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Don't Do That BannerCell Rejection with Source Biasing Gives Wrong Answer.

Anomaly: When cell rejection is used with source biasing, MCNP may return a wrong answer.
History: Submitted by user.
Versions affected: MCNP5, MCNPX, MCNP6
Impact: Potential Wrong Answer
Work Around: Don't use cell rejection with source biasing.  Note warning: "biased source position rejection sampling is dangerous" when running.


Steps required to reproduce this anomaly:


Note:  Run this example in the RSICC release of each of these codes.


About the Input file:

This input file contains a truncated cone.


cell rejection source biasing image


cell rejection source biasing input file


Click here to download the input file, iddt7 (2 kb).



Run the input file in MCNP5 and/or MCNPX and/or MCNP6 verify the result.


Work around:

Do not use cell rejection with source biasing.  Note that MCNP gives the following warning:


warning. biased source position rejection sampling is dangerous.


Modify the input file to remove the source biasing by commenting out the sb2 line.


Click here to download the modified input file, iddt7a (2 kb).


Results obtained from our test:

MCNP6 runs with warnings.

cell rejection source biasing fail_run


Open the .o file and view the results.  The source weight should be 1 and it is 1.24

source weight


The tally value should be 1 and it is 1.24.


tally weight