Add energy-dependent branching ratio

[vivian-salino]

After a reaction, an isotope can be left in its ground state (general case) or in an excited state (also called metastable or isomer) with radically different cross sections. The production towards one or the other is called the branching ratio. By default, DRAGON uses fixed branching ratios contained in the Draglib, given when it is generated. For example:

• Pm147(n,γ) reaction can produce Pm148 or Pm148m (ground state or metastable, respectively),
• Am241(n,γ) reaction can produce Am242 or Am242m,
• Np237(n,2n) reaction can produce Np236 or Np236m.

OpenMC works the same way. To take another example, Serpent also has fixed default branching ratios but only for (n,γ) reactions (MT102). Serpent can also dynamically calculate these branching ratios, as they depend on the neutron spectrum, from the information contained in the MF9 or MF10 of evaluations.

Sadly, the branching ratios are "one of the major sources of uncertainty as well as differences between burnup calculation codes." Indeed, DRAGON doesn't dynamically calculate these values, which often leads to inconsistencies (versus evaluation content, other codes, etc). Moreover, it is impossible to propagate uncertainties on MF9 or MF10 through DRAGON (with Total Monte-Carlo).

In order to have this capability in DRAGON, it would be necessary to provide such MF9 and MF10 data into a Draglib, when it is available. DRAGON will also need to be modified, to take that data into account (instead of a fixed branching ratio).

Validation of such developments will be possible through comparisons with Serpent.