Absorption rate

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1

Hi,

I am running a simple PWR pincell with reflective B.C. and get the following result:

k-effective (Collision) = 1.30104 +/- 0.00207
k-effective (Track-length) = 1.30411 +/- 0.00233
k-effective (Absorption) = 1.30268 +/- 0.00151
Combined k-effective = 1.30224 +/- 0.00171
Leakage Fraction = 0.00000 +/- 0.00000

Now I’m trying to calculate k_eff based on reaction rates, so I add tally scores (with no filters) and get:

Total Material
Fission Rate 0.528691 +/- 0.000847503
Nu-Fission Rate 1.30104 +/- 0.00206856
Absorption Rate 493.162 +/- 0.426163
heating 1.05294e+08 +/- 154104.0
Flux 92.3223 +/- 0.324063
Total Reaction Rate 536.032 +/- 0.454614

Now, as you can see, the nu-fission rate is very similar to the k_eff (up to statistical uncertainty), which means absorption rate should be ~1 (no leakage).
However, the absorption rate is extremely high (493.162), which seems nonphysical.
Any idea about that?

Another question that may be related:

When I calculate the ratio heating/k_eff for different conditions (boron, temperatures) I always get the same number (again, up to statistical uncertainty).
This also suggests that the absorption rate should be around unity, right?

Thanks,
Shai

2

Hi @shaikinast. Yes, generally in a problem with no leakage the absorption rate should be close to unity. The problem of secondary neutrons in (n,2n) and (n,3n) reactions causes the total absorption per source neutron to be slightly greater than unity, but nothing like what you’re seeing. This looks like some kind of bug but hard to say what exactly. I tried doing the same tallies on a pincell problem and I don’t see the same behavior (I get an absorption of 1.001). Would you be willing to share your model so that I can dig into this further? Thanks!

3

Ok, so what’s happening here is that you are running a coupled neutron–photon calculation and tallying the absorption rate, so the tallied value indicates the absorption rate of all particles, not just neutrons. If you add a particle filter:

tally.filters = [openmc.ParticleFilter(['neutron', 'photon'])]

you’ll be able to see the absorption rate for neutrons and photons separately. For neutrons, you should get a value very close to 1. For photons, you will see a much higher value and this is indeed physically correct. In your problem, each neutron generates on average ~10 photons. Each of those photons in turn generates ~40 photons on average through a cascade of electron/positron/photon interactions. Note that most of these photons are X-rays resulting from atomic relaxation. Since each particle produced has to be absorbed eventually, this is why you see a very high absorption rate for photons.

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4

That indeed makes a lot of sense.

Thank you very much for your help Paul!