Etching methods for Electron Microscopy of Actinide Nitride Fuel.

Sammanfattning: This report describes the process of a Bachelor thesis project conducted by two students at the KTH Royal Institute of Technology in Stockholm, Sweden. The project consists of establishing a functioning etching method for enhancing the visibility of grain structure in uranium nitride ( ) and uranium-zirconium nitride ( ). Revealing the grains and grain boundaries is desirable for the evaluation through photo mapping, of actinide nitride fuel pellets that have been produced at KTH. The work process involves an initial literature study to get an idea of earlier employed methods on the subject, followed by experimental work and evaluation as well as modification of the tried procedure. After each tried procedure the obtained results are evaluated using an optical microscope (OM) and the final results as the project approaches its end, a scanning electron microscope (SEM) is employed. The iterative process culminates in the establishment of a number of effective methods. A standard lab protocol is compiled that may be followed for carrying out a suitable etching procedure. Six different compositions of acids are used in order to obtain the best visibility of grains. When the results are evaluated in OM some grains are assumed to be in an oxidised layer, the chemical solutions are changed in order to prevent this oxidisation. The SEM analysis later indicates that they actually are grains of uranium or uranium and zirconium – not grains of an oxidised layer. As a result it is discovered that aqua regia yields the best grain visualisation in both OM and SEM. However as an exclusive etchant aqua regia is only tested once – all other experiments with aqua regia are carried out as an after-treatment for other tried procedures. Furthermore, it is discovered that grains are visible on all tested pellets if they are etched with a two-step (or three-step) method. Step one is a composition of 12.5 ml nitric acid and 12.5 ml lactic acid with a duration time of 30 seconds. Step two is a composition of 12.5 ml nitric acid, 12.5 ml lactic acid and two drops of hydrofluoric acid with a duration time of 60 seconds. If grain structure is not visible in OM at this stage, step three consists of treating the sample with aqua regia, with a duration time of approximately 60 seconds.