Why Are Burnup Numbers Identical for Different Fuels in MCNP?

[Rafimah]

TL;DR Summary

Different fuels are giving the same burnup in MCNP

Hi everyone,

I'm trying to compare 3 different fuels and MCNP and I want to recover the burnup of each. When I do that however, I get identical numbers for burnup, which doesn't make sense to me, as they have different materials (LEU vs LEU+ vs a thorium-based fuel).

Does anyone know what could be the issue here? My understanding is that burnup is the energy reduced per unit mass of isotopes >=90, so it should be pretty different for these 3 cases. I uploaded the 3 out files.

Thanks!

 

[Alex A]

Hi @Rafimah , I can't see the out files.

 

[rpp]

I don't see your output files, but I think you have a conceptualization problem. For burnup calculations, ther burnup is an input (usually specified as energy per mass of heavy metal), and the code will calculate the isotopic distribution and k-effective.

It sounds like you ran three cases and received the same burnup. Not surprising, since this is an input. Look at the isotopics and k-effective, and they should be different.

 

[Alex A]

There is a BURN card in the other thread specified as a power level and number of days to burn for but the reactor isn't finished and has a k of around 0.01, it's actually so under critical the code won't run. I think comparing fuels with BURN is valid, but if the conversion ratio is low any fuel is just going to yield the strictly theoretical amount of time and energy.