# Set up time\_step for depletion

**URL:** <https://openmc.discourse.group/t/set-up-time-step-for-depletion/1837>\
**Category:** User Support\
**Created:** [March 20, 2022, 6:08am UTC](https://openmc.discourse.group/t/set-up-time-step-for-depletion/1837 "2022-03-20T06:08:12Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![zein](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/zein/32/1753_2.png) [@zein](https://openmc.discourse.group/u/zein)\
**Post date:** [March 20, 2022, 6:08am UTC](https://openmc.discourse.group/t/set-up-time-step-for-depletion/1837/1 "2022-03-20T06:08:12Z")

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hello everyone, i want to do a depletion calculation for msr thermal reactor. power output msr is 350 MW. how to set the time step and time steps unit in openmc if i want to use unit MWd/kg?

Thank you,

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**Author:** ![imtiaj\_hossain](https://avatars.discourse-cdn.com/v4/letter/i/90db22/32.png) [@imtiaj\_hossain](https://openmc.discourse.group/u/imtiaj_hossain)\
**Post date:** [March 20, 2022, 12:51pm UTC](https://openmc.discourse.group/t/set-up-time-step-for-depletion/1837/2 "2022-03-20T12:51:25Z")

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You can define time steps in two ways,

one is to define directly

timesteps = [(1, ‘MWd/kg’), (2, ‘MWd/kg’),(2, ‘MWd/kg’) (this is 5MWd/KgHM burnup with 3 time step units)  
integrator=openmc.deplete.PredictorIntegrator(operator, burnup, power = power, timestep\_units=‘MWd/kg’)

& the second one is to use numpy array

import numpy as np  
burnup = np.array([1,2,2]) (this is 5MWd/KgHM burnup with 3 time step units)  
burnup = np.diff(burnup, prepend=0.0)  
integrator=openmc.deplete.PredictorIntegrator(operator, burnup, power = power, timestep\_units=‘MWd/kg’)

then run the depletion calculation. u’ll get the resultatnt keff value changes with time which is needed to be converted into MWd/KgHM

time t obtained from depletion result , (unit in s, convert it into days)  
p=350MW  
m= total mass of your system in Kg. (you can get it using operator.heavy\_metal.

now burnup b = p_t/m (MW_d/Kg)

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**Author:** ![zein](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/zein/32/1753_2.png) [@zein](https://openmc.discourse.group/u/zein)\
**Post date:** [March 21, 2022, 10:30pm UTC](https://openmc.discourse.group/t/set-up-time-step-for-depletion/1837/3 "2022-03-21T22:30:20Z")

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thank you @imtiaj_hossain , your answer is very helpful