Determination of the Uranium Content of Aluminium Alloys

Many materials testing reactors use as fuel an alloy of aluminium and enriched uranium. The amount of U235 in each fuel element must be known accurately. The techniques used to determine this are not only simple in principle but also varied; they include measurement of the alloy density, counting of the γ-activity of the U235, chemical analysis, determination of the isotopic content and evaluation of radiograph blackening. Unfortunately, all these methods possess more or less serious disadvantages when used for inspection on an industrial scale. The measurement of alloy density by Archimedes' method gives sufficiently accurate results if care is taken and the densities of the constituent metals are exactly known. The procedures employed, however, make this a very slow method. Moreover, the isotopic content must be known with great accuracy if the amount of U235 contained is to be determined. Counting the y-activity of the U235 yields a direct evaluation of this isotope, but the many parameters of a single-channel spectrometer require the use of accurately known standards and exceptionally stable counting facilities. Nevertheless, this method is in the authors' opinion the most suitable one for determinations on an industrial scale; and although the choice of standards requires care, and it is a slow and expensive matter to establish them, for any given level of production the choice is made once and for all, the only practical difficulty being to ensure the continued electronic stability of the installation. The other methods mentioned - chemical and isotopic analyses, and densitometry - are not. in common use. Measurement of the blackening of a radiograph is not sufficiently precise and does not permit the determination of uranium content with the required degree of accuracy. Densitometric examination of radiographs is nevertheless very useful, even indispensable, in assessing the homogeneity of an alloy or its mean content compared with two extreme standards on the same negative. Chemical and isotopic analyses can be very accurate, but they are also destructive: certain working precautions must be taken in order on the one hand to eliminate errors due to chemical impurities in the alloy, and on the other to allow for the presence of the different uranium isotopes, the proportions of which must be known for an exact calculation of the U235 content. Finally, the authors show that the measurement of uranium content must be carried out in several stages, each involving all the resources of relatively simple techniques. The value and limits of each technique are discussed. The choice of standards is based on the evidence of radiographs. The exact U235 content of the standards is determined by chemical and isotopic analyses. Production-line inspection is carried out by measuring the alloy density or by counting the γ-activity of the U235. The precision of the two methods is comparable (± 0.5% relative to the uranium content). From the economic point of view the authors recommend determining the U235 content by y-counting whenever a large number of components of a given shape have to be inspected. This subject has already been dealt with. The paper reports and discusses the results of an industrial experiment. (author)