I’ve attached teh images. This only happens when i use the fuel temperature as 900. when its 1200 it is more gradual and there is a small dip. 
This is quite strange indeed. Can you provide the scripts used to generate these plots? And plots for 900 and 1200 K runs would be helpful as well
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HI, Thanks for the reply. I’ve attached the script as .py it wouldn’t let me upload a .html which may have mad it easier for you to see. I hae also attached the graphs i got for 1200 and the single fuel pincell im trying to copy .
Untitled (1).py (4.5 KB)
Thank you for the script. Nothing really jumps out as problematic in your hex fuel pin model. One thing that stands is your material densities aren’t set. I see things like
fuel_inner.add_nuclide('O16',4.1265E-02)
#fuel_inner.set_density('sum',6.18746E-02)
#fuel_inner.set_density('atom/b-cm',6.18746E-02)
meaning nuclides and elements will be added as atom percents, which is probably consistent with the benchmark. But opemnc will create the material density based on the sum of atom densities, which might be introducing some small discrepancies in the data. This might be un-related to your issue but if the specification you’re working with has the material density I’d recommend adding it.
Are you able to provide the depletion result file for both temperatures? And are both input files totally identically except for
fuel_inner.temperature = 1200
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thank you, you are right. There was an error with one of the densities now it gives viable results
and also i realised the geometry is set up as fuel, coolant then cladding rather than fuel clad coolant. and i think case they both in number density so i think it so we are allowed to do that. thank you for the help again.