CREEP-RUPTURE PROPERTIES OF 20Cr-25Ni-Nb STABILIZED STAINLESS STEEL IN IODINE AND TELLURIUM VAPOURS.

Interactions between volatile fission products (e. g. iodine, tellurium, etc. ) and fuel cladding have been observed in a number of reactor systems. To determine the effect of iodine or tellurium on the mechanical properties of the 20Cr-25Ni-Nb steel used as fuel cladding in UK advanced gas-cooled reactors, stress rupture tests have been performed in environments of dynamic vacuum, iodine vapor and tellurium vapor under initial stresses of 47-130 MN/m**2 at 750 degree C. Both iodine and tellurium vapors promoted surface nucleated, intergranular cracking which significantly reduced the specimen rupture life and strain at fracture compared with tests in vacuum. An increase in the iodine vapor pressure from 0. 04 to 56 mbar resulted in further embrittlement. Examination of the surface regions of a specimen after long-term exposure to iodine vapor revealed a porous, duplex scale and significant depletion of chromium from the metal associated with subsurface void formation. Embrittlement mechanisms are discussed.