Why Does MCNP Delete Surfaces in Hexagonal Fuel Element Simulations?

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SUMMARY

The discussion focuses on issues encountered while modeling gas-cooled reactors with hexagonal fuel elements in MCNP (Monte Carlo N-Particle Transport Code). Users reported that MCNP deletes coincident surfaces, which is a standard behavior when multiple surfaces overlap. This leads to simulation failures due to undefined volumes, particularly when particles are lost or geometry errors occur. Solutions include ensuring all cells are properly defined and adding void cells to prevent overlaps.

PREREQUISITES
  • Familiarity with MCNP (version unspecified) for particle transport simulations.
  • Understanding of geometry definitions in computational modeling.
  • Knowledge of gas-cooled reactor design principles.
  • Experience with troubleshooting simulation errors related to geometry.
NEXT STEPS
  • Review MCNP documentation on handling coincident surfaces and geometry definitions.
  • Learn how to utilize the MCNP plotting package to visualize cell definitions.
  • Investigate methods for defining void cells in MCNP simulations.
  • Explore best practices for modeling complex geometries in MCNP.
USEFUL FOR

This discussion is beneficial for nuclear engineers, simulation specialists, and researchers working with MCNP who are involved in modeling gas-cooled reactors and troubleshooting geometry-related simulation errors.

AlexFi
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TL;DR
tried to model gas cooled reactor, MCNP deleting surfaces
Hello
Tried to model gas cooled reactor with hexagonal fuel elements. MCNP keep deleting surfaces (If you could, run my input and check the .txto file) so the simulations won't run
Any advice?
 

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MCNP will delete coincident surfaces, this is normal. It's a bit like you have a table, and a chair, they might be complicated shapes but the bottom of all the legs of the table and the chair will be touching the floor, so you might have a lot of floor level surfaces that are actually all the same. When you build from macro bodies this will happen a lot, MCNP splits these into elementary surfaces and then gets rid of the duplicates.
 
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Thanks for the explanation. It seems to me that MCNP deleted the entire cell and the simulation won't run because particles got lost. What would be a solution for this?
 
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Usually particles getting lost means part of the volume is undefined. In this case there is something beyond cell 9, that isn't defined, and you have no void cell.
 
Made some minor changes, added void cell
Still getting 'particle lost' and 'geometry error:no cell found' error message
 

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Hi Alex,

Have you got the plotting package for MCNP set up? If you view your file you will see you have dotted lines. It means that MCNP thinks that some cells are not properly defined. They may be such that the cells overlap. Or that there are regions with no cell defined.
 

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