MCNP: Integral flux crossing the spherical surface of a spherical cap

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Discussion Overview

The discussion revolves around the use of MCNP (Monte Carlo N-Particle Transport Code) for calculating integral flux across a spherical surface, specifically in the context of a spherical cap. Participants explore the setup of surface tallies and the interpretation of scalar flux in particle transport simulations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests using the tally F1:n 110 and FS1 10 to measure the integral scalar flux through the sphere, indicating that this setup would yield two values based on the defined regions.
  • Another participant expresses uncertainty about the initial question, implying a need for clarification on the intended measurement.
  • A further explanation is provided regarding the nature of scalar flux, where one participant describes how neutrons crossing surfaces and scattering contribute to the scalar flux, emphasizing that they add as scalars rather than canceling out.

Areas of Agreement / Disagreement

Participants appear to have differing levels of understanding regarding surface tallies and scalar flux, leading to some confusion. There is no clear consensus on the initial question or the correct approach to the problem.

Contextual Notes

Some assumptions about the definitions of flux and the setup of the MCNP tallies may be implicit and not fully articulated, which could affect the clarity of the discussion.

xisco
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TL;DR
I want to calculate the integral flux crossing the spherical surface of a spherical cap, which I have defined using a spherical surface and a plane. What tally should I use?
c *************** BLOCK 2: SURFACE CARDS **************
10 PZ 100
110 SO 110
 
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c ---- TALLIES-------------------------------------
F1:n 10
 
I'm not that familiar with surface tallies, so I've checked a few things in the manual. If what you want is an integral of the *scalar* flux in particles (per source particle) then what I think you want is,
Code:
F1:n 110
FS1 10
So that is a tally through the sphere, with the sphere split by surface 10. That should give two values, the integrated flux through the sphere in the region where Z is larger than 100, since the sense is positive in the FS card, and all other flux. All other flux in this simple example is the sphere with Z<100. If you want it the other way around use -10 instead.
 
Thanks for your reply. Maybe I didn't quite understand what the question was.
 
If you have two cells A and B, and a neutron crosses the surface from A to B the flux is now 1 neutron. Say this neutron scatters from something in B, back to A. The scalar flux, as I understand it, is now 2 neutrons. They don't cancel, they don't vector sum, they add like simple scalars. It's the usual meaning of flux, I'm sorry if I made it sound weird. Does that help?
 

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