Means to Avoid Unusually Increased PWR Fuel Rod Corrosion

Analysis of the performance of reload fuel elements as well as of specific experiments have shown that the corrosion behavior can change under certain conditions from the normal thermal behavior to an increased corrosion. There are three different causes for such an increased corrosion: a) A low size of second phase particle (SPP) in the cladding material b) The development of a dense hydride rim at the outer surface of the cladding at a high heat flux above a critical oxide layer thickness c) A concentration of Li in the oxide layer in excess of a critical value Phenomena a and b lead to a particular oxide profile, given by the low temperature dependency buth high neutron flux dependency of this type of corrosion. Phenomenon c results in very localized oxide thickness increases. Phenomena a, the increased corrosion due to fine SPP can be avoided by properly specifying the temperatures and times for the cladding fabrication process via unaccumulated temperature/time parameter the so called A-parameter. The critical A-parameter for increased corrosion was estimated by Siemens with specific irradiation tests. Phenomenon b occur when a dense hydride rim can be formed because some fraction of the corrosion hydrogen is picked up nd because H tends to diffuse to the colder outer side of the cladding due to thermal diffusion. the critical concentration of hydrogen decreases with incraesing health flux as hydrogen redistribution tests and PIE of particular fuel rods have shown. This phenomenon that can arise particularly under full low leakage conditions, can be avoided by use of high corrosion resistant materials as the Optimized Zry-4, the Modified Zry-4 or the ELS DUPLEX claddings developed by Siemens. Phenomenon c occurs when Li from the coolant concentrates in the outer porous part of the oxide at high heat fluxes to a critical concentration. PIE has shown that the concentration factor depends on heat flux, oxide layer thickness and cladding chemical composition. Specific modeling for a modified Li/B strategy has shown that Li enhanced corrosion arises most likely late in the cycle when Li in the coolant is still high and oxide layer thickness has grown to a relativelly high thickness. Therefore, in most cases increased Li contents (e.g. to 2.5 ppm) at the very begin of the cycle comparatively harmless. Alloys enhanced Fe+Cr content compared to Zry-4 as the Modified Zry-4 and the ELS DUPPLEX cladding of Siemens have a higher resistance to Li than Zry-4. (Author)