Hydrogen concentration measurement in fuel cladding by backscattered electron image analysis

For high burn-up fuel, increasing of hydrogen absorption to the fuel cladding is serious problem from the viewpoint of keeping the integrity of fuel cladding. The hydrogen is absorbed to the fuel cladding with the formation of oxide layer during irradiation. When the hydrogen concentration exceeds the solid limit, the excess hydrogen precipitates in a hydride phase. Those hydrides tend to precipitate in the outer circumference side because hydrogen has a characteristic to migrate to the low temperature side, and high concentration of hydride causes the fuel cladding embrittlement. Additionally, the migrated hydrides might become the initiation of fractures. Therefore it is important to evaluate hydrogen concentration at local area in claddings to confirm the safety margin of them. A measurement technique for hydrogen concentration using Backscattered Electron Image analysis (BEI method) had been developed by Studsvik Nuclear AB, Sweden. The hydrides in claddings are identified using BEIs which are imaged Scanning Electron Microscope (SEM), and the hydrogen concentrations are calculated from the area fractions of the hydrides in the matrix. The BEI method is very useful for the measurement of local hydrogen concentrations in fuel claddings. In the Reactor Fuel Examination Facility, a sample preparation, imaging conditions of SEM and image analysis procedures for the BEI method were improved. In addition, the hydrogen concentration obtained by the improved BEI method and Hot Vacuum Extraction (HVE) method were compared to confirm the reliability of the improved BEI method. The results showed, the improved BEI method has the same reliability as that of HVE method and can be applied for the Post-Irradiation Examination. (author)