From the point of view of predicting melting behavior of fuel elements containing fission products after 50 percent burn-up, the fuel can be considered to consist of 2000 moles Th, 150 moles U, 55 moles of rate earth metal, 31 moles of Zr, 25 moles of Mo, 20 moles of Rh-Ru-Tc, and 15 moles of alkaline earth metal. All other fission products are present in too small amounts to have any important effect upon the melting point or will have vaporized. However, the presence of alkali metal vapor should be considered.