Nitrogen Ion Bombardment of Fuel Cladding Materials
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In a nuclear reactor where there are high stresses, temperatures and radiation levels, a number of material problems can occur including corrosion, thermal fatigue cracking, and embrittlement. Material problems are a serious issue in regards to the performance, the advancement, and the safety of nuclear reactors under accident conditions. Fuel cladding materials are of particular significance since cladding is the most important safety barrier, as it contains the majority of radioactive fission products. Nitriding is a method proven to improve many material properties including corrosion and wear resistance. Nitriding works by dissolving or implanting nitrogen into the surface of a material where the nitrogen bonds with the material atoms to form nitrides. The nitriding in these studies is performed by the use of ion bombardment with 40 KeV nitrogen atoms. The purpose of this work is to improve understanding of this type of surface modification on fuel cladding materials. The materials used here are iron, 316L stainless steel, zirconium, and zircaloy-4. The focus of these studies was on fluence dependence and temperature stability of nitride layers induced by ion nitriding. Here it was found that by a fluence of 5E17 N/cm2, both pure Fe and SS316L are saturated and that by a fluence of 1E18 N/cm^2, both zirconium and zircaloy-4 are saturated. By 300°C in both Fe and SS316L and 600°C in zirconium and zircaloy-4, metastable nitrides have become unstable and nitrogen begins to diffuse out. Hardness test show that with a combination of irradiation and annealing, each material’s hardness can greatly be improved. However, the high mobility of nitrogen in iron at temperatures above 300°C limits the uses of iron alloys in very high temperature environments like reactors. Zirconium is shown to be stable up to temperatures approximately twice as high as iron indicating nitrides in zirconium alloys maybe able withstand reactor temperatures.
Castanon, Elizabeth Diane (2016). Nitrogen Ion Bombardment of Fuel Cladding Materials. Master's thesis, Texas A & M University. Available electronically from