Particle biasing

From: David Horvath <>
Date: Sat, 11 Oct 2008 12:19:33 +0200

Dear Fluka users,

I try to calculate a shielding for a spent fuel of a teaching reactor,
and I tried out different ways of importance biasing, but they gave
different results which i can't explain.

The results

usrtrack_21, inside the shielding:

no biasing:
Tot. response (p/cmq/pr) 9.8644884E-04 +/- 0.4479266 %

sliced geometry with importance biasing:
Tot. response (p/cmq/pr) 9.8818401E-04 +/- 0.1642988 %

with usimbs.f:
Tot. response (p/cmq/pr) 9.8289608E-04 +/- 9.9353164E-02 %

These results are quite the same.

usrtrack_22, last layer of shelding:

no biasing:
Tot. response (p/cmq/pr) 4.3660339E-08 +/- 2.982906 %

sliced geometry with importance biasing:
Tot. response (p/cmq/pr) 3.8885091E-08 +/- 9.0743139E-02 %

with usimbs.f:
Tot. response (p/cmq/pr) 5.2892267E-07 +/- 0.2122020 %

These should be also the same.

For each simulation I used 2 runs with 1,000,000 primaries in each run.

To validate the results, I used MCNP4C with sliced geometry with the
same importance biasing used in Fluka.

The mcnp results (5,000,000 primaries)

Inside the shielding:
  cell (30<40[0 0 0]) 1.21985E-03 0.0024
Last layer of shelding:
  cell 109 4.55585E-08 0.0056

Is it a normal behavior of the biasing, or I did something wrong?

Thank you for your answer.

Best regards,

engineering physicist student
Budapest University of Technology and Economics

I attached the input, usrtrack, source.f, usimbs.f files, and the mcnp
input and output zipped.

Received on Sat Oct 11 2008 - 12:26:15 CEST

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