الفهرس 
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Abstract
The uncertainty in the nuclear data in the actinide region should be as small as possible because they are the basis behind all simulations of nuclear reactor core behavior over both long time scale (fuel depletion and waste production) and short time scale (accident scenarios). Large model prediction uncertainties increase design margins, which in turn, increase fuel cycle cost and limit the plant’s power rating. With the core attributes uncertainties understood, optimization upon current design margins could be utilized.
In this paper, a model for very high temperature reactor (VHTR) is developed using Monte Carlo code. The model is used to predict the uncertainty that originates in the reactor parameters due to the uncertainties in reaction cross sections of (U235 - C). U235is the main isotope affect the flux as the U235 is responsible on the number of neutrons and Carbon is the moderator in this type of reactor and occupied the major volume in the core .The effect of the uncertainty in the nuclear cross sections is shown for kelT, the power and the flux.
The flux was found to change linearly with the uncertainty in the fission cross section in the thermal region. Changes in the scattering cross section of carbon also can have significant effect on the flux in the fast, intermediate and thermal regions. The main uncertainty effect on the flux is the fission cross section of U235 in the thermal region.