Allow reactions included in depletion chain to be configured

[paulromano]

This pull request expands the list of (possible) transmutation reactions in the deplete/chain.py module. There is now a global dictionary, REACTIONS, that contains all the necessary information for each transmutation reaction. To actually choose which reactions should be included in a depletion chain, the Chain.from_endf method now has a reactions argument that takes an iterable of reaction names, e.g. ["(n,2n)", "(n,3n)", "(n,gamma)"]. The default list of reactions is still the same as it was before.

I've been able to use these capabilities to generate a depletion chain using TENDL-2019 with 36 reactions present (for fusion applications). The way it works is roughly:

from openmc.deplete import chain

chain.Chain.from_endf(..., reactions=chain.REACTIONS.keys())

A few other small fixes/changes:

• Made the reaction name for MT=34 consistent with others
• Fixed the reaction name for MT=180
• If a reaction product is missing during depletion chain generation, the replace_missing function is used to find a suitable replacement

[drewejohnson]

No major issues on my end. Would like to hear what @shimwell or other fusion people think

[shimwell]

Perhaps something could be added to this doc string so users are more aware of that useful list of reactions in chain.Reactions.keys()

[paulromano]

Good suggestion @shimwell. I've just pushed a commit that adds documentation of REACTIONS and references it here.

[shimwell]

thanks for that

[shimwell]

That looks most useful.

reactions using the same defaults as before but also allowing a much larger list to be added.

Not sure I would have spotted that useful list of reactions available in chain.REACTIONS.keys() without seeing your example and reading the source code. Wondering if that could be hinted at in the documentation or docstring.

[drewejohnson]

Based on the feedback by @shimwell #1618 (comment) and the updated documentation 3d35cd3, this looks good to go. Thanks @paulromano and @shimwell!