Numerical simulation of axial and radial decay heat distribution in spent RBMK-1500 fuel assembly

The RBMK-1500 reactor is a boiling water reactor. Thus, the coolant density and the neutron moderation efficiency vary axially over the fuel assembly height influencing the distribution of decay heat in the discharged fuel. The underestimation of this effect on the decay heat may lead to biases in safety assessments of the spent fuel management. Axial coolant density and power profiles were used in this work to determine axial and radial decay heat distributions in the RBMK-1500 fuel assembly over the first 100 years of cooling using the SCALE code system. It was found that the application of axial profiles influences the increase of estimated total decay heat by 3 % just 2 years after the irradiation is finished. However, the estimation of total decay heat is lower by 2 % after 100 years of cooling compared to the case with no applied axial profiles. The maximum of the normalized decay heat profile also varies between 1.31 and 1.40 over the cooling time. The estimated decay heat in outer pins over the cooling time is higher by 10 divided by 14 % compared to the decay heat produced in inner pins. The variation of results over the cooling phase and reasons behind the difference in estimated decay heat was explained by analysing nuclides which mainly contribute to the total decay heat.