Depletion Assumptions

User Support
1

Hi all,

In Serpent, depletion calculations as a default use a critical spectrum assumption. Does OpenMC do the same or does it have no spectrum modification? I have looked at the OpenMC documentation and the recently published OpenMC depletion paper, but I haven’t found anything about the topic.

Thank you,
Gwendolyn Chee

2

Hey @gwenchee. OpenMC does not do any spectrum modifications. Are you sure that depletion calculations in Serpent use a critical spectrum? Just looking at the input syntax manual, the description for set mdep seems to imply that homogenized cross sections for depletion are calculated using the actual spectrum. We get excellent agreement in depletion calculations between OpenMC and Serpent, so my gut instinct is that Serpent is also not making any spectrum modifications (otherwise it would be reasonable to expect some differences as a result of that).

3

@gwenchee

I would also be interested in hearing more on the critical spectrum assumption in Serpent. This sounds like their fundamental mode calculation (set fum) option, but that is responsible for producing homogenized group constants (macroscopic xs, nu-bar, chi, etc.) not used in depletion.

There is also set xscalc which changes how Serpent obtains the transmutation cross sections (direct tallies vs. spectrum collapsing). This sounds closer but doesn’t seem to indicate any critical spectrum calculation.

4

@paulromano @andrewjohnson

Thanks for your responses. Yes, so I was referring to Serpent’s " set fum" option. The reason why I ask is that I’m participating in the NEA’s FHR Benchmark with OpenMC, and there are some differences in the Serpent and OpenMC keff for the depletion calculation. The benchmark participants are in the process of figuring out what normalizations and settings each participant used so that we can correct for these differences. During our last discussion, the Serpent participants mentioned that they used the critical spectrum assumption and so I was trying to find out if there is an OpenMC equivalent (I am not very familiar with Serpent and so “set fum” is what I assumed they used).

I have pinged one of the Serpent participants for the benchmark to see if he can weigh in here on the settings he used for his Serpent depletion calculations.

Thanks,
Gwen

5

Are the keff differences present at BOL or is it a difference that grows over time?

It can be pretty difficult to get the data/assumptions to match well, even without this new wrinkle of spectrum modification considered.

  1. Are you ensuring that the same cross sections and decay/FPY data are used?
  2. Are you ensuring that the same power normalization is used? The easiest way to achieve this is by calculating observed powers based on fission Q values and ensuring that the Q values are the same between OpenMC and Serpent.
  3. Are you using the “average” fission product yield interpolation mode?
  4. Are you using version 0.12.1? There was a bug in earlier versions that can result in wrong capture reaction rates in the URR.
  5. Are probability tables turned on for Serpent? (they are off by default in Serpent)
  6. Are you using the same depletion time integration method?

I’d encourage you to check out this repo which has matching OpenMC and Serpent models that were used for my own depletion comparisons.

6

The difference grows over time. I used an earlier version of OpenMC to do the depletion calculations (when version 0.12 was still in the dev phase).

Thanks for the excellent recommendations and the repo. I’ll look into each of them.

Best,
Gwen