Virtual Zero Density Detector Materials

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Virtual Zero Density Detector Materials

Postby infoseek » Fri Jul 06, 2018 5:16 am

Hello all,
I'd like to estimate Helium production rates in a steel in a reactor simulation. I have read that Helium is produced primarily through neutron capture in Nickel. Ni-58 undergoes radiative capture to become Ni-59, a synthetic element.
This Ni-59 undergoes an (n,alpha) reaction to produce helium, which will form bubbles in the steel, degrading its material properties.

How should I record the reaction rate of (n,alpha) in Ni-59 if I don't have any Ni-59 in my simulation in the first place?
Do I need to create a 'virtual material' or zero-density placeholder in my steel for Ni-59?
OR do I need to run burnup simulations in order to be able to model the buildup of Ni-59? I'd like to avoid this, if possible.

Here's a pseudo-code version of what I'd like to do, for clarity. Not sure if it's possible though.
--------------------------------------
%%%% HT9 Steel from literature
%%%% Now with separated Ni isotopes for Ni detectors.
%%%% Note that Ni-59 is not present in the initial composition.
mat steel -7.67323857
Cr-nat.09c -0.12
C-nat.09c -0.002
Mo-nat.09c -0.01
V-nat.09c -0.005
Mn-55.09c -0.002
Si-nat.09c -0.0025
Ni-58.09c -0.003359885
Ni-60.09c -0.001338798
Ni-61.09c -5.9168E-05
Ni-62.09c -0.000191741
Ni-64.09c -5.04075E-05
W-nat.09c -0.005
Fe-nat.09c -0.8485

%% Helium production detector materials.
mat Ni58 1.0 Ni-58.09c 1.0
mat Ni59 1.0 Ni-59.09c 1.0 %% Here I've defined the isotope that is not present initially in the steel.

%% Detectors for helium production
det ni59_abs_rate dr -2 Ni59 %% Total macroscopic absorption in Ni-59. Not present initially.
det ni59_alpha_rate dr 107 Ni59 %% helium production in Ni-59. Not present initially.
det ni58_gamma dr 102 Ni58 %% Radiative capture in Ni-58.
--------------------------------------

Will I be able to record the reaction rates in Ni-59 this way?
Also, on a related note, are the negative and positive MTs equivalent here (i.e. micro and macro cross sections), because I've defined the detector materials as having a single nuclide of density of 1.0?
Thanks for your help on this issues.
Best,
Louis Gregg
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Re: Virtual Zero Density Detector Materials

Postby Jaakko Leppänen » Fri Jul 06, 2018 10:27 am

This looks like the right way to calculate the reaction rates. The macroscopic and microscopic reaction rates are equivalent when the atomic density is set to 1. You should note, however, that reaction -2 includes all capture modes, which may or may not include also other reactions in addition to (n,gamma) and (n,alpha).

To calculate the helium production rate for Ni-59 you need to know the density, which depends on the irradiation time. I don't know how large the absorption cross section for Ni-59 is, but since this is a long-lived isotope, its concentration does not reach an equilibrium by radioactive decay.
- Jaakko
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Jaakko Leppänen
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