New reactors presently in design or construction stages, as well as revised operating procedures for existing reactors, have shown an increasing emphasis on extending the exposure time of the reactor fuel elements. However, operating experience at Hanford, as at other installations, has demonstrated that as the amount of burn-up in uranium metal is increased an increase is also noted in operational difficulties resulting from the dimensional behavior of the fuel. During reactor irradiation uranium slugs or rods have been observed to change in length and diameter, to warp, and to develop surface roughening.