Page 1 of 1

Spontaneous fission decay constants

Posted: Fri Apr 10, 2020 8:25 pm
by topefo
Hi folks,

It seems that spontaneous fission decay constants are simply set to the
values of the radioactive decay constants when such data is not available.

For example, in the [input]_mdx1b1.m file I get:

942390 9.11013E-13 8.39745E-13 6 3.10000E-12 0 % Pu-239 spontaneous fission
942390 9.11013E-13 8.39745E-13 4 1.00000E+00 922350 % Pu-239 alpha to U-235

However, in the [input].out file I find:

290 3 94239.03c 03c TRA 94 239 0 239.05218 333.0 9.11013E-13 -BAD--T--F-DU--RN--- plutonium 239 (Pu-239)

In other words, the flag "S" (S - nuclide has spontaneous fission yield data) is missing from
the string "-BAD--T--F-DU--RN---".

How would one obtain these spontaneous fission decay constants, by creating another XS library?
Are such XS libraries available for Serpent2 users?


Re: Spontaneous fission decay constants

Posted: Tue Apr 14, 2020 10:53 am
by Jaakko Leppänen
The S in the .out output refers to spontaneous fission yields (fission product distributions), not the existence of the reaction channel. SF yields are read separately using the "set sfylib" card (similar to set nfy). The contribution of spontaneous fission on fission product distributions is so minute that it can be ignored in all reactor applications.

In the micro-depletion output the decay constants are the same for all decay modes. The probability of the decay mode is given by the 5th parameter. So in your case the fraction of spontaneous fission is 3.1E12 of all decays (the rest are alpha decay).

Re: Spontaneous fission decay constants

Posted: Tue Apr 14, 2020 3:51 pm
by topefo
Many thanks Jaakko!

Best regards,