How to comprehend the leakage fraction in output

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1

I can’t find the definition of leakage fraction.In Effective multiplication factor,it multiply the leakage rate both for fast neutron and thermal neutron,but the leakage fraction here seemingly the sum for the whole energy spectrum section.I know at the end of every batch,openmc code will sample new particles from last batch,did the leakage fraction show the lose particle in ‘vacuum’ boundary type in every batch?

thank you all

2

@fyh_sinap That’s correct; the leakage fraction is simply the fraction of the total number of particles that were killed by vacuum boundaries.

3

Is there some tally or way of preserving the leakage fraction in the tallies.out file so it can be referred to in the future?

Thanks, Eleanor

4

While the leakage fraction isn’t written to the tallies.out file, you can find it in the statepoint HDF5 file that is written out. Fetching it from the statepoint in Python would look like:

with openmc.StatePoint(filename) as sp:
    leakage_mean = sp.global_tallies[3]['mean']
    leakage_stdev = sp.global_tallies[3]['std_dev']
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5

Just to clear something up - how should I interpret a leakage fraction higher than 1? For instance, a point source surrounded by beryllium produced a leakage fraction of ~1.4.
Should I interpret the leakage fraction as the ratio of neutrons killed by vacuum boundaries to neutrons produced by the source? In this case, for each source neutron, multiplication results in 1.4 neutrons being terminated at the vacuum boundary.

6

Yes, that is exactly right. It is the number of particles that leak per source particle, hence why you can end up with a value higher than 1 due to (n,xn) reactions.

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