Re: Disparity in Results between FLUKA & MCNPX (fwd)

From: Mary Chin <mchin_at_mail.cern.ch>
Date: Thu, 7 Jul 2011 17:10:07 +0200

Dear Lewis,

This is very interesting. I would first check the conservations, which
should be convenient to do, given the way you have partitioned the
geometry. The energy has to end up somewhere. If FLUKA is short in the
backscattered component, do you mean FLUKA's neutron fluence is more
forward-scattered than MCNP? Or, do you mean FLUKA and MCNP pass
different proportions of energy to neutrons, photons etc?

I would also check region-by-region the population of neutrons etc. to
get an idea of how they penetrate your beam dump. This naturally
requires applying comparable transport settings in both codes. After
getting these numbers the next step would be to consider their energy
distribution.

> all the neutron doses in MXNPX are up to a factor of 2 higher than in FLUKA,
> whereas the photon doses agree very well.
*Does the total energy balance up? If the neutron component is twice but
everything else agrees, then where is the extra compensated?

> Note that flux-dose conversion factors in MCNPX are in microSv/h and not in
> pSv
*I would first check the energy, before worrying about Sieverts.

Cheers,
mary

On Wed, 6 Jul 2011, Macfarlane, Lewis wrote:

> Dear FLUKA experts,
>
> I've been trying to cross-check some FLUKA calculations I have done
> using MCNPX but I am having problems getting good agreement for some
> scoring regions.
>
> I have a beam dump with a carbon centre, surrounded in iron and then in
> concrete. I have a spherical scoring region around this beam dump that
> has been segmented using cones so that dose & flux can be scored in
> different directions. I am finding that the neutron doses I calculate in
> FLUKA for the directions opposed to the beam direction are up to ~factor
> 3 lower than calculated using MCNPX. Neutron doses in other directions
> (through the beam dump and emerging from the sides) agree fairly well
> and photon doses agree well throughout, it is only the neutrons where
> the disparity is apparent.
>
> I have also attached a simplified version of the same model, without
> biasing and with only the carbon core. Here, all the neutron doses in
> MXNPX are up to a factor of 2 higher than in FLUKA, whereas the photon
> doses agree very well.
>
> FLUKA calculations have been run using Fluka 2011. MCNPX using MCNPX
> Version 2.5.0. All calculations have been run sufficiently long to
> obtain SD < 5% in all scoring regions. I have also found that the
> differences are less apparent in some other materials, i.e. iron and
> aluminium. Note that flux-dose conversion factors in MCNPX are in
> microSv/h and not in pSv so this has to be considered in normalisation
> to compare with the FLUKA.
>
> Any input would be most helpful, many thanks!
> Lewis.
>
Received on Thu Jul 07 2011 - 17:48:26 CEST

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