Method development for the determination of trace impurities in nuclear grade zirconium

Abstract

Zirconium is currently the most suitable cladding material available for nuclear fuel. Its chemical properties allow thermal neutrons to be transmitted through it with minimal loss of these atom-splitting neutrons. However, the purity of the material is of utmost importance as the presence of impurity elements such as hafnium, boron and cadmium among others, may adversely affect the nuclear fission process, by absorbing thermal neutrons. Removal of these impurities from the material continues to be a persistent challenge and the analysis of the purified material is also a challenge. The immediate challenges with the analysis of the material are inadequate, outdated, ineffective analytical methods which are not standardized. Thus the aim of this study was to develop a functional analytical method for the analysis of nuclear grade zirconium using currently available analytical techniques. The research approach involved the development and testing of critical steps performed in the analysis of the material, with a particular focus on the sample preparation step. The dissolution of the material using hydrofluoric acid, including the effect of this mineral acid on each of the impurity elements was studied in detail. The preparation procedure for analysis by inductively coupled plasma –atomic emission spectrometry(ICP-AES)was investigated and evidence to support the effectiveness of the preparation procedure was gathered and evaluated on the basis of recoveries obtained from treated solutions. From this study, the following findings were noted: the use of HF alone will result in non-quantitative determinations of the following elements: Ca, Pb, Sr and the rare earth elements (REEs). The use of a strongly oxidizing acid, HClO4, will prevent the formation of the insoluble fluoride compounds of these elements. Thus, the most effective dissolution media should contain a significant amount of perchloric acid to have a truly quantitative determination of all the elements specified. The preparation and analysis of a reference material was the only means of assessing and validating the effectiveness of the developed method, as there are no available proficiency testing schemes. Lastly, the need to have current literature from others developing similar methods across the world, made available, unhindered by the secrecy that prevails in the nuclear industry. The developed method has been proven to be fit for purpose based on recoveries (Bias) obtained on the reference material NBS 1212a, with the accuracy and precision of measuring 22 of 27 specified elements being within acceptable recovery range of 80-110% and relative standard deviation < 15% RSD. The developed method has quantification limits, for the specified impurity elements, ranging from 0.026 mg L-1 to 0.971 mg L-1 for magnesium and uranium respectively. For elements such as B, Cd, Li and U the LOQ’s obtained were inadequate. The remaining challenge is to quantitatively determine B, Cd, Li and U, which could not be quantified due to lack of sensitivity, by an alternative technique such as ICP-MS. In conclusion, the main challenges of unavailability of standard reference materials; standard methods; and collaborative laboratories for determining the impurity elements in nuclear grade zirconium has been overcome. The analytical method as developed can be implemented as a working procedure that will allow for clear product characterization, furthermore, meaningful contribution to the sparsely available literature on the topic can be made