Abstract: Ab initio calculations based on plane-wave basis pseudopotential method were used to study the effects of helium production from the radioactive decay of tritium in case of TiT_(2).By investigating the electronic structure and elastic constant,it is found that the decay helium occupies the tetrahedron site,instead of the octahedron site though the volume of the octahedron interstice is much larger than that of the tetrahedron interstice.The presence of helium enhances the bonding interaction of the nearest T with Ti.This is the reason of the lowering of the tritium desorption plateau pressure.Accompaning T decay to helium,there is a much large depletion of Ti—T d—s σ bonding bond,and this decreases the bulk modulus largely.