Unexpected up-scatter XS larger than down-scatter XS

Report any suspected bugs and unexpected behavior here
Post Reply
andrew.hall
Posts: 22
Joined: Fri Feb 17, 2012 1:13 am

Unexpected up-scatter XS larger than down-scatter XS

Post by andrew.hall » Mon Dec 08, 2014 10:03 pm

Hi all,

I was running a simple 2 region pin cell problem in Serpent 1.1.19 and discovered that for one of the regions, the up-scatter cross section (2->1) is larger than the down-scatter cross section (1->2). The two regions are stacked on top of each other and I used 2 energy groups with the cutoff between the two at 1.855eV. Reflective boundary conditions were applied to the exterior of the geometry. See below:

Code: Select all

GTRANSFXS                 (idx, [1:   8]) = [  3.38636E-01 8.5E-05  2.90278E-03 0.02983  5.00108E-04 0.00206  5.20311E-01 0.00133 ];
However, the other region does not see this trend as I would expect:

Code: Select all

GTRANSFXS                 (idx, [1:   8]) = [  3.73628E-01 0.00012  3.52152E-04 0.01673  3.21553E-03 0.00110  4.99889E-01 0.00030 ];
Personally I can't think of a reason why up-scatter would be larger than down-scatter. Any help is greatly appreciated.

Thanks,
Andrew

User avatar
Tuomas Viitanen
Posts: 65
Joined: Thu Mar 25, 2010 1:50 pm
Security question 2: 0
Location: VTT Technical Research Centre of Finland, Espoo

Re: Unexpected up-scatter XS larger than down-scatter XS

Post by Tuomas Viitanen » Fri Dec 12, 2014 11:50 am

Hi Andrew,

First a disclaimer: I'm not at all familiar with the scattering matrices or group-generation in general, so I might be wrong. However, I think that the up-scattering cross section being larger than the down-scattering cross section makes sense, but it is the up-scattering reaction rate that should always be smaller than the down-scattering reaction rate.

How do the corresponding reaction rates, i.e. GRANSFX multiplied by the group fluxes, look like?

Best regards,

Tuomas

User avatar
Jaakko Leppänen
Site Admin
Posts: 2202
Joined: Thu Mar 18, 2010 10:43 pm
Security question 2: 0
Location: Espoo, Finland
Contact:

Re: Unexpected up-scatter XS larger than down-scatter XS

Post by Jaakko Leppänen » Tue Dec 16, 2014 10:10 am

You should also check the scattering probabilities (GTRANSFP).
- Jaakko

andrew.hall
Posts: 22
Joined: Fri Feb 17, 2012 1:13 am

Re: Unexpected up-scatter XS larger than down-scatter XS

Post by andrew.hall » Sun Dec 21, 2014 10:12 pm

Hi Tuomas and Jaakko,

So I checked the reaction rates for the up-scatter and down-scatter:

Up-scatter: RR = 3.98263E-02*2.90579E-03 = 1.15727E-04
Down-scatter: RR = 4.46476E+01*4.99760E-04 = 2.23131E-02

In this instance you are right that the reaction rate for down-scatter is higher. However, I also checked the group transfer probabilities as Jaakko suggested:

Code: Select all

GTRANSFP                  (idx, [1:   8]) = [  9.98526E-01 3.0E-06  5.55302E-03 0.03163  1.47358E-03 0.00204  9.94447E-01 0.00018 ];
This shows that there's a higher probability for up-scatter than down-scatter, correct? This seems wrong to me. The only way I can see this possible is if some of my neutrons are born in group 1 in this region. They then stream from this region to the second region where they scatter into group 2. They then stream back into the first region and then possibly up-scatter? All of this while fewer neutrons down-scatter into group 2 within region 1. Maybe I will try running a case with just the first region and see if these cross sections and probabilities still exist.

Thanks,
Andrew

User avatar
Jaakko Leppänen
Site Admin
Posts: 2202
Joined: Thu Mar 18, 2010 10:43 pm
Security question 2: 0
Location: Espoo, Finland
Contact:

Re: Unexpected up-scatter XS larger than down-scatter XS

Post by Jaakko Leppänen » Wed Dec 24, 2014 3:06 pm

On a second thought, the probabilities of up- and down-scattering are independent of each other, because they are related to reactions occurring at different energy groups. So even though it seems a bit suspicious, there is no physical reason why down-scattering probability should be higher than up-scattering probability. For the rates this is the case, as Tuomas pointed out.
- Jaakko

Post Reply