Strange (?) behavior of Am-242 in depletion

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Peter Wolniewicz
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Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 3:57 pm

(Running Serpent 2 version 2.1.27)

I cannot understand the behavior of Am-242 in a modelled neutron start-up source (NSS).
I have modelled a reactor and the NSS and run a depletion calculaton burning the NSS.
Original material composition of the NSS is
mat ambe -1.24 rgb 255 0 0 vol 33.8144 burn 1
4009.70c 0.99339
8016.70c 0.00441
95241.70c 0.00220

The activity for some of the nuclides in the NSS is shown in the plot:
NSS.jpg
NSS.jpg (45.11 KiB) Viewed 3075 times
Burnsteps are before year 2005, decay steps are after year 2005.
Looking at Am-242, there is a large drop in total activity (and also the Am-242 mass) at the depletion step when the reactor power is shut down, i.e., when "decstep" is activated in the input file.

Why is the activity of this nuclide dropping a factor of ~100 between during depletion phase and during decay phase?
Thank you very much!

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Jaakko Leppänen
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Re: Strange (?) behavior of Am-242 in depletion

Post by Jaakko Leppänen » Thu Dec 07, 2017 7:40 pm

Looks like it falls to the level of Am242m. Are you sure you are parsing the output right?
- Jaakko

Peter Wolniewicz
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Re: Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 7:58 pm

I can produce the same result using the following method:
1.) I run the dep.m file in matlab
2.) I check the identifier for Am-242 and Am-242m
3.) The identifiers are (Id's) 327 for 952420, and 328 for 952421
4.) Plotting using semilogy (TOT_A(327,:)) and semilogy(TOT_A(328,:)) produces the attached plots:
952420.jpg
952420
952420.jpg (12.91 KiB) Viewed 3059 times
952421.jpg
952421.jpg (12.9 KiB) Viewed 3059 times
The plots for tot_mass have similar shape.

The steps in the input file are these:

set bunorm 1
dep daystep %xenon compensation
1
dep daystep %xenon compensation
1
dep daystep
89.25
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25

%Year:may 2001
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
%Year:may 2002
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
%Year:may 2003
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
%Year:may 2004
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
dep daystep
91.25
%Year:may 2005
dep daystep
46
%middle june 2005
dep decstep
365 365 365 365 365 365 365 365 365 365 365 200 365 365 365 %decay until january 2017 when measurements where performed plus three extra years

Peter Wolniewicz
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Re: Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 8:02 pm

There is approximately a 0.5% difference between 242 and 242m during the decay steps. They are not identical in value, but close.

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Jaakko Leppänen
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Re: Strange (?) behavior of Am-242 in depletion

Post by Jaakko Leppänen » Thu Dec 07, 2017 8:05 pm

Can you send me the input?
- Jaakko

Peter Wolniewicz
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Re: Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 8:06 pm

Yes, You will have it in your email within 3 minutes. Edit: Ok I sent it now to your serpent vtt email.

Peter Wolniewicz
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Re: Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 8:36 pm

I ran the sim again (but with less population) and I got similar results..

Peter Wolniewicz
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Re: Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 8:50 pm

Perhaps its a normal result? Am-242 has a very short half-life and is produced by capture on Am-241 (which is located in the neutron source). As soon as the reactor is shut down all Am-242 decays rapidly but an equilibrium to Am-242m is kept since it decays into Am-242....

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Jaakko Leppänen
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Re: Strange (?) behavior of Am-242 in depletion

Post by Jaakko Leppänen » Thu Dec 07, 2017 8:58 pm

Yes, I think this is what happens. The equilibrium activity is close to that of the source term (Am242m).
- Jaakko

Peter Wolniewicz
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Re: Strange (?) behavior of Am-242 in depletion

Post by Peter Wolniewicz » Thu Dec 07, 2017 8:59 pm

Jaakko Leppänen wrote:Yes, I think this is what happens. The equilibrium activity is close to that of the source term (Am242m).
Thank you! Problem solved and case closed... Im really happy! :D

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