Abstract
Interactions between volatile fission products, such as iodine, and fuel cladding have been observed in a number of reactor systems. To determine the effect of iodine 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 iodine vapour, air and vacuum under initial stresses of 47 to 130 MN/m 2 at 750° C. Iodine vapour promoted surface-nucleated, intergranular cracking which significantly reduced the specimen rupture life and strain at fracture compared with tests in air or vacuum. Increasing the iodine vapour 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 vapour revealed a porous duplex surface scale and significant depletion of chromium from the metal associated with sub-surface void formation. Embrittlement mechanisms and some possible implications for fuel clad behaviour are discussed.
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