Anomalously high dr -8 output

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gavin.ridley.utk
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Anomalously high dr -8 output

Post by gavin.ridley.utk » Fri Jun 29, 2018 2:39 am

Hi,

I'm trying to calculate heat deposition fractions in a simple geometry using the dr -4 and dr -8 response functions. From what I understand, dr -4 includes all neutron-driven reactions that produce heat, including fission and captures. dr -8 should be just fissions, and thus always less than dr -4. For some reason, using the below input, I see a dr -8 detector giving higher output than a dr -4 on the same material, both with volumes normalized correctly.

In this very simple geometry (3 concentric cubes), I set the power to 20 kilowatts (set power 20e3), and then let serpent run.

Code: Select all

set title "Simple heat deposition"

set acelib "sss_endfb7u.xsdata"
set declib "sss_endfb7.dec"
set nfylib "sss_endfb7.nfy"


% UMo fuel
mat fuel -17.0 vol 512.
92235.06c 80.0
Mo-nat.06c 20.0

% iron metal and some hydrogen
mat iron -13.0 vol 1352.
1001.06c .1
Fe-nat.06c .9

% I acknowledge that water S(a,b) libs shouldnt be used with zirc hydride...
mat zrh -5.6 moder hsab 1001 vol 2232.
Zr-nat.06c 1.0
1001.06c 1.6 
therm hsab lwj3.11t

% Cube of UMo fuel encased in zirc hydride and iron
surf 1 cuboid -4.0 4.0 -4.0 4.0 -4.0 4.0
surf 2 cuboid -7 7 -7 7 -7 7
surf 3 cuboid  -8 8 -8 8 -8 8

set bc 2

cell 1 0 fuel -1
cell 2 0 zrh 1 -2
cell 3 0 iron 2 -3
cell 4 0 outside 3

det modNtPow dr -4 zrh dv 2232.
det fuelNeutPower dr -4 fuel dv 512.
det fuelFissPower dr -8 fuel dv 512.
det shieldNeutPower dr -4 iron dv 1352

% 20 kilowatts
set power 20e3

plot 1 500 500
mesh 1 500 500

set pop 2000 50 10
From this, I get this detector output:

Code: Select all

DETmodNtPow = [
    1    1    1    1    1    1    1    1    1    1  3.09235E-01 0.00465
];


DETfuelNeutPower = [
    1    1    1    1    1    1    1    1    1    1  2.92318E+03 0.00689
];


DETfuelFissPower = [
    1    1    1    1    1    1    1    1    1    1  3.33515E+03 0.00690
];


DETshieldNeutPower = [
    1    1    1    1    1    1    1    1    1    1  3.52345E+01 0.00737
];
This output doesn't quite make sense to me. Serpent should normalize the fission rate using that Q value of 202 MeV, which accounts for additional energy due to decays and gamma captures. If this is the case, why does my fission power detector show 33 kilowatts? Shouldn't this exactly match the prescribed 20 kW?

Any help is greatly appreciated!
Last edited by gavin.ridley.utk on Fri Jun 29, 2018 5:06 pm, edited 1 time in total.
Gavin Ridley

Yellowstone Energy

Ville Valtavirta
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Re: Anomalously high dr -8 output

Post by Ville Valtavirta » Fri Jun 29, 2018 12:12 pm

Hi Gavin,

you were probably trying to use

Code: Select all

det modNtPow dr -4 void dm zrh dv 2232.
det fuelNeutPower dr -4 void dm fuel dv 512.
det fuelFissPower dr -8 void dm fuel dv 512.
det shieldNeutPower dr -4 void dm mercury dv 1352
instead of

Code: Select all

det modNtPow dr -4 zrh dv 2232.
det fuelNeutPower dr -4 fuel dv 512.
det fuelFissPower dr -8 fuel dv 512.
det shieldNeutPower dr -4 mercury dv 1352
The second argument of the dr card specifies which material should be used as the response material each time the detector will be scored. Your input used "fuel" as the target material even if the collision happened in some other material.

The fixed input will yield

Code: Select all

DETmodNtPow = [
    1    1    1    1    1    1    1    1    1    1  1.57796E-01 0.00459 
];


DETfuelNeutPower = [
    1    1    1    1    1    1    1    1    1    1  3.44171E+01 0.00004 
];


DETfuelFissPower = [
    1    1    1    1    1    1    1    1    1    1  3.90625E+01 0.00000 
];


DETshieldNeutPower = [
    1    1    1    1    1    1    1    1    1    1  7.01670E-01 0.00571 
];
Now

Code: Select all

>> 3.90625E+01*512
ans =  20000
and summing up the "dr -4" heating from the other materials yields

Code: Select all

>> 3.44171E+01*512 + 1.57796E-01*2232 + 7.01670E-01*1352
ans =    1.8922e+04
or approximately 95 % of the "dr -8" value.

"dr -8" will give you 202.27 MeV per U-235 fission, whereas "dr -4" will use the neutron KERMA coefficients from the cross section library.

You should note that the recoverable energy from a U-235 fission is approximately 193.4 MeV and the rest of the 202.27 MeV comes from an approximation that neutrons produced in fission are absorbed in the geometry (not necessarily in fuel), and additional binding energy is released in (n,gamma) reactions. I think the 202.27 MeV approximation is generally OK for large LWR systems and may be off for other kinds of systems (as we see here).

We are currently working on more accurate heat deposition modes that are based on using the KERMA data in conjunction with the fission energy release data MT 458 and can be complemented with photon transport to yield accurate spatial distributions as well.

-Ville

MartinJean
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Re: Anomalously high dr -8 output

Post by MartinJean » Mon Sep 17, 2018 8:22 pm

Hello,

I wanted to know if the energy model 2 & 3 presented in the 8th serpent user group meeting in may 2018 was implemented in SERPENT V2 or if any beta version was available upon request. I was interested in taking correctly into account the energy release by capture in non fissile material for depletion. Currently, I have modified my deterministic code to mimic the model 1 of SERPENT.

Thanks for your support!

Martin

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