-on-tritium-transport-through-316-steel-pipe/178/1 "2026-08-06T14:58:40Z")

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Hi all,

I’m new to the community, and am running a simple diffusion problem for a purge gas system. As a first approximation to build up from, I have T2 at 100Pa inside a 5mm thick stainless steel pipe and am finding the time evolution of tritium inventory and flux in and out of the pipe.

 ![image](https://global.discourse-cdn.com/free1/uploads/festim/original/1X/c82720dc8d7dfbdfb8453c6eafaf7aca95ed3cbb.png)

I initially used SievertsBC at the inner surface and FixedConcentrationBC(concentration=0) on the outer surface, and it all ran fine. I now want to explicitly account for dissociation effect at the inner surface and recombination at the outer surface. I believe I can do this by using:

festim.DissociationFlux to replace the SievertsBC at the inner surface

and

festim.RecombinationFlux to replace the FixedConcentrationBC at the outer surface

However, when I tried swapping the boundary conditions, I got the following error:

AttributeError: module ‘festim’ has no attribute ‘RecombinationFlux’

Please could someone tell me -

1. If my approach is sound and makes sense
2. Why I’m getting this error

Thanks very much!

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**Author:** ![remidm](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/remidm/32/4_2.png) [@remidm](https://festim.discourse.group/u/remidm)\
**Post date:** [August 6, 2026, 3:18pm UTC](https://festim.discourse.group/t/how-to-include-dissociation-recombination-effects-on-tritium-transport-through-316-steel-pipe/178/2 "2026-08-06T15:18:36Z")

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Hi @srichr and welcome to the FESTIM discourse! A few questions:

What version of FESTIM are you using?

If you’re using FESTIM2 then I would point you to [this tutorial](https://festim-workshop.readthedocs.io/en/latest/content/boundary_conditions/h_transport_advanced.html#recombination-and-dissociation)

```python
my_bc = F.SurfaceReactionBC(
    reactant=[H, H],
    gas_pressure=P_H2,
    k_r0=...,
    E_kr=...,
    k_d0=...,
    E_kd=...,
    subdomain=boundary,
)

```

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<div class="post-metadata">

**Author:** ![srichr](https://avatars.discourse-cdn.com/v4/letter/s/ecd19e/32.png) [@srichr](https://festim.discourse.group/u/srichr)\
**Post date:** [August 12, 2026, 1:01pm UTC](https://festim.discourse.group/t/how-to-include-dissociation-recombination-effects-on-tritium-transport-through-316-steel-pipe/178/3 "2026-08-12T13:01:42Z")

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Hi @remidm, yes I am using FESTIM2. Thanks very much for the pointer; I’d managed to gloss over that section before! Now all working 🙂