# Prompt neutrons producing fissions over total number of neutrons

**URL:** <https://openmc.discourse.group/t/prompt-neutrons-producing-fissions-over-total-number-of-neutrons/4098>\
**Category:** User Support\
**Created:** [March 28, 2024, 11:48am UTC](https://openmc.discourse.group/t/prompt-neutrons-producing-fissions-over-total-number-of-neutrons/4098 "2024-03-28T11:48:49Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Dimitrius](https://avatars.discourse-cdn.com/v4/letter/d/2acd7d/32.png) [@Dimitrius](https://openmc.discourse.group/u/Dimitrius)\
**Post date:** [March 28, 2024, 11:48am UTC](https://openmc.discourse.group/t/prompt-neutrons-producing-fissions-over-total-number-of-neutrons/4098/1 "2024-03-28T11:48:49Z")

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Hello everyone,

I need to calculate the ratio of Prompt neutrons producing fissions over Total number of neutrons (ratio unitless).

The numerator should be determined by starting running a specific calculation with settings.create\_delayed\_neutrons = False for getting the Total fission reaction rate (score “fission”) [reactions/source - prompt neutron]. I guess the scores “prompt-nu-fission” and “nu-fission” should then be used as well as, for the total number of neutrons, the score “heating” along with the total reactor power, all of this for normalization and ending with a unitless value of the sought ratio.

Nevertheless, even with all these terms, I cannot get any satisfying formulas.

Could anyone please help me?

Thank you in advance.  
Best regards,  
Dimitrius

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**Author:** ![Dimitrius](https://avatars.discourse-cdn.com/v4/letter/d/2acd7d/32.png) [@Dimitrius](https://openmc.discourse.group/u/Dimitrius)\
**Post date:** [March 28, 2024, 1:17pm UTC](https://openmc.discourse.group/t/prompt-neutrons-producing-fissions-over-total-number-of-neutrons/4098/2 "2024-03-28T13:17:49Z")

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For getting this ratio, should it be fine to consider the equivalent in reaction rates, ie Total fission reaction rate (“fission”) normalized with The recoverable fission energy production rate (“fission-q-recoverable”) and Power, over the Total reaction rate (“total”) normalized with the Total nuclear heating (“heating”) and Power?

I do not know MCNP, but I have been told the “neutrons loss to fissions” (the sought ratio of this topic) is a “natural” output of this code. Does someone know much more?

Best

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<div class="post-metadata">

**Author:** ![Dimitrius](https://avatars.discourse-cdn.com/v4/letter/d/2acd7d/32.png) [@Dimitrius](https://openmc.discourse.group/u/Dimitrius)\
**Post date:** [April 3, 2024, 10:45pm UTC](https://openmc.discourse.group/t/prompt-neutrons-producing-fissions-over-total-number-of-neutrons/4098/3 "2024-04-03T22:45:38Z")

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@paulromano

Dear Paul,

The ratio determined only by fission-q-recoverable (eV/source-prompt-neutron, with settings.create\_delayed\_neutrons = False) over heating (eV/source-neutron) would be sufficient?

Would you have any clues for me regarding all of that?

Deep thanks in advance.