Monte Carlo Simulations of Bowing Effects Using Realistic Fuel Data in Nuclear Fuel Assemblies

Sammanfattning: Deformations of nuclear fuel assemblies have been observed in nuclear power plants since the mid-90s. Such deformations are generally called bowing effects. Fuel assemblies under high irradiation undergo growth and creep induced by high loading forces and low skeleton stiffness of the assemblies which gives permanent deformations and modifies moderation regions. Hence, giving an unpredicted neutron flux spectrum, power distribution, and isotopic concentrations in the burnt fuel. The aim of this thesis is to study the effects of local fuel bowing in terms of power distribution and isotopic composition changes through simulations of the reactor core.  The reactor is simulated with realistic bowing maps and previous deterministically simulated realistic fuel data from a present reactor by deploying the Monte Carlo method using the nuclear reactor code Serpent 2. Two subparts of a full reactor core with fuel from separate fuel cycles are investigated in 2D using burnup. To quantify the impact of the bowing, the change in power distribution and the induced isotopic composition change are calculated by a relative difference between a nominal case and a simulation with perturbed fuel assemblies. The results are presented in colormaps, for visualization. The isotopic composition for U235, U238, Pu239, Nd148, and Cm244 are investigated. Also, statistical uncertainty estimations in the composition of the depleted fuel are done by multiple calculations of the same geometry while changing the seed of random variables in the Monte Carlo calculation. The mean value and the standard deviation in the mass density of U235 and Pu239 are calculated for two pins together with histograms with a normal fit for each case to clarify the mathematical distribution of the calculations.  The simulations performed in this thesis have detected clear impacts of the reactor behavior in terms of power distribution and isotopic composition in the burnt fuel introduced by the bowing. Assembly perturbations of about 10 mm may locally introduce a 10 % relative difference in power density and U235 content between the nominal and the bowed case at 15 MWd/kgU burnup. The power and the isotopic composition changes agree with expectations from the bowing maps.