The lattice parameters of simulated DUPIC fuel and UO2 were measured from room temperature to 1273 K using neutron diffraction to investigate the thermal expansion and density variation with temperature. The lattice parameter of simulated DUPIC fuel is lower than that of UO2. and the linear thermal expansion of simulated DUPIC fuel is higher than that of UO2. For the temperature range from 298 to 1273 K, the average linear thermal expansion coefficients for UO2, and simulated DUPIC fuel are 10.471 x 10(-6) and 10.751 x 10(-6) K-1, respectively. (C) 2002 Elsevier Science B,V. All rights reserved.