Bug Description

The get_activity() method of openmc.deplete.Results uses half life values from openmc/data/data.py (ENDF/B-VIII.0).
My expected behavior was that the half life values of the depletion chain file are used.

Steps to Reproduce

The case was also mentioned here https://openmc.discourse.group/t/zircaloy-4-activation-with-jeff33-depletion-chain-nb97-m1/4932

1. Run a OpenMC depletion simulation with the jeff33 depletion chain file.
2. Extract Nb97m atoms and manually calculate activity with jeff33 half life of 52.7s
3. Use get_activity() to get Nb97m activity (half life ENDF/B-VIII.0 58.7s is used)
4. Activity differs by ~10%.

Environment

The discourse post was based on version 0.14.1-dev and it is still valid for 0.15.2.

Ideas for modification

• Inform user in documentation on the level of openmc.deplete.Results about the default half life values from ENDF/B-VIII.0
• Use the half life values from the chain file specified by the user when get_activity is called on openmc.deplete.Results