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Don't Do That Bannerkeff different in MCNP5, MCNPX, and MCNP6

Anomaly: In an input file with more than 100 duplicate unique surfaces, the resulting keff value will be different in MCNP5, MCNPX, and MCNP6.
History: This was a problem submitted to Randy by a user.  That input file was proprietary so this is one Randy created that illustrates the problem.
Versions affected: MCNP5 gives an accurate response, MCNPX aborts with an error, MCNP6 gives a different response.
Impact: In this example, keff for MCNP5 is 1.85133.  For MCNP6, keff is 0.69175.  MCNPX aborts with an error.
Work Around: Don't create input files that have more than 100 duplicate unique surfaces.

Steps required to reproduce this anomaly:

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

About the Input file:

The geometry consists of 192 cells. 96 cells containing 235U/water mix and 96 cells containing 239Pu.

The Uranium oxide cells are at (-100) and the 239Pu at 100.

 

The density in each set of cells is set to randomly change from cell to cell.

 

The complexity of this example is necessary to provide a realistic example of the use of over 200 duplicate surfaces.  While not as common in non-macrobody geometries, duplicate surfaces are very common in geometries that use macrobodies heavily.

 

Click here to download the input file, irand.dat (27 kb).

 

Calculate keff:

In MCNP6, calculate keff.

In MCNP5, calculate keff. 

Notice the different result.

 

Calculate the Total Fission Energy F7:

In MCNP6, calculate the Total Fission Energy (F7) in all cells.

In MCNP5, calculate the Total Fission Energy (F7) in all cells.

Notice the different result.

 

 

Results obtained from our test:

 

MCNP5:

 

keff:

     | the final estimated combined collision/absorption/track-length keff = 1.85133 with an estimated standard deviation of 0.01088

F7:

     cell union total 1.46966E-05 0.0065

 

MCNP6:

 

keff:

     | the final estimated combined collision/absorption/track-length keff = 0.69175 with an estimated standard deviation of 0.00185

F7:

     cell union total 6.71120E-06 0.0056

 

Digging deeper:

To further investigate this problem, try the following changes to the input file:

Change the number of particles per cycle from 100 to some larger number.

Move the source to the Plutonium 239

Use and SDEF volume source to sample all fissionable cells.