Energy-depentdent gamma source from decay calculations

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keckler
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Energy-depentdent gamma source from decay calculations

Post by keckler » Tue Jul 02, 2019 10:57 pm

Hello,

In Serpent, I have a core which has been burned up for some amount of time, having built up a significant inventory of fission products. I am wanting to extract from this fission product inventory the gamma activity as a function of energy within each material. Ideally, I would like to be able to use this energy-dependent gamma distribution as a source definition for gamma transport calculations in Serpent.

I have tried a number of things, but it is not clear to me that this is possible with Serpent. Using a decay step, I am able to obtain the gamma source in each material, but it has no energy-dependence. Adding energy-dependent detectors does not seem to change this either.

I assume that Serpent is aware of this information because it would be important in calculations of decay heat or if somebody wanted to shield some spent fuel. But I cannot find any way to access the energy-dependent gamma source through any of the results files. Do you have any suggestions about how I might be able to accomplish this task?

Thanks!

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Re: Energy-depentdent gamma source from decay calculations

Post by Ville Valtavirta » Thu Jul 04, 2019 11:14 am

Hi,

do you mean something like the radioactive decay source?

-Ville

keckler
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Re: Energy-depentdent gamma source from decay calculations

Post by keckler » Thu Jul 04, 2019 7:13 pm

That is definitely exactly what I mean. Thanks for pointing this out. Makes my life very easy!

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Re: Energy-depentdent gamma source from decay calculations

Post by keckler » Sat Jul 27, 2019 2:07 am

Hello,

I have been using this function for a few different purposes recently, and I think I may have discovered a bug. Your opinion on this would be much appreciated.

I have been working with two different cases: one that is 2D (an infinitely tall fuel bundle that is repeated in a lattice) and one that is infinite (a fuel pebble repeated in an infinite cubic lattice). In the 2D case, the radioactive decay source appears to work fine. But in the infinite case, I am getting an issue.

Basically, I go through some process outlined in the link that you posted a few posts up, but when I tally results in the restart file containing the decay source, the tallies appear to be divided by the volume of the material containing the decay source. To get the correct answer, I need to multiply by the material volume an additional time.

What makes me think that there might be a bug is the fact that I get the expected answer when I follow the same procedure for the 2D case, without having to multiply by the volume an additional time. Is it possible that the code treats infinite cases differently than it treats 2D cases? And if so, might the code be internally dividing by the material volume too many times during the conversion from the original transport calc to the decay source restart?

Edit: I should note, I am not using the 'vol' option in any of my tallies. I am not actually interested in the absolute value of any tally, but rather I am interested in the ratio of two separate tallies, R1/R2. R1 comes from the restart calculation using the decay source, while R2 comes from the original burnup calculation that is used to generate the restart file. So the fact that my answer is off by a factor equal to the material volume makes me think there is some inconsistency between the normalization in the two calculations.

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Jaakko Leppänen
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Re: Energy-depentdent gamma source from decay calculations

Post by Jaakko Leppänen » Mon Jul 29, 2019 9:30 am

I think it might be related to the volumes. Normalization depends on the volume of the radioactive material. Serpent may be able to calculate the volume correctly in one case, but not in the other. Have you tried the -checkvolumes command line option to see which values are used in the calculation?
- Jaakko

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Re: Energy-depentdent gamma source from decay calculations

Post by keckler » Mon Jul 29, 2019 7:33 pm

I have checked, and Serpent uses the same volumes between both cases. In fact the geometry is unchanged between the criticality and the fixed source calculations, so it would seem strange if Serpent could calculate the volumes in one but not the other.

I suppose my other line of thinking is that the tally normalization is just different between the criticality and fixed source calculations. When I have a criticality calculation in an infinite lattice of pebbles and I make some tallies, what exactly are the tallies normalized to? Because if I specify power as the normalization, obviously absolute power does not make sense in the case of an infinite sized core. In this case, I am specifying a power of 1 W. But meanwhile the fixed source photon calculation in the infinite pebble lattice must be inherently normalized to the volume of the source (i.e. the fuel volume). So if the tallies in the infinite pebble lattice criticality calculation are somehow not normalized to the correct fuel volume, this would account for the discrepancy I am seeing.

Does this line of thinking make sense?

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Re: Energy-depentdent gamma source from decay calculations

Post by keckler » Mon Jul 29, 2019 8:39 pm

To back this up a little bit, I did some messing around with the material volumes in my infinite pebble lattice model to try to confirm this.

If I change the fuel volume in the decay source calc using the 'vol' option in the material definition, the tally results change, as expected since the strength of the source is dependent on the volume of the radioactive material. But if I change the fuel volume in the criticality calc, it has no impact on the tally results, which I suppose is also expected since the material volumes typically only impact burnup. But to me, this implies that the two calculations are not really consistent with each other, since tallies in the decay source calculation are impacted by volume while those in the criticality calculation are not.

My other question is why I wouldn't see this same issue in my other model with 2D geometry. As noted previously, I am working with another model that is an infinitely tall fuel bundle repeated in a lattice, and I have gone through the same process and examined the same ratio R1/R2. In that case, I get the expected answer without the volume discrepancy. Might this be because in 2D calculations all results are per unit length?

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Re: Energy-depentdent gamma source from decay calculations

Post by Jaakko Leppänen » Tue Jul 30, 2019 8:48 am

If the criticality calculation is normalized to power, then fuel volume does not affect your tally results. However, the volume affects the results of your burnup calculation if your depletion steps are given in units of burnup. And this will then affect your gamma source in the second calculation.

Did you check that the volumes are correct in both the 2D and the infinite case?
- Jaakko

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Re: Energy-depentdent gamma source from decay calculations

Post by keckler » Wed Jul 31, 2019 2:52 am

I have checked, and the volumes are correct in all of the calculations that I performed.

In all of my cases, depletion steps are given in units of days, not burnup.

However, for the 2D case, I use power density for the normalization in the criticality calculation. I suppose this implies some sort of normalization to volume, and therefore the tallies in both the criticality and subsequent decay source calculations will be normalized to volume. I tested this out, and it gives the expected results. So the root of the original problem is that in my infinite case, I am setting the fission source normalization to just power, not power density.

This is rather interesting and unexpected to me, but I suppose that it does make sense in the end. It might be worthwhile to put a note about this on the page that outlines the radioactive decay source.

Thanks for all your help in getting this understood!

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