Transfer rates with multiple destinations

Hello all,

In the most recent version of OpenMC, the transfer rate functionality is implemented as a dictionary with the string source material index as the key, and a dictionary as the value. Within the value dictionary, the key is the string element symbol and the value is a tuple of the rate and destination material.

I think this may be more effective modified slightly, as currently users cannot have transfer rates from a single material lead to more than one destination. If this is attempted using set_transfer_rate or add_transfer_rate, it will simply overwrite the previous value. I’ve given a brief demonstration of what this looks like in the integrator.transfer_rates.transfer_rates variable.

{'2': {'Xe': (0.1, '4')}, '3': {}, '4': {'Xe': (4e-05, '3')}}
{'2': {'Xe': (0.1, '4')}, '3': {}, '4': {'Xe': (0.05, '2')}}

In this example, I would like xenon to be transferred from material 4 to both material 3 and material 2 simultaneously. Additionally, in this case, I cannot add the rates together and just use one destination material, as that would not provide the behavior I seek.

One way to implement this that seems the most straightforward to me would be to change the tuple into a list of tuples, as demonstrated below.

{'2': {'Xe': [(0.1, '4')]}, '3': {}, '4': {'Xe': [(4e-05, '3'), (0.05, '2')]}}

Has anyone already looked into implementing this?

Hi @LukeSeifert,
You’re right, the current implementation of TransferRates only allows to set transfer between one source material and one destination material.

To implement more destinations in the code you would need to modify form_rr_term in and the transfer rates matrices definition in, accordingly.

Otherwise a crude way to do it with the current implementation would be to define a buffer region and set more transfer rates, in your specific case could be:

{'4': {'Xe': (4.05e-05, 'buffer')}, 'buffer': {'Xe': (4e-05, '3')}, 'buffer': {'Xe': (0.05, '2')},}
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