Temperature-dependent thermal conductivity and fuel performance of UN-UO_(2) and UN-X-UO_(2)(X=Mo,W)composite nuclear fuels by finite element modeling

The temperature-dependent effective thermal conductivity of UN-X-UO_(2)(X=Mo,W)nuclear fuel composite was estimated.Following the experimental design,the thermal conductivity was calculated using Finite Element Modeling(FEM),and compared with analytical models for 10%,30%,50%,and 70%(in mass)uncoated/coated UN microspheres in a UO2 matrix.The FEM results show an increase in the fuel thermal conductivity as the mass fraction of the UN microspheres increases from 1.2 to 4.6 times the UO2 reference at 2,000 K.The results from analytical models agree with the thermal conductivity estimated by FEM.The results also show that Mo and W coatings have similar thermal behaviors,and the coating thickness influences the thermal conductivity of the composite.At higher weight fractions,the thermal conductivity of the fuel composite at room temperature is substantially influenced by the high thermal conductivity coatings approaching that of UN.Thereafter,the thermal conductivity from FEM was used in the fuel thermal performance evaluation during LWR normal operation to calculate the maximum centerline temperature.The results show a significant decrease in the fuel maximum centerline temperature ranging from−94 K for 10% UN to−414 K for 70%(in mass)UN compared to UO2 under the same operating conditions.

Proton irradiation-induced cracking and microstructural defects in UN and (U,Zr)N composite fuels

Proton irradiation with a primary ion energy of 2 MeV was used to simulate radiation damage in UN and(U,Zr)N fuel pellets.The pellets,nominally at room temperature,were irradiated to peak levels of 0.1,1,10 dpa and 100.0 dpa resulting in a peak hydrogen concentration of at most 90 at.%.Microstructure and mechanical properties of the samples were investigated and compared before and after irradiation.The irradiation induced an increase in hardness,whereas a decrease in Young’s modulus was observed for both samples.Microstructural characterization revealed irradiation-induced cracking,initiated in the bulk of the material,where the peak damage was deposited,propagating towards the surface.Additionally,transmission electron microscopy was used to study irradiation defects.Dislocation loops and fringes were identified and observed to increase in density with increasing dose levels.The high density of irradiation defects and hydrogen implanted are proposed as the main cause of swelling and consequent sample cracking,leading simultaneously to increased hardening and a decrease in Young's modulus.