Fission spectra, CHIT vs CHIP vs CHID

[arfc]

Fission spectra, CHIT vs CHIP vs CHID

[smpark7]

CHI values here to show that the differences are significant.

[smpark7]

Also, I'm not sure about SCALE output format and whether it has the same issue.

[katyhuff]

The actual problem is not quite what you have described here.
The correction is made, appropriately, if accounting for delayed neutrons with the approach:

$$\Chi_p = (1-\beta) \Chi_d$$

We do this in the if block related to _account_delayed.

As discussed in our meeting, though, we should probably do this individually for each group, though... That's a separate issue.

[smpark7]

It should be that because the values I provided above show that for some groups, , while for others. If the equality were true, \chi^p_g should always be smaller than \chi^t_g because (1-\beta) is always less than 1.

Instead, from the Stacey Nuclear Reactor Physics textbook (equation 16.20a) and Alex's paper, the fission source term for neutron group g should be

The current implementation of CoupledFissionKernel for neutron group g with _account_delayed = true is:

.

The implementation differs from the standard formulation by the type of \chi terms used.

[gridley]

Hey @smpark7, I don't know if you ever fixed this, but I just had to do the math on this for another project and thought I'd share my result with you. You're definitely correct that the current approach is incorrect. If there is a separate delayed neutron emission spectrum for each delayed group, the relation should be

Where the subscript j indexes all the delayed neutron groups. This confirms what you're saying. Sorry that this was not put in correctly. I may have written that code... so, apologies!

[smpark7]

Hi @gridley, I have not implemented a fix for this but it should be a relatively simple task of swapping the CHI_T's for CHI_P's under a conditional that checks whether CHI_D data exists. I will self-assign this as a backlog task to be completed in the next week or so but feel free to make the changes if you want to jump in earlier.