Re: [fluka-discuss]: Pu239 fission and number of secondaries neutron

From: <kem002_at_campus.mephi.ru>
Date: Fri, 19 Apr 2019 18:02:54 +0300

Dear Mr Fontana,

Thank you, I've read the topic. As I understand from Alfredo Ferrari
reply
http://www.fluka.org/web_archive/earchive/new-fluka-discuss/2403.html
> Note that even for a (slightly) subcritical system, fluctuations among
> events (if you start at high energy) can still trigger a stack
> overflow.

In my case, when I have a supercritical system ( 7 cm radius sphere,
22.5 kg Pu239), the program must give me that error "Stack overflow in
Feeder. Execution terminated", even with 5.5 cm radius should give me
that. But, nothing happens, just calculating as normal with ~11 fission
neutrons per primary particle.

Sincerely,
Evgeny Kolodin
postgraduate student, MEPhI

Andrea Fontana wrote 2019-04-19 15:44:
> Dear Evgeny,
>    the conditions foir particle stack overflow were discussed in the
> forum a while ago. This thread might be useful:
>
> http://www.fluka.org/web_archive/earchive/new-fluka-discuss/2399.html
>
> Kind regards,
> Andrea
>
> On 19/04/2019 14:26, kem002_at_campus.mephi.ru wrote:
>> Dear Mr Fontana,
>>
>> Thanks for your response. I also calculated that problem with
>> PRECISION defaults, but the results are the same.
>>
>> I agree with you that comparison with some other codes may be wrong
>> because I could make mistakes in them also.
>>
>> To abstract from other codes and the possibility of mistakes, I
>> suppose to conduct simulations with critical Pu assembly. For Pu239
>> critical mass is close to 11 kg. For 15.7 g/cm3 density, it gives us a
>> critical radius of Pu sphere of 5.51 cm.
>> Next, I increase the radius of Pu sphere in my problem to 7.0 cm. For
>> now, I expect the program termination in reason of particles stack
>> overflow. But calculations are going on. I can't understand why.
>>
>> Sincerely,
>> Evgeny Kolodin
>> postgraduate student, MEPhI
>>
>> Andrea Fontana wrote 2019-04-18 18:13:
>>> Dear Evgeny,
>>>    I am checking your input file and, at a first sight, I would
>>> recommend you to repeat the calculation by using the PRECISION
>>> defaults,
>>> instead of the NEUTRONS defaults, as you did.
>>>
>>> If you look in the Fluka manual, with PRECISION Fluka will perform
>>> a more detailed simulation, in particular with a "fully analogue
>>> absorption for low-energy neutrons", lower thresholds for several
>>> reactions and several other more precise default options. With
>>> NEUTRONS you have instead "Non analogue absorption for low-energy
>>> neutrons with probability 0.95 for the last (thermal) groups".
>>>
>>> Moreover, I am not sure about the Geant4 and MCNP settings, but
>>> these kind of comparisons are always "tricky" and require some
>>> experience in the tuning of the different codes...
>>>
>>> Hope this helps.
>>> Andrea
>>>
>>> On 18/04/2019 14:18, kem002_at_campus.mephi.ru wrote:
>>>> Dear FLUKA users,
>>>>
>>>> I am a beginner, so I'm sorry for the potential silly question.
>>>>
>>>> The problem is shown in the following situation:
>>>> 1. I use low energy pencil neutron beam (2 MeV)
>>>> 2. In distance of 20 cm, I dispose of a sphere with a radius of 4.55
>>>> cm filled by Pu239 material with a density of 15.7 g/cm3.
>>>> 3. Behind the plutonium, I also set the detector in a distance of 40
>>>> cm. It represents water sphere with a radius of  20 cm.
>>>> The geometry has axial symmetry. (Fig. 1)
>>>>
>>>> I calculated the energy spectrum in the detector with USRTRACK card.
>>>> I also calculate this problem with Geant4 toolkit and asked my
>>>> colleague to calculate this on MCNP.
>>>>
>>>> I obtained different spectra between Fluka and MCNP/Geant4. The
>>>> difference appears in neutron amount, the Fluka has lower than
>>>> MCNP/Geant4, but spectrum form is the same. (Fig. 2)
>>>>
>>>> I suppose it is a result of lower secondary neutron production in
>>>> Fluka for comparison with MCNP (3.42 vs 6.45 [fission neutrons /
>>>> primary particle]). After normalizing spectrums on these
>>>> coefficients, they convergent quite well. (Fig. 3)
>>>>
>>>> I can't understand where is the problem. I suppose I could miss some
>>>> cards in Fluka, that 'turn on' some physic processes or something
>>>> like this.
>>>>
>>>> I also concerned about the percentage of secondaries in Fluka
>>>> output. It shows 100% as the number of all secondaries but in the
>>>> listing below it hardly half of this amount. (Fig. 4)
>>>>
>>>> I attach my input file and images to this message.
>>>>
>>>> Sincerely,
>>>> Evgeny Kolodin
>>>> postgraduate student, MEPhI
>>

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Received on Fri Apr 19 2019 - 18:14:53 CEST

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