Apparent temperature and time of annealing of fuel particles ejected from the Chernobyl reactor during the accident

Migration parameters of cesium in UO2 at 1270 and 1670 K and the radius of equivalent sphere (grain or crystallite of UO2) L = 2.4+/-0.7 mu m of the Chernobyl nuclear fuel have been determined from experimental data on the dynamics of specific leaching of Cs-137 from Chernobyl fuel particles. The apparent time and temperature of annealing of fuel particles during the accident have been estimated using the fractionation coefficients of Cs-137 and Sr-90 in real Chernobyl fuel particles and the temperature dependence of the apparent diffusion coefficient. The apparent temperatures and times of high-temperature annealing of fuel particles ejected from the reactor as well as their distributions (median values of T = 2400 K and t = 3.4 s, respectively) show that fuel particles (greater than 20 mu m in size) were formed explosively under conditions of rapidly increased annealing temperature during the accident.