I’m interested in studying heating in a reactor core.

I ran my model in dev version (0.12.0) twice, with two different commits:

A. From Jan 17 - after the correction of delayed gamma energy (#1449).

B. From Feb 4 - after the correction of photon energy, including separation into photon/electron/positron (#1462).

See tallies below.

My questions are:

In Case B (Feb 4) I observe negative positron energy, why is that?

While neutron heating are same in both cases, when I compare heating of Case A (photon) to Case B (photon+electron, ignoring negative positron energy), it turns out that Case A is ~5% higher. Is that due to TTB?
If so, is Case B more accurate for photon heating?

The reason that the case A results in a higher value is actually due to a bug (#1453) that caused double counting of some of the energy deposition. That bug was fixed with pull request 1462 which is why you see a lower (and more correct) overall heating value from case B.

The fact that you can get negative values for positron heating has to do with how thick-target bremsstrahlung works in OpenMC. Photon energies are randomly sampled between 0 and the energy of the electron/positron. When multiple photons are created from TTB, you can end up getting a total photon energy that exceeds the original energy of the electron/positron. I wouldn’t worry about the negative values – if you sum up the photon, electron, and positron heating, you should get an accurate value of the overall photon-related heating.

Just to make sure I understand - in order to compute total photon-related heating, do you suggest to include the negative positron energy in the sum (photon+electron+positron), or to ignore it?
In the above case, positron energy magnitude is anyway negligible comparing to electron energy, but I assume that it may not always be like that (?).

Yes, I do suggest taking the sum of photon+electron+positron. If the positron energy is negative, that means that too much energy was given to photons, so when you add them together it should balance out. In most cases I’ve looked at, the positron energy is not negative; I’m not sure if there is something about your particular case that causes that behavior.

I also found negative values of positron in my simulations (version 0.13.0). Since I want to consider the contribution of photons in the heating deposition, i.e. photon heating/total heating, how could I proceed?
Since, considering that too much energy is given to photons, this fraction can result wrong.

thank you very much for your answer. My concern was about the evaluation of the single contribution of photons but, as I understand, by summing together the photon+electron+positron heating, I have a compensation of errors to get the right value of photon-related contribution.