I’m trying to study OpenMC depleting calculation function and coupled it with my small PWR core simulation in the next step. But I meet a problem when I repeated the Pincell depletion example. The value of k-eff is going to 0.85 form 0.77 but not going down. It’s obviously can’t make sense.

I don’t know which part of this model has a problem. The most codes are the same as the example. The only different part is the chain file, I used PWR spectrum simplified chain which downloaded form OpenMC database website in the depletion calculation. Will that affect the calculation results?

Please contact me if you know the reasons for this problem. Thank you very much!

So it’s definitely strange, but not impossible. Since the only differences are in particle statistic (your notebook using 1000 particles per generation vs. 100 in test notebook) and depletion chain, the growth in k must come from one of those. My suspicion is that this pin cell has a faster spectrum (average neutron energy higher than typical light water reactor) and is turning in to a breeder. Subcritical system implies that neutron leakage, non-fission absorption, or both are increasing. If there are more high energy absorptions in U238, it is not impossible that Pu is being produced as the system is depleted.

Can you provide concentrations of the following isotopes over time: U235, U238, Pu239, and Pu240?

Also, to remedy the initially subcritical system, I would recommend adding axially reflective boundary conditions from https://docs.openmc.org/en/latest/examples/pincell.html. This way you should see an initial k of ~1.4. I hope this helps,

It appears there is an error in your code in how you are using the rectangular_prism function. You have a width and height of 0.62 cm, which means bounds of -0.31 and 0.31 cm in the x and y directions. Since the fuel radius is 0.42 cm, it extends beyond these bounds, and effectively your model is almost entirely fuel and no water. The width really should be twice as much, 1.24 cm. Thus, as Andrew suggested, you’ve turned your pincell into a breeder because of the fast spectrum, which results in an increasing keff.

One other comment is that you have 30 day timesteps, which are probably too long. This is especially true at the beginning where you generally need small timesteps to resolve the initial buildup of Xe135.

Thanks for your messages. I redo the depletion calculation with a modified fuel pin model, and the results look more reasonable than the previous simulation. But the k-ff value turns up at the end of the life-time. I think it’s because of the build-up of Pu isotope atoms but can not make sure of that. The K-eff waving plot and the concentrations of U235, U238, Pu239, and Pu240 atoms over time plots are attached in this mail.

Can you help me take a look of this? Thanks for your help!

The uncertainties on keff are really large, so that increase on the last timestep doesn’t appear to be statistically meaningful. Two general comments here:

The number of particles you are running is really small. This will result in reaction rates that have pretty big uncertainties, which will propagate into the depletion calculations.

Your timesteps are still very large (30 days) which can lead to errors.

Using more particles and shorter timesteps should give you a better sense of the true behavior.